A computer’s a useless hunk of metal & plastic until somebody feeds it a program, which is a list of commands teaching the computer how to perform a task. A person who writes a program is called a programmer. Now you’ll learn how to write programs, so you become a programmer.

You learned to buy & use programs such as Microsoft Office. Now you’ll learn how to invent your own programs, so you can become the computer’s master and make it do whatever you wish, not limited to the creations of other programmers.

Programming the computer can be easy. You’ll write your own programs just a few minutes from now, when you read the next page. Then on your résumé you can brag you’re a “programmer.” As you read farther, you’ll learn how to become a good programmer, by writing programs that are more sophisticated.

Learning to program is fun, an adventure that expands your mind and turns you into a brilliant thinker.

To program the computer, feed it a list of commands written in a computer language. Each computer language is a small part of English. The easiest popular computer language is Basic.

Basic was invented in New Hampshire by 2 Dartmouth College professors
(John Kemeny & Tom Kurtz) in 1964 and improved afterwards. Basic consists of words such as “print”, “input”, and “if”. I’ll explain how to program the computer by using those Basic words.

Different computers speak different dialects of Basic.

For example, a popular dialect was invented in 1975 by a 19-year-old kid, Bill Gates. Since he developed software for microcomputers, he called himself Microsoft and called his Basic dialect Microsoft Basic.

A different dialect of Basic was invented in 1981 in England by the British Broadcasting Company (BBC) then greatly improved by Richard Russell, who’s amazingly brilliant. His newest and best version is called BBC Basic for Windows. In many ways, it’s better than Microsoft Basic. You can download it from his Website for free (unless you need advanced features that cost extra).

This chapter explains BBC Basic for Windows, because it’s the most pleasant programming language for beginners.

After reading this chapter, you can explore Visual Basic, which is explained in a later chapter. Visual Basic, invented by Microsoft, is harder than BBC Basic for Windows but more powerful: it lets you teach the computer to create windows & buttons and handle mouse clicks.

The commands of BBC Basic for Windows are explained on these pages:

The functions of BBC Basic for Windows are explained on these pages:

Those are the best commands & functions for beginners, but BBC Basic for Windows includes many more!

 

Let’s have fun programming! If you have any difficulty, phone me at 603-666-6644 (day or night) for free help.

Get Basic

Here’s how to copy the nicest Basic (BBC Basic for Windows, version 6.14a) from the Internet to a Windows 10 or 11 computer, free (using Microsoft Edge):

Start Basic

Double-tap “BBC BASIC for Windows” (which is on the desktop screen). Tap “OK”.

You see the program window, titled “BBC BASIC for Windows 6.14a (trial)”.

To make your life easy (and follow the examples in this book), make sure the computer allows lowercase typing, by doing this:

Type your program

Now you’re ready to type your first program!

For example, type this program:

Here’s how:

At the end of that line, press the Enter key. (You must press the Enter key at the end of each line.) Pressing the Enter key makes the computer indent the line and makes the word “print” become orange.

A program is a list of commands that you want the computer to obey. The sample program you typed contains one command, which tells the computer to do some math: it tells the computer to compute 4+2, get the answer (6), and print the answer onto the screen.

Run your program

To make the computer obey the program you wrote, tap the “4” (which is green). That tells the computer to run the program: the computer will run through the program and obey any commands in it. The computer obeys the “print 4+2” command and prints this answer onto the
output window:

Congratulations! You’ve written your first program! You’ve programmed the computer to compute the answer to 4+2! You’ve become a programmer! Now you can put on your résumé: “programmer!”

When you finish admiring the computer’s answer, close the output window, by tapping its X button.

Edit your program

After you’ve typed your program, try typing another one. For example, create a program that makes the computer print the answer to 79+2. To do that, make this program appear on the screen:

To make that program appear, just edit the program you typed previously (which said print 4+2). To edit, do this:

If you’ve edited the program successfully, the screen shows just the new program —

and you don’t see the old program anymore.

When you’ve finished editing the program, run it by tapping the green triangle. Then the computer will print the answer:

Your program can contain several lines. Each line is a separate command. For example, your program can be:

When you run that program (by tapping the green triangle), the computer will obey both commands, so the output window will show both answers:

While editing, use these tricks.…

Fix your errors

What happens if you misspell a computer word, such as “print”? For example, what happens if you accidentally say “primpt” instead of “print”?

Here’s the result:

Abbreviation

Instead of typing the word “print”, you can abbreviate: type just the letter “p” and a period. When you press the Enter key at the end of the line, the computer will change the “p.” to “print”.

So instead of typing —

you can type just:

Immediate mode

To make Basic do math, you’ve learned to type a program then make the computer run it (by tapping the green triangle), so the answer appears in the output window. But here’s a faster way: type directly in the output window! That technique is called immediate mode. Here’s how to do it.

Make the output window appear. If it hasn’t appeared yet, make it appear by using one of these methods:

In the output window, you see this symbol:

Next to that symbol, type any command you wish, such as:

At the end of that line, press the Enter key. The computer will obey that line immediately, so it will show:

Try it!

Math

This program makes the computer add 4+2:

When you run the program (by tapping the green triangle), the computer will print the answer:

If you want to subtract 3 from 7, type this command instead:

(When typing the minus sign, do not press the Shift key.) The computer will print:

You can use decimal points and negative numbers. For example, if you type this —

the computer will print:

Multiplication To multiply, use an asterisk. So to multiply 2 by 6, type this:

The computer will print:

Division To divide, use a slash. So to divide 8 by 4, type this:

The computer will print:

To divide 2 by 3, type this:

The computer will round the answer and print just:

If you try to divide by 0, by typing —

the computer will refuse. Instead, it will print this gripe:

Avoid commas Do not put commas in big numbers. To write four million, do not write 4,000,000; instead, write 4000000.

E notation If the computer’s answer is huge (more than a million) or tiny (less than .0001), the computer might print an E in the answer. The E means “move the decimal point”.

For example, suppose the computer says the answer to a problem is:

The E means, “move the decimal point”. The plus sign means, “towards the right”. Altogether, the E12 means “move the decimal point towards the right, 12 places.” So look at 8.516743297 and move the decimal point towards the right, 12 places; you get 8516743297000.

So when the computer says the answer is 8.516743297E12, the computer really means the answer is 8516743297000, approximately. The exact answer might be 8516743297000.2 or 8516743297000.79 or some similar number, but the computer prints just an approximation.

Suppose your computer says the answer to a problem is:

After the E, the minus sign means, “towards the left”. So look at 9.23 and move the decimal point towards the left, 6 places. You get:

.00000923

So when the computer says the answer is 9.23E-6, the computer really means the answer is:

.00000923

You’ll see E notation rarely: the computer uses it just if an answer is at least a billion (1,000,000,000) or tiny (tinier than .0001). But when the computer does use E notation, remember to move the decimal point!

If you want to write E notation yourself, you must capitalize the E.

Rounding To print a long number, the computer will round it and print just 9 significant digits.

The highest number The highest number the computer can handle well is about 1E4932, which is 1 followed by 4932 zeros. If an answer is much higher, the computer will say:

The tiniest decimal The tiniest decimal the computer can handle easily is about 4E-4932, which is a decimal point followed by 4932 digits (4931 zeros then a 4). If you try to go much tinier, the computer will say 0.

Order of operations What does
“2 plus 3 times 4” mean? The answer depends on whom you ask.

To a clerk, it means “start with 2 plus 3, then multiply by 4”; that makes 5 times 4, which is 20. But to a scientist, “2 plus 3 times 4” means something different: it means “2 plus three fours”, which is
2+4+4+4, which is 14.

Since computers were invented by scientists, computers think like scientists. If you type —

the computer will think you mean “2 plus three fours”, so it will do 2+4+4+4 and print this answer:

The computer will not print the clerk’s answer, which is 20. So if you’re a clerk, tough luck!

Scientists and computers follow this rule: do multiplication and division before addition and subtraction. So if you type —

the computer begins by hunting for multiplication and division. When it finds the multiplication sign between the 3 and the 4, it multiplies 3 by 4 and gets 12, like this:

So the problem becomes 2+12, which is 14, which the computer prints.

For another example, suppose you type:

The computer begins by doing all the multiplications and divisions. So it does 2*3 (which is 6) and does 72/9*5 (which is 8*5, which is 40), like this:

So the problem becomes 10-6+40, which is 44, which is the answer the computer prints.

You can use parentheses the same way as in algebra. For example, if you type —

the computer will compute 5-2 and print:

You can put parentheses inside parentheses. If you type —

the computer will compute 10-(5-2), which is 10-3, and will print:

Strings

Let’s make the computer fall in love. Let’s make it say, “I love you”.

Type this program:

Here’s how to type the second line:

When you run that program (by tapping the green triangle), the computer will obey your command; it will print:

You can change the computer’s personality. For example, if you give this command —

the computer will reply:

Notice that to make the computer print a message, you must put the message between quotation marks. The quotation marks make the computer copy the message without worrying about what the message means. For example, if you misspell “I love you”, and type —

the computer will still copy the message (without worrying about what it means); the computer will print:

Jargon The word “joy” consists of 3 characters: j and o and y. Programmers say that the word “joy” is a string of 3 characters.

Strings versus numbers The computer can handle two types of expressions: strings and numbers. Put strings (such as “joy” and “I love you”) in quotation marks. Numbers (such as
4+2) do not go in quotation marks.

Accidents Suppose you accidentally put the number 2+2 in quotation marks, like this:

The quotation marks make the computer think “2+2” is a string instead of a number. Since the computer thinks “2+2” is a string, it copies the string without analyzing what it means; the computer will print:

It will not print 4.

Suppose you want the computer to print the word “love” but you accidentally forget to put the string “love” in quotation marks, and type this instead:

Since you forgot the quotation marks, the computer thinks love is a number instead of a string but doesn’t know which number, since the computer doesn’t know the meaning of love. Whenever the computer is confused, it gripes at you. In this particular example, when you run the program the computer will print this gripe:

So if you incorrectly tell the computer to proclaim its love, it will say no.

Longer programs You can program the computer to say it’s madly in love with you!

Let’s make the computer say:

To make the computer say all that, just run this program:

To run that program, type it then tap the green triangle. Try it!

To have even more fun, run this program:

It makes the computer print “I long”, then print the answer to 2+2 (which is 4), then print “U”. So altogether, the computer prints:

Yes, the computer says it longs for you!

Adding strings You can add strings together: “hot”+“dog” is the same as “hotdog”. If you say —

the computer will print:

 

Tricky printing

Printing can be tricky! Here are the tricks.

Indenting Suppose you want the computer to print this letter onto the screen:

This program prints it:

In the program, each line contains 2 quotation marks. To make the computer indent a line, put blank spaces AFTER the first quotation mark.

Blank lines Life consists sometimes of joy, sometimes of sorrow, and sometimes of a numb emptiness. To express those feelings, run this program:

Altogether, the computer will print:

Semicolons Run this program:

The top line, which makes the computer print “fat”, ends with a semicolon. The semicolon makes the computer print the next item on the same line; so the computer will print “her” on the same line, like this:

The next program shows what happens to an evil king on a boat:

The computer will print “sin”, and will print “king” on the same line, like this:

Notice that in a print statement, you can type several items (such as “sin” and “king”). Between the items, type a semicolon.

Spaces around numbers To print a number, the computer wants to print 10 characters. If the number is shorter than 10 characters, the computer puts blank spaces before the number, to make the total number of characters be 10. Exception: if the “print” statement has a semicolon before the number, the computer does not put blank spaces before the number.

For example, “print 2+3” makes the computer print 5 with blank spaces before it —

but “print ;2+3” makes the computer print 5 without blank spaces before it:

Suppose you want the computer to print the answer to 2+2 and also the answer to 5+4, both on the same line. If you write —

the computer will print the answer to 2+2 (which is 4) and put blank spaces before it, then print the answer to 5+4 (which is 9) without blank spaces, like this:

To separate those two answers from each other, you can tell the computer to print a space between the numbers, like this:

That makes the computer print:

This command is correct:

The computer will print “eat ” (which includes a blank space in its ending), then 3+2 (which is 5), then “ apples” (which begins with a blank space), so the computer will print:

If you accidentally put no spaces in the strings, like this —

the computer will print no spaces in the answer, so it prints this jumble:

Double quotation marks Let’s make the computer print this:

To make the computer print quotation marks around “wow”,
put DOUBLE quotation marks around that word:

If you accidentally put just single quotation marks, like this —

the computer will print “You really ” then complain it doesn’t know how to wow. The computer will print:

Copy program onto paper To print a copy of your program onto paper, make sure the printer is turned on. Then tap the Printer icon (which is above your program) then “OK”.

For example, if you program says —

the printer will print this copy onto paper:

The printer automatically underlines all orange words (such as “print”). It also prints today’s date on the paper’s bottom-right corner, so you can brag about when you printed it.

Copy answers onto paper To copy the computer’s answers onto paper, make the program’s top line say:

Make sure the printer is turned on, then run the program (by tapping the green triangle).

The answers will still appear in the ouput window, as usual; but afterwards, when you close the output window (by clicking its X), the printer will print a copy of the answers.

 

For example, if the program is —

the printer will print 6 onto paper (although the 6 won’t be quite as far to the right as you expect).

If you want the printer to copy just some of the answers, type “vdu 2” above the lines whose answers you want copied and type “vdu 3” below them. The “vdu 2” means “copy to paper”; “vdu 3” means “hide from paper”; but all printing gets delayed until you close the output window (or until the computer encounters a line saying “vdu 12”, which forces the printer to spit out the answers immediately).

Save

If you want the computer to copy the program onto drive C, tap the Save icon. (It’s above your program. It’s left of the Printer icon. It’s below the “U” in “Utilities”. It looks like a floppy disk, which is square.)

To make sure things stay simple, do this: tap “This PC” then double-tap “Documents”.

If you haven’t invented a name for the program yet, here’s what happens:

Exception: if the name you invented was already used by another program, the computer asks you, “Do you want to replace it?” Tap the “Yes” button if you want the new program to replace the old program, so the old program disappears. If you do not want the new program to replace the old program, tap the “No” button instead of “Yes” then invent a different name for your new program.

Suppose you’re creating a program that’s so long it takes you several hours to type. You’ll be upset if, after several hours of typing, your town suddenly has a blackout that makes the computer forget what you typed. To protect yourself against such a calamity, tap Save every 15 minutes. Then if your town has a blackout, you’ll lose just a few minutes of work; the rest of your work will have already been saved on the disk. Saving your program every 15 minutes protects you against blackouts and also “computer malfunction” and any careless errors you might make.

New When you’ve finished inventing and saving a program, here’s how to erase everything in the program window, so you can start writing a different program instead: tap “File” (which is near the program window’s top-left corner) then “New”.

(If you didn’t save the program you worked on, the computer asks, “Save current changes?” If you want to save the program you worked on, tap the “Yes” button; if you do not want to save the program you worked on, tap the “No” button instead.)

Here’s a faster way to erase everything in the program window:

Load If you saved a program onto the hard disk (drive C), here’s how to use it again: tap the Load icon. (It’s above your program. It looks like a file folder opening up, so it’s also called the Open icon. It’s below the “E” in “Edit”.) If the computer asks “Save current changes?” tap the “No” button (unless you really want “Yes”).

The computer says “Load”.

The first box should say:

If it doesn’t say that yet, do this:

Then you see a list of Basic programs (and other files) in drive C’s Documents folder. Double-tap the program you want.

Then the program appears in the program window. To run it, tap the green triangle.

Save As Suppose you’ve saved a program called Joe. Here’s how to create a variant of it, called Joe2, so your hard disk contains both versions, Joe and Joe2.

First, create Joe and save it. Then make sure the program window still shows Joe’s program lines. Edit those program’s lines, to create whatever variant you want. Then name that variant “Joe2” by doing this:

Exit

When you’ve finished using Basic, close its window (the program window) by tapping its X button.

(If you didn’t save the program you worked on, the computer asks, “Save current changes?” If you want to save the program you worked on, tap the “Yes” button; if you do not want to save the program you worked on, tap the “No” button instead.)

Then the computer will exit from Basic.

Line numbers

You can number the lines in your program. For example, instead of typing —

you can type:

Then when you’re discussing your program with another programmer, you can talk about “line 2” instead of having to talk about “the line that mentions sandwiches”.

Selective numbering You can number just the lines you’re planning to discuss.

For example, if you’re planning to discuss just lines 1 and 3, you can number just those lines:

Or if you prefer, number them like this:

(When you type an unnumbered line, the computer automatically indents it to look like that.)

Number by tens Instead of making line numbers be 1, 2, 3, etc., make the line numbers be 10, 20, 30, etc., like this:

Then you can insert an extra numbered line without changing your numbers:

Number automatically Above your program, you see this icon:

If you click it and then press the Enter key (while letting the “First line” box say 10, the “Increment” box say 10, and the “Remove unused line numbers box” be blank), the computer will automatically number all the lines in your program (10, 20, 30, etc.).

Multi-statement line

In your program, a line can contain several statements separated by colons, like this:

When you run that program, the computer will print “I dream” then print “of you”, like this:

If you want to number the line, put the number at the far left, like this:

Become an expert

Congratulations! You’ve learned how to program!

C’mon, write some programs! It’s easy! Try it. You’ll have lots of fun!

A person who writes a program is called a programmer. Congratulations: you’re a programmer!

Write several programs like the ones I’ve shown you already. Then you can put on your résumé that you have “a wide variety of programming experience”, and you can talk your way into a programming job!

The rest of this chapter explains how to become a good programmer.

Practice Programming the computer is like driving a car:
the only way to become an expert is to put your hands on that mean machine and try it yourself.

If you have a computer, put this book next to the computer’s keyboard. At the end of each paragraph, type the examples and look, look, see the computer run! Invent your own variations: try typing different numbers and strings. Invent your own programs: make the computer print your name or a poem; make it solve problems from your other courses and the rest of your life. The computer’s a fantastic toy. Play with it.

If you’re a student, don’t wait for your instructor to give lectures and assign homework. Act now. You’ll learn more from handling the computer than from lectures or readings. Experience counts.

Hang around your computer. Communicate with it every day. At first, that will be even harder than talking with a cat or a tree, because the computer belongs to a different species, a different kingdom; but keep trying. Get to know it as well as you know your best friend.

If you’re taking a French course, you might find French hard; and if you’re taking a computer course, you might find computers hard also. But even a stupid 3-year-old French kid can speak French, and even kindergarten kids can program the computer. They have just one advantage over you: practice!

Be bold In science fiction, computers blow up; in real life, they never do. No matter what keys you press, no matter what commands you type, you won’t hurt the computer. The computer is invincible! So go ahead and experiment. If it doesn’t like what you type, it will gripe at you, but so what?

Troubles When you try using the computer, you’ll have trouble — because you’re making a mistake, or the computer is broken, or the computer is weird and works differently from the majority computers discussed in this book. (Each computer has its own “personality”, its own quirks.)

Whenever you have trouble, laugh about it, and say, “Oh, boy! Here we go again!” (If you’re Jewish, you can say all that more briefly, in one word: “Oy!”) Then get some help.

Get help For help with your computer, read this book! For further help, read the manuals that came with your computer or ask the genie who got you the computer (your salesperson or parent or boss or teacher or friend).

If you’re sitting near computers in your office, school, or home, and other people are nearby, ask them for help. They’ll gladly answer your questions because they like to show off and because the way they got to know the answers was by asking.

Computer folks like to explain computers, just as priests like to explain religion. You’re joining a cult! Even if you don’t truly believe in “the power and glory of computers”, at least you’ll get a few moments of weird fun. Just play along with the weird computer people, boost their egos, and they’ll help you get through your initiation rite. Assert yourself and ask questions. “Shy guys finish last.” To get your money’s worth from a computer course, ask your teacher, classmates, lab assistants, and other programmers lots of questions!

Your town might have a computer club. (To find out, ask the local schools and computer stores.) Join the club and tell the members you’d like help with your computer. Probably some computer hobbyist will help you.

Call me anytime at 603-666-6644: I’ll help you, free!

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

A letter can stand for a number. For example, x can stand for the number 47, as in this program:

The top line says x stands for the number 47; x is a name for the number 47.

The bottom line says to print x+2. Since x is 47, the x+2 is 49; so the computer will print 49. That’s the only number the computer will print; it will not print 47.

Jargon

A letter that stands for a number is called a numeric variable. In that program, x is a numeric variable; it stands for the number 47. The value of x is 47.

In that program, the statement “x=47” is called an assignment statement, because it assigns 47 to x.

A variable is a box

When you run that program, here’s what happens inside the computer.

The computer’s random-access memory (RAM) consists of electronic boxes. When the computer encounters the line “x=47”, the computer puts 47 into box x, like this:

box x          47

Then when the computer encounters the line “print x+2”, the computer prints what’s in box x, plus 2; so the computer prints 49.

Undefined variable

In that program, if you forget to type “x=47”, here’s what happens:

More examples

Here’s another example:

The top line says y is a numeric variable that stands for the number 38.

The bottom line says to print y-2. Since y is 38, the y-2 is 36; so the computer will print 36.

Example:

The top line says b is 8. The bottom line says to print b*3, which is 8*3, which is 24; so the computer will print 24.

One variable can define another:

The top line says n is 6. The next line says d is n+1, which is 6+1, which is 7; so d is 7. The bottom line says to print n*d, which is 6*7, which is 42; so the computer will print 42.

Changing a value

A value can change:

The top line says k’s value is 4. The next line changes k’s value to 9, so the bottom line prints 18.

When you run that program, here’s what happens inside the computer’s RAM. The top line (k=4) makes the computer put 4 into box k:

box k           4

The next line (k=9) puts 9 into box k. The 9 replaces the 4:

box k           9

That’s why the bottom line (print k*2) prints 18.

After that program runs, the computer will keep remembering k is 9. In the output window, if you type —

the computer will print 9. The computer will keep remembering k is 9, until you close the output window. Closing the output window makes the computer forget the values of all variables.

Hassles

When writing an equation (such as x=47), here’s what you must put before the equal sign: the name of just one box (such as x). So before the equal sign, put one variable:

The variable on the left side of the equation is the only one that changes. For example, the statement d= n+1 changes the value of d but not n. The statement b=c changes the value of b but not c:

The third line changes b, to make it equal c; so b becomes 7. Since both b and c are now 7, the bottom line prints 14.

“b=c” versus “c=b” Saying “b=c” has a different effect from “c=b”. That’s because “b=c” changes the value of b (but not c); saying “c=b” changes the value of c (but not b).

Compare these programs:

In the left program (which you saw before), the third line changes b to 7, so both b and c are 7. The bottom line prints 14.

In the right program, the third line changes c to 1, so both b and c are 1. The bottom line prints 2.

While you run those programs, here’s what happens inside the computer’s RAM. For both programs, the second and third lines do this:

box b           1

box c           7

In the left program, the fourth line makes the number in box b become 7 (so both boxes contain 7, and the bottom line prints 14). In the right program, the fourth line makes the number in box c become 1 (so both boxes contain 1, and the bottom line prints 2).

 

Modifying itself A variable can modify itself:

The top line says d is 5. The next line says d’s new value is what d was before, plus 2, so d is 5+2, which is 7. The bottom line prints 7.

Instead of typing “d=d+2”, you can type just “d+=2”, which means “d is increased by 2”.

Here are similar statements:

Example:

The top line says d is 100. The next line says d’s new value is d’s old value divided by 4 (so d is divided by 4), so d becomes 25. The bottom line prints 25.

When to use variables

Here’s a practical example of when to use variables.

Suppose you’re selling something that costs $1297.43, and you want to do these calculations:

To do those four calculations, you could run this program:

But that program’s silly, since it contains the number 1297.43 four times. This program’s briefer, because it uses a variable:

It’s also easier to update, if later your boss says you must change 1297.43 to a different number.

So whenever you need to use a number several times, turn the number into a variable, which will make your program briefer and easier to update.

String variables

A string is any collection of characters, such as “I love you”. Each string must be in quotation marks.

A letter can stand for a string — if you put a dollar sign after the letter, like this:

The top line says g$ stands for the string “down”. The bottom line prints:

In that program, g$ is a variable. Since it stands for a string, it’s called a string variable.

Every string variable must end with a dollar sign. The dollar sign is supposed to remind you of a fancy S, which stands for String. The second line is pronounced, “g String is down”.

If you’re paranoid, you’ll love this program:

The top line says t$ stands for the string “They’re laughing at you!” The later lines make the computer print:

Spaces between strings

Examine this program:

The bottom line says to print “sin” and then “king”, so the computer will print:

Let’s make the computer leave a space between “sin” and “king”, so the computer prints:

To make the computer leave that space, choose one of these methods.…

 

Nursery rhymes

The computer can recite nursery rhymes:

The top line says p$ stands for “Peas porridge ”. The later lines make the computer print:

This program prints a fancier rhyme:

Lines 1-3 define h$, m$, and c$. The later lines make the computer print:

Long variable names

A numeric variable’s name can be a letter (such as x) or a longer combination of characters, such as:

The variable’s name must begin with a letter. It can include small letters, capital letters, digits, and underlines. It can be as long as you wish!

It must not begin with a word that has a special meaning to the computer. For example, it cannot begin with the letters “print”.

If the variable stands for a string, it must end in a dollar sign.

Beginners are usually too lazy to type long variable names, so beginners use variable names that are short. But when you become a pro and write a long, fancy program containing hundreds of lines and hundreds of variables, you should use long variable names to help you remember each variable’s purpose.

In this book, I’ll use short variable names in short programs (so you can type those programs quickly) but long variable names in long programs (so you can keep track of which variable is which).

Programmers employed at Microsoft capitalize each word’s first letter and can’t use underlines. So instead of writing —

they write:

That’s harder to read; but if you’re programming for Microsoft, you have no choice.

 

 

Humans ask questions; so to turn the computer into a human, you must make it ask questions too. To make the computer ask a question, use the word “input”.

This program makes the computer ask for your name:

When the computer sees that input line, the computer asks “What is your name?” then waits for you to answer the question. Your answer will be called n$. For example, if you answer Maria, then n$ is Maria. The bottom line makes the computer print:

When you run that program, here’s the whole conversation that occurs between the computer and you; I’ve underlined the part typed by you.…

Try that example. Be careful! When you type the input line, make sure you type the two quotation marks and the semicolon. You don’t have to type a question mark: when the computer runs your program, it will automatically put a question mark at the end of the question.

Just for fun, run that program again and pretend you’re somebody else.…

When the computer asks for your name, if you say something weird, the computer will give you a weird reply.…

Abbreviation

Instead of typing the word “input”, you can abbreviate: type just the letter “i” and a period. When you press the Enter key at the end of the line, the computer will change the “i.” to “input”.

So instead of typing —

you can type just:

 

College admissions

This program prints a letter, admitting you to the college of your choice:

When the computer sees the INPUT line, the computer asks “What college would you like to enter?” and waits for you to answer. Your answer will be called c$. If you’d like to be admitted to Harvard, you’ll be pleased.…

You can choose any college you wish:

That program consists of three parts:

Input versus print

The word “input” is the opposite of the word “print”.

The word “print” makes the computer print information out. The word “input” makes the computer take information in.

What the computer prints out is called the output. What the computer takes in is called your input.

Input and Output are collectively called I/O, so the input and print statements are called I/O statements.

Once upon a time

Let’s make the computer write a story, by filling in the blanks:

To write the story, the computer must ask for your name, your friend’s name, and a verb. To make the computer ask, your program must say “input”:

Then make the computer print the story:

Here’s a sample run:

Here’s another run:

Try it: put in your own name, the name of your friend, and something you’d like to do to your friend.

Contest

The following program prints a certificate saying you won a contest. Since the program contains many variables, it uses long variable names to help you remember which variable is which:

Here’s a sample run:

Numeric input

This program makes the computer predict your future:

Here’s a sample run:

Suppose you’re selling tickets to a play. Each ticket costs $2.79. (You decided $2.79 would be a nifty price, because the cast has 279 people.) This program finds the price of multiple tickets:

This program tells you how much the “oil crisis” costs you when you drive your car:

Here’s a sample run:

Conversion

This program converts feet to inches:

Here’s a sample run:

Trying to convert to the metric system? This program converts inches to centimeters:

Nice day today, isn’t it? This program converts the temperature from Celsius to Fahrenheit:

Here’s a sample run:

See, you can write the Guide yourself! Just hunt through any old math or science book, find any old formula (such as f=c*1.8+32), and turn it into a program.

 

You can control how your computer goes and stops.

Wait

A centisecond is a very short period of time: just a hundredth of a second!

If you say —

the computer will wait for 600 centiseconds, which is 6 seconds. That’s how to make the computer pause for 6 seconds.

Give the 6-second pause before you reveal a joke’s punch line:

That program makes the computer print the joke’s setup (“Human, your intelligence is amazing! You must be an M.D.”), then pause for 6 seconds, then reveal the joke’s punch line, so the screen finally shows:

If you want the computer to pause for 9 seconds instead of 6, say “wait 900” instead of “wait 600”.

This program makes the computer brag, then confess:

The computer begins by bragging:

But then the computer pauses for 10 seconds and finally admits:

This program makes the computer change its feelings, in surprising ways:

The computer will print —

then pause 3 seconds and change it to —

then pause 4 seconds and change it to —

then pause 5 seconds and change it to —

then pause 6 seconds and change it to:

Experiment: invent your own jokes, and make the computer pause before printing the punch lines.

Repeat

This program makes the computer print the word “love” once:

This fancier program makes the computer print the word “love” three times:

When you run that program, the computer will print:

Let’s make the computer print the word “love” many times. To do that, we must make the computer obey this line repeatedly:

To make the computer obey the line repeatedly, say “repeat” above the line, so the program looks like this:

The computer automatically indents the print line for you, when you press the Enter key at that line’s end.

How often do you want the computer to repeat? Underneath all that, say “until false” (which means “forever”), so the program looks like this:

The computer automatically unindents the “until” for you, when you press the Enter key at that line’s end.

When you run that program, the computer will print “love” many times, so it will print:

The computer will print “love” on every line of the output window.

But even when the output window is full of “love”, the computer won’t stop: the computer will try to print even more loves onto your screen! The computer will lose control of itself and try to devote its entire life to making love! The computer’s mind will spin round and round, always circling back to the thought of making love again!

Since the computer’s thinking keeps circling back to the same thought, the computer is said to be in a loop. In that program, “repeat” means “repeat what’s underneath & indented”; “until false” means “do it forever”. The lines that say “repeat” and “until false” — and the lines between them — form a loop, which is called a repeat loop.

To stop the computer’s lovemaking madness, you must give the computer a “jolt” that will put it out of its misery and get it out of the loop. To jolt the computer out of the program, abort the program by using one of these methods:

In that program, since the computer tries to go round and round the loop forever, the loop is called infinite. The only way to stop an infinite loop is to abort it.

Semicolon For more lovely fun, put a semicolon after “love”, so the program looks like this:

The semicolon makes the computer print “love” next to “love”, so the output window looks like this:

If you put a space after love, like this —

the computer will put a space after each love:

Bigger loop Run this program:

Lines 2 & 3 (which say print “dog” and print “cat”) make the computer print “dog” and then print “cat” next to it. Since those lines are between the words “repeat” and “until”, the computer does them repeatedly — print “dog”, then print “cat”, then print “dog” again, then print “cat” again — so the screen looks like this:

The computer will keep printing “dog” and “cat” until you abort the program.

Cls

To make the computer erase everything that was printed in the output window, type type “cls” (which means “CLear the Screen”).

You can type “cls” in the output window or make “cls” be a line in your program (so the computer will erase what’s in the output window). Typing “cls” lets you hide whatever naughty thing you put in the output window.

Blinking Let’s make the screen say “Stop pollution!” and make that message blink.

To do that, flash “Stop pollution!” onto the screen for 2 seconds, then turn that message off for 1 second (so the screen is blank), then flash that message on again. Here’s the program:

Lines 2 & 3 (which say to print “Stop pollution!” and wait for 2 seconds) flash the message “Stop pollution!” onto the output window and keep it there for 2 seconds. The next pair of lines (cls and wait for 1 second) make the output window become blank for 1 second. Since those lines are all in a repeat loop, the computer does them repeatedly — flash message then blank, flash message then blank, flash message then blank — so your output window becomes a continually flashing sign. It will keep flashing until you abort the program.

Instead of saying “Stop pollution!”, edit that program so it flashes your favorite phrase instead, such as “Save the whales!” or “Marry me!” or “Keepa youse hands offa my computer!” or “Jesus saves — America spends!” or “In God we trust — all others pay cash” or “Please wait — Dr. Doom will be with you shortly” or “Let’s rock!” or whatever else turns you on. Make the computer say whatever you feel emotional about. Like a dog, the computer imitates its master’s personality. If your computer acts “cold and heartless”, it’s because you are!

In the program, you typed just a few lines; but since the top line says “repeat” (and the bottom line says “until false”), the computer does an infinite loop. By saying “repeat”, you can make the computer do an infinite amount of work. Moral: the computer can turn a finite amount of
human energy into an infinite amount of good. Putting it another way:
the computer can multiply your abilities by infinity.

 

Goto

This program makes the computer print the words “dog” and “cat” repeatedly:

It makes the computer print:

This program does the same thing:

The top line (which is numbered 10) makes the computer print “dog”. The next line makes the computer print “cat”. The bottom line makes the computer go back to line 10, so the computer will print “dog” again, then “cat again”, then go back to line 10 again, then print “dog” again, then “cat” again, etc. The computer will print “dog” and “cat” repeatedly, until you abort the program.

This program does the same thing:

The top line (named “joe”) makes the computer print “dog”. The next line makes the computer print “cat”. The bottom line makes the computer go back to the line named “joe”. In that program, “joe” is called the top line’s label.

One word “goto” is one word. You’re supposed to type “goto”, not “go to”. If you accidentally type “go to” instead of “goto”, the computer will gripe by saying “Mistake” when you run the program.

Skip ahead Did you ever dream about having a picnic in the woods? This program expresses that dream:

It makes the computer print:

Let’s turn that dream into a nightmare where we all become giant termites. To do that, insert the shaded items:

The computer begins by printing “Let’s munch”. Then the computer does goto 40, which makes the computer GO skip down TO line 40, which prints “the trees!” So the program makes the computer print just this:

Is “goto” too powerful? The word “goto” gives you great power: if you say GO back TO line 10, the computer will create a loop (as if you’d said “repeat”if you say GO skip down TO line 40, the computer will skip over lines of your program.

Since the word “goto” is so powerful, programmers fear it! Programmers know that the slightest error in using that powerful word will make the programs act very bizarre! Programmers feel more comfortable using milder words instead (such as “repeat”), which are safer and rarely get botched up. Since the word “goto” is scary, many computer teachers prohibit students from using it, and many companies fire programmers who say “goto” instead of “repeat”.

But saying “goto” is fine when you’ve learned how to control the power! Though I’ll usually say “repeat” instead of “goto”, I’ll say “goto” in certain situations where saying “repeat” would be awkward.

Life as an infinite loop

A program that makes the computer repeat the same thing again and again forever is an infinite loop.

Some humans act just like computers. Those humans do the same thing again and again.

Go to your bathroom, get your bottle of shampoo, and look at the instructions on the back. A typical bottle has three instructions:

Those instructions say to lather, then rinse, then repeat — which means to lather again, then rinse again, then repeat again — which means to lather again, then rinse again, then repeat again.… If you follow those instructions, you’ll never finish washing your hair! The instructions are an infinite loop! The instructions are a program: they program you to use lots of shampoo! That’s how infinite loops help sell shampoo.

End

To make the computer skip the bottom part of your program, say “end”:

When you run that program (by tapping the green triangle), the computer will print “She smells” and then end, without printing “of perfume”.

Suppose you write a program that prints a long message, and you want to run the program several times (so several of your friends get the message). If one of your friends would be offended by the end of your message, send that friend an abridged message! Here’s how: put “end” above the part of the message that you want the computer to omit — or skip past that part by saying “goto”.

Instead of writing “end”, you can write “stop” (which acts like “end” but also makes the computer print “STOP”) or write “quit” (which acts like “end” but also makes the computer close the output window).

Here’s how to restrict the computer, so it performs certain lines only under certain conditions.…

If

Let’s write a program so that if the human is less than 18 years old, the computer will say:

Here’s the program:

The top line makes the computer ask “How old are you” and wait for the human to type an age. Since the symbol for
“less than” is “<”, the bottom line says: if the age is less than 18, then print “You are still a minor”.

Go ahead! Run that program! The computer begins the conversation by asking:

Try saying you’re 12 years old, by typing a 12, so the screen looks like this:

When you finish typing the 12 and press the Enter key at the end of it, the computer will reply:

Try running that program again, but this time try saying you’re 50 years old instead of 12, so the screen looks like this:

When you finish typing the 50 and press the Enter key at the end of it, the computer will not say “You are still a minor”. Instead, the computer will say nothing — since we didn’t teach the computer how to respond to adults yet!

In that program, the most important line says:

In that line, “age<18” is the condition. The whole line is called a conditional statement. If the condition is true (if age is really less than 18), the computer does the action, which comes afterwards:

Else

Let’s teach the computer how to respond to adults.

Here’s how to program the computer so that if the age is less than 18, the computer will say “You are still a minor”, but if the age is not less than 18 the computer will say “You are an adult” instead:

In programs, the word “else” means “otherwise”. That program’s bottom line means: if the age is less than 18, print “You are still a minor”; otherwise (if the age is not less than 18), print “You are an adult”. So the computer will print “You are still a minor” or else print “You are an adult”, depending on whether the age is less than 18.

Try running that program! If you say you’re 50 years old, so the screen looks like this —

the computer will reply by saying:

Daddy’s always right A Daddy wrote a program for his 5-year-old son, John. When John runs the program and types his name, the computer asks “What’s 2 and 2?” If John answers 4, the computer says “No, 2 and 2 is 22”. If he runs the program again and answers 22, the computer says “No, 2 and 2 is 4”. No matter how many times he runs the program and how he answers the question, the computer says he’s wrong. But when Daddy runs the program, the computer replies, “Yes, Daddy is always right”.

Here’s how Daddy programmed the computer:

Multi-line if

If the age is less than 18, here’s how to make the computer print “You are still a minor” and also print “Ah, the joys of youth”:

Here’s a more sophisticated way to say the same thing:

That sophisticated way (in which you type 4 short lines instead of a single long line) is called a multi-line if (or a block if).

In a multiline if:

In the middle of a multi-line if, you can say “else”:

That means: if the age is less than 18, then print “You are still a minor” and “Ah, the joys of youth”; otherwise (if age not under 18) print “You are an adult” and “We can have adult fun”.

Case

Let’s turn your computer into a therapist!

To make the computer ask the patient, “How are you?”, begin the program like this:

Make the computer continue the conversation by responding this way:

To accomplish all that, type this:

Like a multi-line if, a case statement consumes several lines. The top line of that case statement tells the computer to analyze a$ and select one of the cases from the list underneath. That list is indented (automatically by the computer) and says:

Every case statement’s bottom line must say “endcase”.

Complete program Here’s the complete program:

The top line makes the computer ask the patient, “How are you?” The next several lines are the case statement, which makes the computer analyze the patient’s answer and print “That’s good!” or “Too bad!” or else “I feel the same way!”

Regardless of what the patient and computer said, that program’s bottom line always makes the computer end the conversation by printing:

In that program, try changing the strings to make the computer print smarter remarks, become a better therapist, and charge even more money.

Vertical case You can write the case statement vertically, like this:

That’s useful if you want the computer to print many things.

Error trap This program makes the computer discuss human sexuality:

Line 1 makes the computer ask, “Are you male or female?”

The remaining lines are a case statement that analyzes the human’s response. If the human claims to be “male”, the computer prints “So is Frankenstein!” If the human says “female” instead, the computer prints “So is Mary Poppins!” If the human says anything else (such as “not sure” or “super-male” or “macho” or “none of your business”), the computer does the “otherwise”, which makes the computer say “Please say male or female!” and then go back to line 1, which makes the computer ask again, “Are you male or female?”

In that program, the “otherwise” is called an error handler (or error-handling routine or error trap), since its only purpose is to handle human error (a human who says neither “male” nor “female”). Notice that the error handler begins by printing a gripe message (“Please say male or female!”) and then lets the human try again (goto 1).

In Basic, “goto” is used rarely: it’s used mainly in error handlers, to let the human try again.

Let’s extend that program’s conversation. If the human says “female”, let’s make the computer say “So is Mary Poppins!”, then ask “Do you like her?”, then continue the conversation this way:

To accomplish all that, put the shaded lines into the program:

Weird programs The computer’s abilities are limited only by your own imagination — and your weirdness. Here are some weird programs from weird minds.…

Like a human, the computer wants to meet new friends. This program makes the computer show its true feelings:

When you run that program, the computer asks “Are you my friend?” If you say “yes”, the computer says “That’s swell.” If you say “no”, the computer says “Go jump in a lake.”

The most inventive programmers are kids. A sixth-grade girl wrote this program, to test your honesty:

When you run that program, the top 3 lines print nonsense. Then the computer asks whether you understand that stuff. If you’re honest and answer “no”, the computer will apologize. But if you pretend that you understand the nonsense and answer “yes”, the computer will print more nonsense, challenge you to translate it, wait for you to fake a translation, and then scold you for lying.

 

Fancy conditions

You can make the “if” clause very fancy:

In the IF statement, the symbols =, <, >, <=, >=, and <> are called relations.

When writing a relation, mathematicians and computerists habitually put the equal sign last:

To say “not equal to”, say “less than or greater than”, like this: <>.

Or The computer understands the word “or”. For example, here’s how to say, “If x is either 7 or 8, print the word wonderful”:

That example is composed of two conditions: the first condition is “x=7”; the second condition is “x=8”. Those two conditions combine, to form “x=7 or x=8”, which is called a compound condition.

If you use the word “or”, put it between two conditions.

And The computer understands the word “and”. Here’s how to say, “If p is more than 5 and less than 10, print tuna fish”:

Here’s how to say, “If s is at least 60 and less than 65, print you almost failed”:

Here’s how to say, “If n is a number from 1 to 10, print that’s good”:

Can a computer be President?

To become President of the United States, you need 4 basic skills:

So altogether, to become President you need to be a good talker and listener and decision maker and also have the patience to put up with monotonous repetition.

Those are exactly the 4 qualities the computer has!

So by using the words “print”, “input”, “if”, and “goto”, you can make the computer imitate any intellectual human activity. Those 4 magic words — “print”, “input”, “if”, and “goto” — are the only concepts you need, to write whatever program you wish!

Yes, you can make the computer imitate the President of the United States, do your company’s payroll, compose a beautiful poem, play a perfect game of chess, contemplate the meaning of life, act as if it’s falling in love, or do whatever other intellectual or emotional task you wish, by using those 4 magic words. The only question is: how? This book teaches you how, by showing you many examples of programs that do those remarkable things.

What programmers believe Yes, we programmers believe that all of life can be explained and programmed. We believe all of life can be reduced to just those 4 concepts: “print”, “input”, “if”, and “goto”. Programming is the ultimate act of scientific reductionism: programmers reduce all of life scientifically to just 4 concepts.

The words that the computer understands are called keywords. The 4 essential keywords are “print”, “input”, “if”, and “goto”.

The computer also understands extra keywords, such as:

Those extra keywords aren’t necessary: if they hadn’t been invented, you could still write programs without them. But they make programming easier.

A Basic programmer is a person who translates an ordinary English sentence (such as “act like the President” or “do the payroll”) into a series of Basic statements, using keywords such as “print”, “input”, “if”, “goto”, “wait”, “repeat”, etc.

The mysteries of life Let’s dig deeper into the mysteries of “print”, “input”, “if”, “goto”, and the extra keywords. The deeper we dig, the more you’ll wonder: are you just a computer, made of flesh instead of wires? Can everything you do be explained in terms of “print”, “input”, “if”, and “goto”?

By the time you finish this book, you’ll know!

 

Exiting a loop

This program plays a guessing game, where the human tries to guess the computer’s favorite color, which is pink:

The “input” line asks the human to guess the computer’s favorite color; the guess is called guess$.

If the guess is “pink”, the computer prints:

But if the guess is not “pink”, the computer will instead print “No, that’s not my favorite color” and then go back to line 1, which asks the human again to try guessing the computer’s favorite color.

End Here’s how to write that program without saying “goto”:

That new version of the program contains a “repeat” loop. That loop makes the computer repeat this repeatedly: ask “What’s my favorite color?” and then print “No, that’s not my favorite color.”

The only way to stop the loop is to guess “pink”, which makes the computer print “Congratulations!” and end.

Exit repeat Here’s another way to write that program without saying “goto”:

That program’s “repeat” loop makes the computer do this repeatedly: ask “What’s my favorite color?” and then print “No, that’s not my favorite color.”

The only way to stop the loop is to guess “pink”, which makes the computer exit from the “repeat” loop; then the computer proceeds to the line underneath the “repeat” loop. That line prints:

Until condition Here’s a shorter way to program the guessing game:

That program’s “repeat” loop makes the computer do this repeatedly: say “You haven’t guessed my favorite color yet!” and then ask “What’s my favorite color?”

The “until” line makes the computer repeat the indented lines again and again, until the guess is “pink”. When the guess is “pink”, the computer proceeds to the line underneath the loop and prints “Congratulations!”.

The “until” condition (guess$=“pink”) is called the loop’s goal. The computer does the loop repeatedly, until the loop’s goal is achieved. Here’s how:

Saying —

is just a briefer way of saying this pair of lines:

 

Let’s make the computer print every number from 1 to 20, like this:

Here’s the program:

The top line (for x=1 to 20) says x will be every number from 1 to 20; so x will be 1, then 2, then 3, etc. The line underneath, which is indented, says what to do about each x; it says to print each x.

Whenever you write a program that contains the word “for”, you must say “next”; so the bottom line says “next”.

The indented line, which is between the “for” line and the “next” line, is the line that the computer will do repeatedly; so the computer will repeatedly print x. The first time the computer prints x, the x will be 1, so the computer will print:

The next time the computer prints x, the x will be 2, so the computer will print:

The computer will print every number from 1 up to 20.

When men meet women

Let’s make the computer print these lyrics:

To do that, type these lines —

and make x be every number from 2 up to 5:

At the end of the song, print the closing couplet:

That program makes the computer print the entire song.

Here’s an analysis:

Since the computer does the indented lines repeatedly,
those lines form a loop. Here’s the general rule:
the statements between “for” and “next” form a loop. The computer goes round and round the loop, for x=2, x=3, x=4, and x=5. Altogether, it goes around the loop 4 times, which is a finite number. Therefore, the loop is finite.

If you don’t like the letter x, choose a different letter. For example, you can choose the letter i:

When using the word “for”, most programmers prefer the letter i; most programmers say “for i” instead of “for x”. Saying “for i” is an “old tradition”. Following that tradition, the rest of this book says “for i” (instead of “for x”), except in situations where some other letter feels more natural.

Print the squares

To find the square of a number, multiply the number by itself. The square of 3 is “3 times 3”, which is 9. The square of 4 is “4 times 4”, which is 16.

Let’s make the computer print the square of 3, 4, 5, etc., up to 20, like this:

To do that, type this line —

and make i be every number from 3 up to 20, like this:

Count how many copies

This program, which you saw before, prints “love” on every line of your screen:

That program prints “love” again and again, until you abort the program (by closing the output window or pressing the Esc key).

But what if you want to print “love” just 20 times? This program prints “love” just 20 times:

As you can see, “for..next” resembles “repeat…until false” but is smarter: while doing for…next, the computer counts!

Count to midnight

This program makes the computer count to midnight:

The computer will print:

 

Print horizontally Let’s make the computer print those numbers horizontally, like this:

Here’s the program:

In the “print” line, the first semicolon prevents the computer from putting lots of spaces before each number. The " " makes the computer put a single space after each number. The final semicolon makes the computer print the output all on the same line instead of pressing the Enter key after each number.

If you want the computer to press the Enter key before “midnight”, insert a “print” line:

That extra “print” line makes the computer press the Enter key just before “midnight”, so the computer will print “midnight” on a separate line, like this:

Nested loops Let’s make the computer count to midnight 3 times, like this:

To do that, put the entire program between the words “for” and “next”:

That version contains a loop inside a loop: the loop that says “for i” is inside the loop that says “for j”. The j loop is called the outer loop; the i loop is called the inner loop. The inner loop’s variable must differ from the outer loop’s. Since we called the inner loop’s variable “i”, the outer loop’s variable must not be called “i”; so I picked the letter j instead.

Programmers often think of the outer loop as a bird’s nest, and the inner loop as an egg inside the nest. So programmers say the inner loop is nested in the outer loop; the inner loop is a
nested loop.

Abnormal exit

Earlier, we programmed a game where the human tries to guess the computer’s favorite color, pink. Here’s a fancier version of the game, in which the human gets just 5 guesses:

The top line warns the human that just 5 guesses are allowed. The “for” line makes the computer count from 1 to 5; to begin, i is 1. The “input” line asks the human to guess the computer’s favorite color; the guess is called guess$.

If the guess is “pink”, the computer jumps down to the line numbered 1, prints “Congratulations!”, and tells how many guesses the human took. But if the guess is not “pink”, the computer will print “No, that’s not my favorite color” and go on to the next guess.

If the human guesses 5 times without success, the computer proceeds to the line that prints “Sorry, … You lose.”

For example, if the human’s third guess is “pink”, the computer prints:

If the human’s very first guess is “pink”, the computer prints:

Saying “1 guesses” is bad grammar but understandable.

That program contains a “for…next” loop. The “for” line says the loop will normally be done 5 times. The line below the loop (which says to print “Sorry”) is the loop’s normal exit. But if the human happens to input “pink”, the computer jumps out of the loop early, to line numbered 1, which is the loop’s abnormal exit.

Here’s how to write that program without saying “goto”:

Step

The “for” statement can be varied:

To count down, you must use the word “step”. To count from 17 down to 5, give this instruction:

This program prints a rocket countdown:

The computer will print:

This statement is tricky:

It says to start i at 5, and keep adding 3 until it gets past 16. So i will be 5, then 8, then 11, then 14. The i won’t be 17, since 17 is past 16. The first value of i is 5; the last value is 14.

In the statement “for i=5 to 16 step 3, the first value or initial value of i is 5, the limit value is 16, and the step size or increment is 3. The i is called the counter or index or
loop-control variable. Although the limit value is 16, the
last value or terminal value is 14.

Programmers usually say “for i” instead of “for x”, because the letter i reminds them of the word index.

Round-off errors

If the step size is a decimal, the computer might make small errors (called round-off errors), which can add up to a result that’s very wrong.

For example, suppose you say:

That means you want the last few values of i to be 16.8, 16.9, and 17; but the computer will accidentally make the step size be slightly more than .1, so the computer’s last few values of i will be about 16.80000000000000003 and 16. 90000000000000003. The computer will refuse to do the next number (which would be about 17. 00000000000000003), since you said not to go past 17; so the last i will be 16. 90000000000000003, which isn’t at all what you wanted for the last value!

To make the last i be about 17, make the limit value be slightly more than 17, like this —

or, better yet, avoid a decimal step size by using this pair of lines instead:

That makes i indeed be 5 then 5.1 then 5.2, etc., up to 17.

 

Let’s make the computer print this message:

That message concerns this list of food: meat, potatoes, lettuce, tomatoes, honey, cheese, onions, peas. That list doesn’t change: the computer continues to love those foods throughout the entire program.

A list that doesn’t change is called data. So in the message about food, the data is meat, potatoes, lettuce, tomatoes, honey, cheese, onions, peas.

Whenever a problem involves data, put the data at the program’s top, ike this:

You must tell the computer to read the data:

That “read” line makes the computer read the first datum (“meat”) and call it a$. So a$ is “meat”.

Since a$ is “meat”, this shaded line makes the computer print “I love meat”:

Hooray! We made the computer handle the first datum correctly: we made the computer print “I love meat”.

To make the computer handle the rest of the data (potatoes, lettuce, etc.), tell the computer to read and print the rest of the data, by putting the “read” and “print” lines in a loop. Since we want the computer to read and print all 8 data items (meat, potatoes, lettuce, tomatoes, honey, cheese, onions, peas), put the read and print lines in a loop that gets done 8 times, by making the loop say “for i =1 to 8”:

Since that loop’s main purpose is to read the data, it’s called a read loop.

When writing that program, make sure the “for” line’s last number (8) is the number of data items. If the “for” line accidentally says 7 instead of 8, the computer won’t read or print the 8th data item. If the “for” line accidentally says 9 instead of 8, the computer will try to read a 9^th data item, realize no 9^th data item exists, and gripe by saying:

Let’s make the computer end by printing “Those are the foods I love”, like this:

To make the computer print that ending, put a PRINT line at the end of the program:

 

End mark

When writing that program, we had to count the data items and put that number (8) at the end of the “for” line.

Here’s a better way to write the program, so you don’t have to count the data items:

The second line (data end) is called the end mark, since it marks the end of the data. The “read” line means:

When the computer exits from the “repeat” loop, the computer prints “Those are the foods I love”. So altogether, the entire program makes the computer print:

The routine that says:

is called the end routine, because the computer does that routine when it reaches the data’s end.

Henry the Eighth Let’s make the computer print this nursery rhyme:

If you own a jump rope, have fun: try to recite that poem while skipping rope!

This program makes the computer recite the poem:

Since the data’s too long to fit on a single line, I’ve put part of the data in the top line and the rest in line 2. Each line of data must begin with the word data. In each line, put commas between the items. Do not put a comma at the end of the line.

The program resembles the previous one. The new line
(if a$=“to wed” print) makes the computer leave a blank line underneath “to wed”, to mark the first verse’s bottom.

Pairs of data

Let’s throw a party! To make the party yummy, let’s ask each guest to bring a kind of food that resembles the guest’s name. For example, let’s have Sal bring salad, Russ bring Russian dressing, Sue bring soup, Tom bring turkey, Winnie bring wine, Kay bring cake, and Al bring Alka-Seltzer.

Let’s send all those people invitations, in this form:

Here’s the program:

The data comes in pairs. For example, the first pair consists of “Sal” and “salad”; the next pair consists of “Russ” and “Russian dressing”. Since the data comes in pairs, you must make the end mark also be a pair (data end,end).

To make the output appear on paper, say “vdu 2”.

Since the data comes in pairs, the “read” line says to read a pair of data (person$ and food$). The first time that the computer encounters the “read” line, person$ is “Sal”; food$ is “salad”. Then the print lines print this message onto the screen and paper:

The “vdu 12” makes the computer eject the paper from the printer.

Then the computer comes to “until false”, which sends the computer back to the word “repeat”, which sends the computer to the “read” line again, which reads the next pair of data, so person$ becomes “Russ” and food$ becomes “Russian dressing”. The “print” lines print onto paper:

The computer prints similar letters to all the people.

After all people have been handled, the “read” statement comes to the end mark (data end,end), so that person$ and food$ both become “end”. Since person$ is “end”, the “if” statement makes the computer exit the “repeat” loop, so the computer does “vdu 3” (which turns off the printer’s output) and prints this message onto just the screen:

In that program, you need two ends to mark the data’s ending, because the “read” statment says to read two strings (person$ and food$).

 

Debts Suppose these people owe you things:

Let’s remind those people of their debt, by writing them letters, in this form:

To start writing the program, begin by feeding the computer the data. The final program is the same as the previous program, except for the part I’ve shaded:

Triplets of data

Suppose you’re running a diet clinic and get these results:

This program makes the computer print a nice report on your screen:

The top line contains the data, which comes in triplets. The first triplet consists of Joe, 273, and 219. Each triplet includes a string (such as Joe) and two numbers (such as 273 and 219), so the second line’s end mark also includes a string and two numbers: it’s the word “end” and two zeros. (If you hate zeros, you can use other numbers instead; but most programmers prefer zeros.)

The “read” line says to read a triplet: a string (person$) and two numbers (weight_before and weight_after). The first time the computer comes to the “read” statement, the computer makes person$ be “Joe”, weight_before be 273, and weight_after be 219. The “print” lines print this:

 

Restore

Examine this program:

The first “read” makes the computer read the first datum (love), so the first “print” makes the computer print:

The next “read” would normally make the computer read the next datum (death); but the ”restore 1” tells the “read” to skip ahead to the data line numbered 1, so the “read” line reads “chocolate” instead. The entire program prints:

So saying “restore 1” makes the next “read” skip ahead to the data line numbered 1. If you write a new program, saying “restore 2” makes the next “read” skip ahead to data line numbered 2. Saying just “restore” makes the next “read” skip back to the beginning of the first data line.

Continents This program prints the names of the continents:

That program makes the computer print this message:

Let’s make the computer print that message twice, so the computer prints:

To do that, put the program in a loop saying “for i=1 to 2”, like this:

After that program says to print “Those are the continents”, the program says to print a blank line and then restore. The word “restore” makes the “read” go back to the data’s beginning, so the computer can read and print the data a second time without saying “Out of data”.

 

Search loop

Let’s make the computer translate colors into French. For example, if the human says “red”, we’ll make the computer say the French equivalent, which is:

Let’s make the computer begin by asking “Which color interests you?”, then wait for the human to type a color (such as “red”), then reply:

The program begins simply:

Next, we must make the computer translate the requested color into French. To do so, feed the computer this English-French dictionary:

That dictionary becomes the data:

The data comes in pairs; each pair consists of an English word (such as “white”) followed by its French equivalent (“blanc”). To make the computer read a pair, say:

To let the computer look at all the pairs, put that “read” statement in a “repeat” loop. Here’s the complete program:

Since the “read” line is in a “repeat” loop, the computer does the “read” line repeatedly. So the computer keeps reading pairs of data, until the computer find the pair of data the human requested. For example, if the human requested “red”, the computer keeps reading pairs of data until it finds a pair whose English word matches the requested word (“red”). When the computer finds that match, the english$ is equal to the request$, so the “until” line says to stop repeating. The computer proceeds to the next line, which makes the computer print:

So altogether, when you run the program the chat can look like this:

Here’s another sample run:

Here’s another:

The computer says “Out of data” because it can’t find “pink” in the data.

Avoid “Out of data” Instead of saying “Out of data”, let’s make the computer say “I wasn’t taught that color”. To do that, put an end mark at the data’s end; and when the computer reaches the end mark, make the computer say “I wasn’t taught that color”:

In that program, the “repeat” loop’s purpose is to search through the data, to find data that matches the input. Since the “repeat” loop’s purpose is to search, it’s called a search loop.

The typical search loop has these characteristics:

Auto rerun At the end of the program, let’s make the computer automatically rerun the program and translate another color.

To do that, make the program’s bottom say to go back to the “input” line:

The word “restore”, which is above the search loop, makes sure the computer’s search through the data always starts at the data’s beginning.

Press Q to quit That program repeatedly asks “Which color interests you” until the human aborts the program. But what if the human’s a beginner who hasn’t learned how to abort?

Let the human stop the program more easily by pressing just the Q key to quit:

Here are some hints to help you master programming.

Constants & beyond

A numeric constant is a simple number, such as:

Another example of a numeric constant is 1.3E5, which means, “take 1.3, and move its decimal point 5 places to the right”.

A numeric constant does not include any arithmetic. For example, since 7+1 includes arithmetic (+), it’s not a numeric constant. 8 is a numeric constant, even though 7+1 isn’t.

A string constant is a simple string, in quotation marks:

A constant is a numeric constant or a string constant:

Variables A variable is something that stands for something else. If it stands for a string, it’s called a string variable and ends with a dollar sign, like this:

If the variable stands for a number, it’s called a numeric variable and lacks a dollar sign, like this:

So all these are variables:

Expressions A numeric expression is a numeric constant (such as 8) or a numeric variable (such as b) or a combination of them, such as 8+z, or 8*a, or z*a, or 8*2, or 7+1, or even z*a-(7+z)/8+1.3E5*(-524.6+b). A string expression is a string constant (such as “I love you”) or a string variable (such as a$) or a combination (such as “hot”+“dog”).

An expression is a numeric expression or a string expression.

Statements At the end of a “goto” statement, the line number must be a numeric constant or numeric variable.

The “input” statement’s prompt must be a string constant.

In a data statement, you must have constants or numeric expressions.

In the data statement, if the constant is a string, you can omit the quotation marks (unless the string contains a comma).

Debugging

If you write and run your own program, it probably won’t work.

Your first reaction will be to blame the computer. Don’t!

The probability is 99.99% that the fault is yours. Your program contains an error. An error is called a bug. Your next task is to debug the program, which means get the bugs out.

Bugs are common; top-notch programmers make errors all the time. If you write a program that works perfectly on the first run and doesn’t need debugging, it’s called a gold-star program and means you should have tried writing a harder one instead!

It’s easy to write a program that’s nearly correct but hard to find the little bug fouling it up. Most time you spend at the computer will be devoted to debugging.

Debugging can be fun. Hunting for the bug is like going on a treasure hunt – or solving a murder mystery. Pretend you’re Sherlock Holmes. Your mission: to find the bug and squish it! When you squish it, have fun: yell out, “Squish!”

How can you tell when a roomful of programmers is happy? Answer: when you hear continual cries of “Squish!”

To find a bug, use three techniques:

Here are the details.…

Inspect the program Take a good, hard look at the program. If you stare hard enough, maybe you’ll see the bug.

Usually, the bug will turn out to be just a typing error, a typo. For example.…

You must put quotation marks around each string, and a dollar sign after each string variable:

Here are 2 reasons why the computer might print too much:

Trace the computer’s thinking If you’ve inspected the program thoroughly and still haven’t found the bug, the next step is to trace the computer’s thinking. Pretend you’re the computer. Do what your program says. Do you find yourself printing the same wrong answers the computer printed? If so, why? To help your analysis, make the computer print everything it’s thinking while it’s running your program. For example, suppose your program uses the variables b, c, and x$. Insert lines such as these into your program:

Then run the program. Those extra lines tell you what the computer is thinking about b, c, and x$ and also tell you how many times the computer reached lines 10 and 20. For example, if the computer prints what you expect in line 10 but prints strange values in line 20 (or doesn’t even get to line 20), you know the bug occurs after line 10 but before line 20.

Here’s a good strategy. Halfway down your program, insert a line that says to print all the values. Then run your program. If the line you inserted prints the correct values, you know the bug lies underneath that line; but if the line prints wrong values (or if the computer never reaches that line), you know the bug lies above that line. In either case, you know which half of your program contains the bug. In that half of the program, insert more lines, until you finally zero in on the line containing the bug.

Shorten the program When all else fails, shorten the program.

Hunting for a bug in a program is like hunting for a needle in a haystack: the job is easier if the haystack is smaller. So make your program shorter: delete the last half of your program. Then run the shortened version. That way, you’ll find out whether the first half of your program is working the way it’s supposed to. When you’ve perfected the first half of your program, tack the second half back on.

Does your program contain
a statement whose meaning you’re not completely sure of? Check the meaning by reading a book or asking a friend; or write a tiny experimental program that contains the statement, and see what happens when you run it.

Hint: before you shorten your program (or write tiny experimental ones),
save the original version (by tappng the Save icon), even though the version contains a bug. After you’ve played with the shorter versions, retrieve the original (by tapping the Load icon) and fix it.

To write a long, correct program easily, write a short program first and debug it, then add a few more lines and debug them, add a few more lines and debug them, etc. So start with a small program, perfect it, then gradually add perfected extras so you gradually build a perfected masterpiece. If you try to compose a long program all at once – instead of building it from perfected pieces – you’ll have nothing more than a mastermess – full of bugs.

Moral: to build a large masterpiece, start with a small masterpiece. To build a program so big that it’s a skyscraper, begin by laying a good foundation; double-check the foundation before you start adding the program’s walls and roof.

Error messages

If the computer can’t obey your command, the computer will print an error message. The following error message are the most common.…

If you say “prind” instead of “print”, the computer will say:

That means the computer hasn’t the faintest idea of what you’re talking about!

If the computer says you have a “Mistake”, it’s usually because you spelled a word wrong, or forgot a word, or used a word the computer doesn’t understand. It can also result from wrong punctuation: check your commas, semicolons, and colons.

If you type a left parenthesis but forget to type the right parenthesis that matches it, the computer will say:

If you try to say “print 5+2” but forget to type the 2, the computer will say:

Numeric errors If the answer to a calculation is a bigger number than the computer can handle, the computer will say:

If you try to divide by zero, the computer will say:

Logic errors If you say “goto 10”, the computer tries to find a line numbered 10. If there’s no line numbered 10, the computer will say:

If a line says to use x (such as “print x” or “print 3+x” “y=3+x” or “goto x”) but you haven’t said how much x is yet, the computer will say:

If you say “read” but the computer can’t find any more data to read (because the computer has read all the data already), the computer will say:

The computer handles 2 major types of info: numbers & strings. If you feed the computer the wrong type of info – if you feed it a number when you should have fed it a string, or you feed it a string when you should have fed it a number – the computer will say:

When you feed the computer a string, you must put the string in quotation marks, and put a dollar sign after the string’s variable. If you forget to type the string’s quotation marks or dollar sign, the computer won’t realize it’s a string; the computer will think you’re trying to type a number instead; and if a number would be inappropriate, the computer will give that gripe. So when the computer gives that gripe, it usually means you forgot a quotation mark or a dollar sign.

Pause

Magicians often say, “The hand is quicker than the eye.” The computer’s the ultimate magician: the computer can print info on the output window much faster than you can read it.

When the computer is printing faster than you can read, make it pause. Here’s how.…

Tap the program window (which is partly hidden behind the output window). Then tap the Pause icon (which looks like “;”).

The computer will pause, to let you read what’s on the screen.

Tap the output window. Read what’s in it.

When you’ve finished reading what’s in the output window and want the computer to stop pausing, tap the Pause icon again. Then the computer will continue printing rapidly, where it left off.

If your eyes are as slow as mine, you’ll need to use the Pause icon often! You’ll want the computer to pause while you’re running a program containing many “print” statements (or a “print” statement in a loop).

Apostrophe

Occasionally, jot a note to remind yourself what your program does and what the variables stand for. Slip the note into your program by putting an apostrophe before it:

When you run that program, the computer ignores everything that’s to the right of an apostrophe. So the computer ignores lines 1 & 2; in lines 3 & 4, the computer ignores the “because…”. Since c is 40, and h is 23, the bottom line makes the computer print:

Everything to the right of an apostrophe is called a comment (or remark). While the computer runs the program, it ignores the comments. But the comments remain part of the program; they appear in the program window the rest of the program. Though the comments appear in the program, they don’t affect the run.

When the computer sees an apostrophe, it thinks the apostrophe marks the beginning of a comment (unless the apostrophe is in a string or “data” statement or “print” statement).

Loop techniques

Here’s a strange program:

The second line (x=4+x) means: the new x is 4 plus the old x. So the new x is 4+9, which is 13. The bottom line prints:

Let’s look at that program more closely. The top line (x=9) puts 9 into box x:

box x           9

When the computer sees the next line (x=4+x), it examines the equation’s right side and sees the 4+x. Since x is 9, the 4+x is 4+9, which is 13. So the line “x=4+x” means x=13. The computer puts 13 into box x:

box x          13

The program’s bottom line prints 13.

Here’s another weirdo:

The second line (b=b+1) says the new b is “the old b plus 1”. So the new b is 6+1, which is 7. The bottom line prints:

In that program, the top line says b is 6; but the next line increases b, by adding 1 to b; so b becomes 7. Programmers say that b has been increased or incremented. In the third line, the “1” is called the increase or the increment.

The opposite of “increment” is decrement:

The top line says j starts at 500; but the next line says the new j is “the old j minus 1”, so the new j is 500-1, which is 499. The bottom line prints:

In that program, j was decreased (or decremented). In the third line, the “1” is called the decrease (or decrement).

Counting Suppose you want the computer to count, starting at 3, like this:

This program does it, by a special technique:

In that program, c is called the counter, because it helps the computer count.

The top line says c starts at 3. The “print” line makes the computer print c, so the computer prints:

The next line (c=c+1) increases c by adding 1 to it, so c becomes 4. The “until” line sends the computer back to the “print” line, which prints the new value of c:

Then the computer comes to the “c=c+1” again, which increases c again, so c becomes 5. The “until” line sends the computer back again to the “print” line, which prints:

The program’s an infinite loop: the computer will print 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and so on, forever, unless you abort it.

Here’s the general procedure to make the computer count:

This program makes the computer count, starting at 1:

The computer will print 1, 2, 3, 4, etc.

This program makes the computer count, starting at 0:

The computer will print 0, 1, 2, 3, 4, etc.

 

Quiz Let’s make the computer give this quiz:

To make the computer score the quiz, we must tell it the correct answers:

So feed the computer this data:

In the data, each pair consists of a question and an answer. To make the computer read the data, tell the computer to read a question and an answer, repeatedly:

Here’s the complete program:

The lines underneath “read” make the computer print the question, wait for the human to input a response, and check if the human’s response matches the correct answer. Then the computer will either print “Correct!” or print “No” and reveal the correct answer. When the computer reaches the data’s end, the computer exits the “repeat: loop and prints “I hope you enjoyed the quiz!”

Here’s a sample run, where I’ve underlined the parts typed by the human:

To give a quiz about different topcs, change the data.

Let’s make the computer count how many questions the human answered correctly. To do that, we need a counter. As usual, let’s call it c:

At the program’s beginning, the human hasn’t answered any questions correctly yet, so the counter begins at 0 (by saying “c=0”). Each time the human answers a question correctly, the computer does “c=c+1”, which increases the counter. The program’s bottom line prints the counter, by printing a message such as:

It would be nicer to print —

or, if the quiz were changed to include 8 questions:

To make the computer print such a message, we must make the computer count how many questions were asked. So we need another counter. Since we already used c to count the number of correct answers, let’s use q to count the number of questions asked. Like c, q must start at 0; and we must increase q, by adding 1 each time another question is asked:

 

Summing Let’s make the computer imitate an adding machine, so a run looks like this:

Here’s the program:

The top line starts the sum at 0. The “print” line prints the sum. The “input” line asks the human what number to add to the sum; the human’s number is called x. The next line (s=s+x) adds x to the sum, so the sum changes. The “until” line sends the computer back to the “print” line, which prints the new sum. The program’s an infinite loop, which you must abort.

Here’s the general procedure to make the computer find a sum:

Checking account If your bank’s nasty, it charges you 20¢ to process each good check you write, a $25 penalty for each check that bounces, and pays no interest on money you’ve deposited.

This program makes the computer imitate such a bank.…

In that program, the total amount of money in the checking account is called the sum, s. The top line (s=0) starts that sum at 0. The first “print” line prints the sum. The next line asks the human to press “d” (to make a deposit) or “c” (to write a check).

If the human presses “d” (to make a deposit), the computer asks “How much money do you want to deposit?” and waits for the human to type an amount to deposit. The computer adds that amount to the sum in the account (s=s+d).

If the human presses “c” (to write a check), the computer asks “How much money do you want the check for?” and waits for the human to type the amount on the check. The computer adds the 20¢ check-processing fee to that amount (c=c+.2). Then the computer reaches the line saying “if c<=s”, which checks whether the sum s in the account is big enough to cover the check (c). If c<=s, the computer says “Okay” and processes the check, by subtracting c from the sum in the account. If the check is too big, the computer says “That check bounced!” and decreases the sum in the account by the $25 penalty.

That program is nasty to customers:

That nasty program makes customers hate the bank — and hate the computer! The bank should make the program friendlier. Here’s how:

Also, to be friendly, the bank should ignore the 20¢ service charge when deciding whether a check will clear. So the bank should eliminate the line saying “c = c+.2”. On the other hand, if the check does clear, the bank should impose the 20¢ service charge afterwards, by changing the “s=s-c” to “s = s-c-.2”.

So if the bank is kind, it will make all those changes. But some banks complain that those changes are too kind! For example, if a customer whose account contains just 1¢ writes a million-dollar check (which bounces), the new program charges him just 1¢ for the bad check; $25 might be more reasonable.

Moral: the hardest thing about programming is choosing your goal — deciding what you WANT the computer to do.

Series Let’s make the computer add together all the numbers from 7 to 100, so that the computer finds the sum of this series: 7 + 8 + 9 + ... + 100. Here’s how.

Let’s make the computer add together the squares of all the numbers from 7 to 100, so that the computer finds the sum of this series: (7 squared) + (8 squared) + (9 squared) +… + (100 squared). Here’s how:

It’s the same as the previous program, except that indented line says to add i*i instead of i. The bottom line prints the final sum, which is 338259.

 

Data sums This program adds together the numbers in the data:

The “data” line contains the numbers to be added. The “data 0” is an end mark. The line saying “s=0” starts the sum at 0. The “read” statement reads an x from the data. The next line (s=s+x) adds x to the sum. The “until” line makes the computer repeat that procedure for every x. When the computer has read all the data and reaches the end mark (0), the x becomes 0, so the computer will exit the “repeat” loop and print the final sum, s.

 

Here’s how to make your output prettier.

Zones

The output window is divided into zones. Each zone is 10 characters wide. A comma makes the computer hop to the next zone:

That makes the computer print "sin" then hop to the next zone, where it prints “king”, like this:

Here are the words of a poet who drank too much and is feeling spaced out:

The computer will print “love” in the first zone, “cries” in the second zone, and “out” in the third zone, so the words are spaced out like this:

This program tells a bad joke:

The computer will print “You’re ugly!” That string has 12 characters. Its first 10 characters are in the first zone; its remaining 2 characters are in the second zone. Then the comma makes the computer hop to the next zone (the third zone), where the computer prints “I’m joking!”, like this:

When you combine commas with semicolons, you can get weird results:

That line contains commas and semicolons. A comma makes the computer hop to a new zone, but a semicolon does not make the computer hop. The computer will print “eat”, then hop to the second zone, then print “me” and “at” and “ball”, then hop to the third zone, then print “no” and “w”. Altogether, the computer will print:

Here’s a strange example:

The computer prints “underwater”, which contains 10 characters, so it fills up the first zone. Then the computer is supposed to print “ing” at the beginning of a new zone, but the computer’s already at the beginning of the second zone, so the computer prints:

Skip a zone You can make the computer skip over a zone:

The computer will print “Joe” in the first zone, a blank space in the second zone, and “loves Sue” in the third zone, like this:

Numbers Compare these 4 “print” statements:

The first “print” statement makes the computer print the number 53. The computer wants to make every number be 10 characters wide, so it puts 8 spaces before the digits, like this:

In the second “print” statement, the semicolon prevents the computer from putting blank spaces before the 986, so the computer prints:

In the third “print” statement, the " " forces the computer to put a blank space before the 986, so the computer prints:

In the fourth “print” statement, the comma makes the computer go to the beginning of a new zone; but after printing the 53, the computer’s already at the beginning of the second zone, so the computer prints 986 without hopping to another zone. Since there’s no semicolon, the computer prints 986 as 10 characters, by putting 7 spaces before it, like this:

Tables This program prints a list of words and their opposites:

The top line makes the computer print “good”, then hop to the next zone, then print “bad”. Altogether, the computer will print:

The first zone contains a column of words; the second zone contains the opposites. Altogether, the computer’s printing looks like a table. So to make a table easily, use zones, by putting commas in your program.

 

Let’s make the computer print this table:

Here’s the program:

In the “for…next” loop, the “print” line makes the computer print each number in the first zone and its square in the second zone. (The computer puts enough spaces before the digits to make each number consume 10 characters.) The top line makes the computer print the headings (“Number” and “Square”) with enough spaces so the headings are centered over the numbers below.

Tab

When the computer puts a line of information on your screen, the leftmost character in the line is said to be at position 0. The next character in the line is said to be at position 1. Then comes position 2.

This program makes the computer skip to position 6 and then print “hot”:

The computer will print:

Here’s a fancier example:

The computer will skip to position 6, then print “hot”, then skip to position 13, then print “buns”:

Diagonal This program prints a diagonal line:

The “for” line says to do the indented line many times. The first time the computer does the indented line, the i is 0, so the computer prints an asterisk at position 0:

The next time, the i is 1, so the computer skips to position 1 and prints an asterisk:

The next time, the i is 2, so the computer skips to position 2 and prints an asterisk:

Altogether, the program makes the computer print this picture:

2-dimensional tab In the output window, the top line is called line 0; underneath it is line 1; then comes line 2; etc.

To make the computer print the word “drown” so that “drown” begins at position 6 of line 2, type this:

The computer will print the word’s first letter (d) at the 6^th position of line 2. The computer will print the rest of the word afterwards, still on line 2.

If words were already printed on the output screen there, “drown” will cover them up. Covering up can be done by a
2-dimensional tab, such as tab (6,2), but not by a plain tab, such as tab(6), which refuses to erase previous typing.

Pixels

The image on screen is called the picture. If you stare at the picture closely, you’ll see the picture’s composed of thousands of tiny dots. Each dot, which is a tiny rectangle, is called a picture’s element, or pic’s el, or pixel, or pel.

Coordinates The dot in the output window’s bottom-left corner is called pixel (0,0). Just to the right of it is pixel (1,0). Then comes pixel (2,0), etc.

Above pixel (0,0) is pixel (0,1). Farther up is pixel (0,2).

Each pixel’s name consists of two numbers in parentheses. The first number is the X coordinate; the second number is the
Y coordinate. For example, if you’re talking about pixel (4,3), its X coordinate is 4; its Y coordinate is 3.

The X coordinate tells how far to the right the pixel is. The Y coordinate tells how far up. So pixel (4,3) is the pixel that’s 4 to the right and 3 up.

Circle This command draws a circle whose center is pixel (200,300) and radius is 50:

Ellipse This command draws an ellipse (oval) whose center is pixel (200,300), horizontal radius is 50, and vertical radius is 25:

Rectangle This command draws a rectangle (box) whose bottom-left corner is pixel (200,300), width is 80, and height is 50:

To draw a square, give the rectangle command but don’t mention the rectangle’s height:

The computer will assume you want the height to be the same as the width.

Fill If you say “circle fill” instead of “circle” or say
“ellipse fill” instead of “ellipse” or say “rectangle fill” instead of “rectangle”, the computer will draw the shape but also fill in the shape’s middle (the shape’s interior).

For example, this command draws a circle whose center is pixel (200,300) and radius is 50 and will also fill the circle’s middle:

Line This command draws a diagonal line from pixel (200,300) to pixel (500,350):

This command draws a diagonal line from the previous line’s end to pixel (600,400):

Plot This command puts a tiny dot at pixel (100,200):

Colors

Normally, the computer prints & graphs everything in dark black, but you can switch to a different color. You can use 16 colors:

This program prints “I’m sad” in dark blue (which is color #4):

Since BBC Basic was invented by the British, it wants you to write “colour”, not “color” (which is American). If you accidentally write “color”, BBC Basic will automatically change it to “colour” when you press the Enter key.

Once you pick a color, the computer will keep printing in that color until you switch to a different color (or close the output window).

This program prints “I love you” in bright red (which is color #9):

Background color Normally, the computer prints on a pure white background, but you can change the background color by adding 128. This program prints “I love you” in bright red on a bright yellow background:

Graphics colors The “colour” command alters the colors of text, not graphics. To alter the colors of graphics (pixel pictures), say “gcol” instead of “colour”. To change the graphics’ background color, you must also say “clg” (which means “clear the graphics screen”). This program creates a bright red circle on a bright yellow background:

Sounds

This program produces a normal sound:

The sound is loud, middle C (on the piano), and lasts for 3 seconds. Try it: run that program!

Here’s what those shaded numbers mean.…

The “15” is how loud the sound will be.

The “3” is how long the sound will last: how many seconds.

The “25” is the sound’s pitch (how high the note will be).

The “1” is the sound’s voice number. BBC Basic can produce 3-part harmony. This program makes the first voice sing middle C (pitch #25) while the second voice sings E (pitch #29) and the third voice sings G (pitch # 32), to produce a chord:

Decimal places

Suppose you want to add $12.47 to $1.03. The correct answer is $13.50. This almost works:

It makes the computer print:

But instead of 13.5, let’s make the computer print 13.50.

This Print Format command makes the computer print
2 digits after each decimal point —

so this program makes the computer print 13.50:

The computer will print:

Here’s a contradiction:

The bottom line says to print 86.239, but the top line says to print just 2 digits after the decimal point, so the computer will round 86.239 and print:

In the Print Format command, the last digit (the 2) says how many digits to print after the decimal point. If you want 3 digits after the decimal point instead just 2, change the 2 to 3, like this:

The Print Format command affects all future printing, until you close the output window or give a different Print Format command or give this Cancel Print Format command —

which returns the printing to normal.

 

You can do fancy calculations — easily!

Pi

A circle’s circumference divided by its diameter is called “pi” (which Americans pronounce the same as the word “pie”). It’s approximately 3.14159265. This program makes the computer print 3.14159265:

Exponents

Try typing this program:

To type the symbol ^, do this: while holding down the Shift key, tap this key:

^ 6

That symbol (^) is called a caret.

In that program, the “4^3” makes the computer use the number 4, three times. The computer will multiply together those three 4’s, like this: 4 times 4 times 4. Since
“4 times 4 times 4” is 64, the computer will print:

In the expression “4^3”, the 4 is called the base; the 3 is called the exponent.

Here’s another example:

The “10^6” makes the computer use the number 10, six times. The computer will multiply together those six 10’s (like this:
10 times 10 times 10 times 10 times 10 times 10) and print the answer:

Here’s another example:

The “3^2” makes the computer use the number 3, two times. The computer will multiply together those two 3’s (like this:
3 times 3) and print the answer:

Order of operations The symbols +, -, *, /, and ^ are all called operations.

To solve a problem, the computer uses the three-step process taught in algebra and the “new math”. For example, suppose you say:

The computer will not begin by subtracting 3 from 70; instead, it will use the three-step process:

In each step, it looks from left to right. For example, in step 2, it sees / and gets rid of it before it sees *.

Speed Though exponents are fun, the computer handles them slowly. For example, the computer handles 3^2 slower than 3*3. So for fast calculations, say 3*3 instead of 3^2.

Square roots What positive number, when multiplied by itself, gives 9? The answer is 3, because 3 times itself is 9.

3 squared is 9. 3 is called the square root of 9.

To make the computer deduce the square root of 9, type this:

The computer will print 3.

The symbol “sqr” is called a function. The number in parentheses (9) is called the function’s input (or argument or parameter). The answer, which is 3, is called the function’s output (or value).

The sqr(9) gives the same answer as 9^.5. The computer handles sqr(9) faster than 9^.5.

BBC Basic lets you omit the parentheses: instead of typing “sqr(9)”, you can type just “sqr9” or “sqr 9”, and the computer will still print the correct answer (which is 3).

Cube roots What number, when multiplied by itself and then multiplied by itself again, gives 64? The answer is 4, because 4 times 4 times 4 is 64. The answer (4) is called the cube root of 64.

Here’s how to make the computer find the cube root of 64:

The computer will print 4.

Exp The letter “e” stands for a special number, which is approximately 2.718281828459045. You can memorize that number easily, if you pair the digits:

That weird number is important in calculus, radioactivity, biological growth, and other areas of science. It’s calculated by this formula:

Therefore:

Exp(x) means e^x. For example, exp(3) means e³, which is e*e*e, which is:

Exp(4) means e⁴, which is e*e*e*e. Exp(3.1) means e^3.1, which is more than e³ but less than e⁴.

Here’s a practical application. Suppose you put $800 in a savings account, and the bank promises to give you 5% annual interest “compounded continuously”. How much money will you have at the end of the year? The answer is 800*exp(.05).

Logarithms Suppose you write 1 with a lot of zeros afterwards, like this:

The logarithm of that number is how many zeros you wrote. So if you say —

the computer will count the zeros and print how many there are; the computer will print:

Here are more examples:

How much is log(7852)? Since 7852 is between 1000 and 10000, its log is between 3 and 4. If you say —

the computer will print a number between 3 and 4; it will print:

I’ve been explaining the common log, which is used in chemistry. It’s also called the log base 10. Mathematicians write it as “log₁₀”. For example, they say log₁₀ 100000 is 5.

A different kind of log, used in calculus, is called the
natural log. It’s also called the log base e. Mathematicians write it as “log_e” or “LN” or “ln”. It’s about 2.3 times the common log. For example, if you say —

the computer will print a number that’s about 2.3 times log(100000), so it will print about “2.3 times 5”, which is 11.5. It will print:

Here are some powers of 2:

To compute the logarithm-base-2 of a number, find the number in the right-hand column; the answer’s in the left column. For example, the logarithm-base-2 of 32 is 5. The logarithm-base-2 of 15 is slightly less than 4.

The logarithm-base-2 of 64 is 6. That fact is written:

It’s also written:

To make the computer find the logarithm-base-2 of 64, say:

The computer will print 6.

Contrasts

The computer’s notation resembles that of arithmetic and algebra, but beware of these contrasts.…

Multiplication To make the computer multiply, you must type an asterisk:

Exponents Put an exponent in parentheses, if it contains an operation:

Fractions Put a fraction’s numerator in parentheses, if it contains addition or subtraction:

Put a denominator in parentheses, if it contains addition, subtraction, multiplication, or division:

Mixed numbers A mixed number is a number that contains a fraction. For example, 9½ is a mixed number. When you write a mixed number, put a plus sign before its fraction:

If you’re using the mixed number in a further calculation, put the mixed number in parentheses:

Stripping

Sometimes the computer prints too much info: you wish the computer would print less, to save yourself the agony of reading excess info irrelevant to your needs. Whenever the computer prints too much info about a numerical answer, say “abs”, “int”, or “sgn”.

Abs, int, and sgn are called stripping functions or strippers or diet functions or diet pills, because they strip away the number’s excess fat and reveal just the fundamentals that interest you.

Here are more details about those three functions.…

Abs To find the absolute value of a negative number, just omit the number’s minus sign. For example, the absolute value of -7 is 7.

The absolute value of a positive number is the number itself. For example, the absolute value of 7 is 7. The absolute value of 0 is 0.

To make the computer find the absolute value of -7, type this:

The computer will print:

Like sqr, abs is a function: you’re supposed to put parentheses after the “abs”, though in BBC Basic the parentheses are optional.

Since abs omits the minus sign, abs turns negative numbers into positive numbers. Use abs whenever you insist that an answer be positive.

For example, abs helps solve math & physics problems about “distance”, since the “distance” between two points is always a positive number and cannot be negative.

This program computes the distance between two numbers:

When you run that program, suppose you say that the first number is 4 and the second number is 7. Since x is 4, and y is 7, the distance between those two numbers is abs(4-7), which is abs(-3), which is 3.

If you reverse those two numbers, so x is 7 and y is 4, the distance between them is abs(7-4), which is abs(3), which is still 3.

Int Int turns a number into an integer (a number without decimal digits).

Int rounds a number down to an integer that’s lower. For example:

To explore further the mysteries of rounding, run this program:

The top line asks you to type a number x.

The next line prints your number rounded down. For example, if you input 3.9, the computer prints 3.

The next line, PRINT -INT(-x), prints your number rounded up. For example if you input 3.9, the computer prints 4.

The bottom line prints your number rounded to the nearest integer. For example, if you input 3.9, the computer will print 4.

Here’s the rule: if x is a number, int(x) rounds x down; -int(-x) rounds x up; int(x+.5) rounds x to the nearest integer.

Rounding down and rounding up are useful in the supermarket:

By using int, you can do fancier kinds of rounding:

This program rounds a number, so that it will have just a few digits after the decimal point:

Here’s a sample run:

Sgn If a number is negative, its sign is -1. For example, the sign of -546 is -1.

If a number is positive, its sign is +1. For example the sign of 8231 is +1.

The sign of 0 is 0.

The computer’s abbreviation for “sign” is “sgn”. So if you say —

the computer will print the sign of -546; it will print -1.

If you say —

the computer will print the sign of 8231; it will print 1.

If you say —

the computer will print the sign of 0; it will print 0.

Sgn is the opposite of abs. Let’s see what both functions do to -7.2. Abs removes the minus sign, but leaves the digits:

Sgn removes the digits, but leaves the minus sign:

The Latin word for sign is signum. Most mathematicians prefer to talk in Latin — they say “signum” instead of “sign” — because the English word “sign” sounds too much like the trigonometry word “sine”.
So mathematicians call sgn the
signum function.

Random numbers

Usually, the computer is predictable: it does exactly what you say. But sometimes, you want the computer to be unpredictable.

For example, if you’re going to play a game of cards with the computer and tell the computer to deal, you want the cards dealt to be unpredictable. If the cards were predictable — if you could figure out exactly which cards you and the computer would be dealt — the game would be boring.

In many other games too, you want the computer to be unpredictable, to “surprise” you. Without an element of surprise, the game would be boring.

Being unpredictable increases the pleasure you derive from games — and from art. To make the computer act artistic, and create a new original masterpiece that’s a “work of art”, you need a way to make the computer get a “flash of inspiration”. Flashes of inspiration aren’t predictable: they’re surprises.

Here’s how to make the computer act unpredictably.…

Random integers This program makes the computer print an unpredictable number from 1 to 5:

Unpredictable numbers are called random numbers. That program makes the computer print a random number from 1 to 5, so the computer will print 1, 2, 3, 4, or 5. The computer’s choice will be a surprise.

For example, when you run that program, the computer might print 3. If you run the program a second time, the computer might print a different number
(1 or 2 or 4 or 5), or it might print the same number (3). You can’t predict which number the computer will print. The only thing you can be sure of is: the number will be from 1 to 5.

To make the computer print many such random numbers, say “goto”:

The computer will print many numbers, like this:

Each number will be 1 or 2 or 3 or 4 or 5. The order in which the computer prints them is unpredictable. The pogram’s an infinite loop: it won’t stop until you abort it. If you run the program again, the pattern will be different; for example, it might be:

When you run that program, the numbers will fly up the screen faster than you can read. To make the numbers easier to read, make the computer pause after each number, by telling the computer to wait a second (100 centiseconds):

Guessing game This program plays a guessing game:

The top line makes the computer say:

The next line makes the computer think of a random number from 1 to 10; the computer’s number is called “computer_number”. The “input” line asks the human to guess the number.

If the guess is less than the computer’s number, the first “if” line makes the computer say “Your guess is too low” and then goto 1, which lets the human guess again. If the guess is greater than the computer’s number, the bottom “if” line makes the computer say “Your guess is too high” and then goto 1.

When the human guesses correctly, the computer arrives at the bottom line, which prints:

Here’s a sample run:

Dice This program makes the computer roll a pair of dice:

Line 2 makes the computer say:

Each of the dice has 6 sides. The next line, a=rnd(6), rolls one of the dice, by picking a number from 1 to 6. The line saying “b=rnd(6)” rolls the other. The bottom line prints the total.

Here’s a sample run:

Here’s another run:

 

Coin flipping This program makes the computer flip a coin:

Rnd(2) is a random number from 1 to 2, so it’s either 1 or 2. If it’s 1, the program makes the computer say “heads”; if it’s 2 instead, the program makes the computer say “tails”.

Until you run the program, you won’t know which way the coin will flip; the choice is random. Each time you run the program, the computer will flip the coin again; each time, the outcome is unpredictable.

Here’s how to let the human bet on whether the computer will say “heads” or “tails”:

The top line makes the computer ask:

The next line makes sure the human says “heads” or “tails”: if the human’s answer isn’t “heads” and isn’t “tails”, the computer gripes. The bottom three lines make the computer flip a coin and determine whether the human won or lost the bet.

Here’s a sample run:

Here’s another:

Here’s another:

 

Here’s how to let the human use money when betting:

The top line gives the human a $100 bankroll, so the human starts with $100. The third line makes the computer say:

The next line makes the computer ask:

The number that the human inputs (the number of dollars that the human bets) is called the human’s stake. The next three lines (which say “if stake”) make sure the stake is reasonable.

The line numbered 2 gets the human to bet on heads or tails. The next few lines flip the coin, determine whether the human won or lost the bet, and then send the computer back for another round (if the human isn’t broke yet). The bottom three lines say good-bye to the human.

Here’s a sample run:

Daily horoscope This program predicts what will happen to you today:

The computer will say —

or —

or some in-between comment. That’s because the “case” line makes the computer pick a random integer from 1 to 5.

For inspiration, run that program when you get up in the morning. Then notice whether your day turns out the way the computer predicts!

Random decimals You’ve seen that rnd(5) is a random number from 1 to 5: it’s 1 or 2 or 3 or 4 or 5. To get a random decimal between 0 and 1, say “rnd(1)” instead of “rnd(5)”.

The decimal that rnd(1) produces is at least 0 and is less than 1, so it can be any decimal from 0.00000000 to 0.99999999. For example, the decimal might be 0.28459187.

Suppose you want the computer to maybe print “love”. Here’s how to make the probability of printing “love” be 37 percent:

 

Character codes

You can use these code numbers:

Alt key Here’s how to type the symbol ñ, whose code number is 241. Hold down the Alt key; and while you keep holding down the Alt key, type 0241 by using the numeric keypad (the number keys on the far right side of the keyboard). When you finish typing 0241, lift your finger from the Alt key, and you’ll see ñ on your screen!

The Alt key works for all numbers in that chart. Remember to type 0 before the number.

You can use the Alt key in your program. For example, try typing this program:

While typing that program, make the symbol ñ by typing 0241 on the numeric keypad while holding down the Alt key. When you run that program, the computer will print:

The Alt key works well in Windows 10, such as when you’re typing in the program window, but not if you’re typing weirdly (such typing in the output window).

Chr$ Here’s another way to type the symbol ñ:

When you run that program, the computer will print the character whose code number is 241. The computer will print:

This program makes the computer print “In Spanish, tomorrow is mañana”:

That makes the computer print “In Spanish, tomorrow is ma”, then print character 164 (which is ñ), then print “ana”.

This program prints, on your screen, all the symbols in the chart:

Low codes Codes below 33 do special things.

In a “print” statement, chr$(7) makes the computer play a 3-note melody. Try it! Run this program:

Chr$(32) is a blank space. It’s the same as “ ”.

Chr$(12) erases the entire screen. Saying —

has the same effect as saying:

Chr$(10) make the computer move down on the screen. Saying —

makes the computer print “hot” then move down then print “dog”, so you see:

Asc The code numbers from 33 to 126 are for characters that you can type on the keyboard easily. Established by a national committee, those code numbers are called the American Standard Code for Information Interchange, which is abbreviated Ascii, which is pronounced “ass key”.

Programmers say, “the Ascii code number for A is 65”. If you say —

the computer will print the Ascii code number for “A”. It will print:

If you say to print asc(“B”), the computer will print 66. If you say to print asc(“b”), the computer will print 98.

If you say to print asc(“ñ”), the computer will print 241 (which is the code number for ñ), even though ñ isn’t an Ascii character.

String analysis

Let’s analyze the word “smart”.

Length Since “smart” has 5 characters in it, the length of “smart” is 5. If you say —

the computer will print the length of “smart”; it will print:

Left, right, middle The leftmost character in “smart” is “s”. If you say —

the computer will print:

Try this program:

The top line says a$ is “smart”. The bottom line says to print the leftmost character in a$, which is “s”. The computer will print:

If a$ is “smart”, here are the consequences.…

You can change a string’s left part, like this:

The top line says a$ is “bunkers”. The left$ line changes the left part of a$ to “hoo”. The “hoo” covers up the “bun”, so the bottom line prints:

Here’s a variation:

The top line says a$ is “bunkers”. The left$ line steals the left 2 characters from “hoo”, so it steals “ho”. The “ho” covers up “bu”, so the bottom line prints:

You can change a string’s right part, like this:

The top line says a$ is “bunkers”. The right$ line changes the right 4 characters of “bunkers”, by stealing them from the beginning of “tingly”, which is “ting”. The “ting” covers up “kers”, so the bottom line prints:

You can change a string’s middle, like this:

The top line says a$ is “bunkers”. The mid$ line changes the middle of a$ to “owl”; the change begins at the 2^nd character of a$. The bottom line prints:

Here’s a variation:

The top line says a$ is “bunkers”. The mid$ line says to change the middle of a$, beginning at the 2^nd character of a$. But “ad agency” is too long to become part of “bunkers”. The computer uses as much of “ad agency” as will fit in “bunkers”. The computer will print:

Another variation:

The top line says a$ is “bunkers”. The mid$ line says to change the middle of a$, beginning at the 2^nd character of a$. But the “,1” makes the computer use just 1 letter from “owl”. The bottom line prints:

Adding strings You can add strings together, to form a longer string:

The top line says a$ is “father”. The bottom line makes the computer print:

Searching in a string You can make the computer search in a string to find another string. To make the computer search in the string “needed” to find “ed”, say:

Since “ed” begins at the third character of “needed”, the computer will print:

If you say —

the computer will search in the string “needed” for “ey”. Since “ey” is not in “needed”, the computer will print:

If you say —

the computer will hunt in the string “needed” for “ed”; but the hunt will begin at the 4th character of “needed”. The computer finds the “ed” that begins at the 5th character of “needed”. The computer will print:

String-number conversion This program converts a string to a number:

The top line says a$ is the string “52.6”. The next line says b is the numeric value of a$, so b is the number 52.6. The bottom line prints:

Val converts a string to a number. The opposite of val is str$, which converts a number to a string. For example, str$(-7.2) is the string “-7.2”. Str$(81.4) is the string “81.4”.

Repeating characters Suppose you love the letter b (because it stands for big, bold, and beautiful) and want to print “bbbbbbbbbbbbbbbbbbbb”. Here’s a short-cut:

That tells the computer to print a string of 20 b’s.

This makes the computer print “fat” 10 times:

The computer will print:

Let’s make the computer draw a dashed line containing 50 dashes, like this:

Here’s how: just say print string$(50, “-”).

 

Let’s make the computer print this triangle:

To do that, we want the computer to print 1 asterisk on the first line, then 2 asterisks on the next line, then 3 asterisks on the next line, and so on, until it finally prints 12 asterisks on the bottom line. Here’s the program:

The “for” line makes i be 1, then 2, then 3, and so on, up to 12. When i is 1, the “print” line makes the computer print one asterisk, like this:

When i is 2, the “print” line makes the computer print a line of 2 asterisks, like this:

The “for” line makes i be every number from 1 up to 20, so computer will print 1 asterisk, then underneath print a line of 2 asterisks, then underneath print a line of 3 asterisks, and so on, until the entire triangle is printed.

Trigonometry

The study of triangles is called trigonometry — and the computer can do it for you!

For example, look at this triangle:

 

 

 

 

 

In that triangle, the left angle is 30°, the bottom-right angle is 90°, and the longest side (the hypotenuse) is 1 inch long.

The side opposite the 30° angle is called the sine of 30°; the remaining side is called the cosine of 30°:

 

 

 

 

 

 

How long is the sine of 30°? How long is the cosine of 30°?

Since the longest side (the hypotenuse) is 1 inch long, and since the sine and the cosine are shorter sides, the sine and the cosine must each be shorter than 1 inch. So the lengths of the sine and cosine are each less than 1. But which decimals are they?

To find out, you can use a ruler. You’ll discover that the sine is half an inch long, and the cosine is nearly seven-eighths of an inch long. But a faster and more accurate way to measure the sine and cosine is to let the computer do it! Yes, the computer can calculate triangles in its mind!

This program makes the computer measure the sine and cosine of 30°:

The top line is a special formula that defines d to mean degrees. The first “print” line prints the sine of 30 degrees:

The bottom line prints the cosine of 30°, which is a decimal that’s slightly less than .87.

The computer can measure the sine and cosine of any size angle. Try it! For example, to make the computer print the sine and cosine of a 33° angle, say:

If you choose an angle of -33° instead of 33°, the triangle will dip down instead of rising up, and so the sine will be a negative number instead of positive.

In those “print” lines, the “*d” is important: it tells the computer that you want the sine of 33 degrees. If you accidentally omit the “*d”, the computer will print the sine of 33 radians instead. (A radian is larger than a degree. A radian is about 57.3 degrees. More precisely, a radian is 180/p degrees.)

Tangent The sine divided by the cosine is called the tangent. For example, to find the tangent of 33°, divide the sine of 33° by the cosine of 33°.

To make the computer print the tangent of 33°, you could tell the computer to print sin(33*d)/cos(33*d). But to find the tangent more quickly and easily, say just print tan(33*d).

Arc functions The opposite of the sine is called the arcsine:

Similarly, the opposite of the cosine is called the arccosine, and the opposite of the tangent is called the arctangent.

This program prints the the arcsine of .5, the arccosine of .87, and the arctangent of .58:

Line 2 prints the arcsine of .5, in degrees. (If you omit the “/ degrees”, the computer will print the answer in radians instead of degrees.) Line 3 prints the arccosine of .87. Line 4 prints the arctangent of .58. The answer to each of the three problems is about 30 degrees.

 

Instead of being a single string, x$ can be a whole list of strings, like this:

Here’s how to make x$ be that list of strings.…

Begin your program by saying:

That line says x$ will be a list of 7 strings. DIM means dimension; the line says the dimension of x$ is 7.

Next, tell the computer what strings are in x$. Type these lines:

That says x$’s first string is “love”, x$’s second string is “hate”, etc.

If you want the computer to print all those strings, type this:

That means: print all the strings in x$. The computer will print:

That program includes a line saying x$(1)=“love”. Instead of saying x$(1), math books say:

The “1” is called a subscript.

Similarly, in the line saying x$(2)=“hate”, the number 2 is a subscript. Some programmers pronounce that line as follows: “x string, subscripted by 2, is hate”. Hurried programmers just say: “x string 2 is hate”.

In that program, x$ is called an array (or matrix). Definition: an array (or matrix) is a variable that has subscripts.

Each item in an array is called an element of the array. For example, “love” is an element of that x$, and so is “hate”.

Subscripted data

That program said x$(1) is “love”, and x$(2) is “hate”, and so on. This program does the same thing, more briefly:

The “dim” line says x$ will be a list of 7 strings. The “data” line contains a list of 7 strings. The first “for…next” loop makes the computer read those strings and call them x$. The bottom “for…next” loop makes the computer print those 7 strings.

In that program, the first 3 lines say:

Most practical programs begin with those 3 lines.

Let’s lengthen the program, so the computer prints all this:

That consists of two verses. The second verse resembles the first, except each line of the second verse begins with “why”.

To make the computer print all that, just add the shaded lines to the program:

The shaded “print” line leaves a blank line between the first verse and the second. The shaded “for…next” loop, which prints the second verse, resembles the “for…next” loop that printed the first verse but prints “why” before each x$(i).

Let’s add a third verse, which prints the words in reverse order:

Before printing that third verse, print a blank line:

Then print the verse itself. To print the verse, you must print x$(7), then print x$(6), then print x$(5), etc. To do that, you could say:

But this way is shorter:

 

Numeric arrays

Let’s make y be this list of five numbers: 100, 94, 201, 8.3, and -7. To begin, tell the computer that y will consist of five numbers:

Next, tell the computer what the six numbers are:

Make the computer read all that data:

To make the computer print all that data, type this:

If you want the computer to add those 5 numbers together and print their sum, say:

Strange example

Getting tired of x and y? Then pick another letter! For example, you can play with z:

The computer will print:

Problems and solutions

Suppose you want to analyze 20 numbers. Begin your program by saying:

Then type the 20 numbers as data:

Tell the computer to read the data:

Afterwards, do one of the following, depending on which problem you want to solve.…

Print all x values Solution:

Print all x values, in reverse order Solution:

Print the sum of all x values In other words, print x(1)+x(2)+x(3)+…+x(20). Solution: start the sum at 0 —

and then increase the sum, by adding each x(i) to it:

Finally, print the sum:

Find the average of x In other words, find the average of the 20 numbers. Solution: begin by finding the sum —

then divide the sum by 20:

Find whether any x value is 79.4 In other words, find out whether 79.4 is a number in the list. Solution: if x(i) is 79.4, print “Yes” —

otherwise, print “No”:

In x’s list, count how often 79.4 appears Solution: start the counter at zero —

and increase the counter each time you see the number 79.4:

Finally, print the counter:

Print all x values that are negative In other words, print all the numbers that have minus signs. Solution: begin by announcing your purpose —

then print the values that are negative; in other words, print each x(i) that’s less than 0:

Print all x values that are above average Solution: find the average —

then announce your purpose:

Finally, print the values that are above average; in other words, print each x(i) that’s greater than average:

Find x’s biggest value In other words, find which of the 20 numbers is the biggest. Solution: begin by assuming that the biggest is the first number —

but if you find another number that’s even bigger, change your idea of what the biggest is:

Afterwards, print the biggest:

Find x’s smallest value In other words, find which of the 20 numbers is the smallest. Solution: begin by assuming that the smallest is the first number —

but if you find another number that’s even smaller, change your idea of what the smallest is:

Afterwards, print the smallest:

Check whether x’s list is in strictly increasing order In other words, find out whether the following statement is true: x(1) is a smaller number than x(2), which is a smaller number than x(3), which is a smaller number than x(4), etc. Solution: if x(i) is not smaller than x(i + 1), print “No” —

otherwise, print “Yes”:

Test yourself: look at those problems again, and see whether you can figure out the solutions without peeking at the answers.

Multiple arrays

Suppose your program involves three lists. Suppose the first list is called a$ and consists of 18 strings; the second list is called b and consists of 57 numbers; and the third list is called c$ and consists of just 3 strings. To say all that, begin your program with this statement:

Double subscripts

You can make x$ be a table of strings, like this:

Here’s how to make x$ be that table.…

Begin by saying:

That says x$ will be a table having 2 rows and 3 columns.

Then tell the computer what strings are in x$. Type these lines:

That says the string in x$’s first row and first column is “dog”, the string in x$’s first row and second column is “cat”, etc.

If you’d like the computer to print all those strings, type this:

That means: print all the strings in x$. The computer will print:

Most programmers follow this tradition: the row’s number is called i, and the column’s number is called j. That program obeys that tradition. The “for i=1 to 2” means “for both rows”; the “for j=1 to 3” means “for all 3 columns”.

Notice i comes before j in the alphabet; i comes before j in x(i,j); and “for i” comes before “for j”. If you follow the i-before-j tradition, you’ll make fewer errors.

At the end of the first “print” line, the comma makes the computer print each column in a separate zone; the semicolon makes the computer keep printing on the same line. The other “print” line makes the computer press the Enter key at the end of each row. The x$ is called a table or 2-dimensional array or
doubly subscripted array.

The sum of all numbers in the table is sum(x()).

Multiplication table

This program prints a multiplication table:

The top line says x will be a table having 10 rows and 4 columns.

The line saying “x(i,j)=i*j” means the number in row i and column j is i*j. For example, the number in row 3 and column 4 is 12. Above that line, the program says “for i=1 to 10” and “for j=1 to 4”, so that x(i,j)=i*j for every i and j, so every entry in the table is defined by multiplication.

The computer prints the whole table:

Instead of multiplication, you can have addition, subtraction, or division: just change the line saying “x(i,j)=i*j”.

Summing a table

Suppose you want to analyze this table:

Since the table has 9 rows and 4 columns, begin your program by saying:

Each row of the table becomes a row of the data:

Make the computer read the data:

To make the computer print the table, say this:

Here are some problems, with solutions.…

Find the sum of all the numbers in the table Solution: start the sum at 0 —

and then increase the sum, by adding each x(i,j) to it:

Finally, print the sum:

The computer will print:

Find the sum of each row In other words, make the computer print the sum of the numbers in the first row, then the sum of the numbers in the second row, then the sum of the numbers in the third row, etc. Solution: the general idea is —

Here are the details:

The computer will print:

Find the sum of each column In other words, make the computer print the sum of the numbers in the first column, then the sum of the numbers in the second column, then the sum of the numbers in the third column, etc. Solution: the general idea is —

Here are the details:

The computer will print:

In all the other examples, “for i” came before “for j”; but in this unusual example, “for i” comes after “for j”.

Secrets about subscripts

When you say “dim x(4)”, the computer creates x(1), x(2), x(3), and x(4) but also secretly creates x(0), which you can use the same way as the other elements, so altogether the array has 5 elements.

The computer secretly makes all 5 of those elements be temporarily 0, until you change them, by giving commands such as x(2)=9.4.

You can change all 5 of them fast by giving a command such as:

That makes x(0) be 138, x(1) by 2049, x(2) be 9.4, x(3) be -4997, and x(4) be 120.

To make all 5 of them be 9.4, you can say just:

The sum of all 5 numbers is automatically called sum(x()), so to print their sum you can say just:

That’s much easier than computing the sum by writing a “for…next” loop.

Similarly, saying “dim x$(4)” creates an array of 5 strings, beginning with x$(0). The computer makes all 5 of those strings be temporarily “” (which is an empty string), until you give commands such as x$(2)=“grape”. You can change all 5 of them fast by giving a command such as”

To make all 5 of them be “grape”, say x$()=”grape”. The sum of all 5 strings is called sum(x$()), which you can print by saying:

The sum is computed using string arithmetic: “hot”+”dog” is “hotdog”.

Saying “dim x(2,3)” creates a table that includes 2 rows and 3 columns but also secretly includes a row #0 and column #0, so altogether the table has 3 rows and 4 columns, a total of 12 elements. All numbers in the table are temporarily 0, until you change them. You can change all of them fast by giving a command such as:

That makes:

 

Here’s a sick program:

It makes the computer print this message:

So the computer prints “We all know...”, then insults the human (“You are stupid! You are ugly!”), then prints “...and yet we love you.”

Here’s a more sophisticated way to write that program:

In the sophisticated version, the top 3 lines tell the computer to print “We all know...”, then do a procedure to insult the human, then print “...and yet we love you.” But the computer doesn’t know how to insult yet.

The bottom 4 lines teach the computer how to insult: they say “insult” means to print “You are stupid!” and “You are ugly!” Those bottom 4 lines define the word insult; they’re the definition of insult.

That program is divided into two procedures. The top 4 lines are called the main procedure (or main routine or main module). The bottom 4 lines (which just define “proc_insult”) are called the subprocedure (or subroutine or submodule).

The subprocedure’s first line (“def proc_insult”) means “here’s the definition of proc_insult”. The subprocedure’s bottom line (“endproc”) means “this is the end of the subprocedure”.

The main procedure’s bottom line (“end”) means “this is the end of the main procedure”.

When you run the program, the computer will say:

Refrains

This is chanted by boys playing tag — and protesters fearing dictators:

In that chant, this refrain is repeated:

This program prints the entire chant:

Big love

This program prints a love poem:

In that program, many of the lines make the computer print the word LOVE. Let’s make those lines print the word LOVE bigger, like this:

To make LOVE be that big, run this version of the program:

In that version, the lines say proc_big_love instead of print “LOVE”. The subprocedure teaches the computer how to make big_love.

Changed variable

Suppose you run this program:

The computer begins by doing the main procedure, which says x is 4. Then the computer does procedure joe, which adds 1 to x, so x becomes 5. Then the computer finishes the main procedure, which says to print x, so the computer prints:

 

To become a good programmer, write your programs using a good style. Here’s how.…

Design a program

First, decide on your ultimate goal. Be optimistic. Maybe you’d like the computer to play the perfect game of chess? or translate every English sentence into French?

Research the past Whatever you want the computer to do, someone else probably thought of the same idea already and wrote a program for it.

Find out. Ask your friends. Ask folks in nearby schools, computer stores, computer centers, companies, libraries, and bookstores. Look through books and magazines. There are even books that list what programs have been written. Ask the company you bought your computer from.

Even if you don’t find exactly the program you’re looking for, you may find one that’s close enough to be okay, or that will work with just a little fixing or serve as part of your program or at least give you a clue as to where to begin. In a textbooks or magazines, you’ll probably find a discussion of the problem you’re trying to solve and the pros and cons of various solutions to it — some methods are faster than others.

If you keep your head in the sand and don’t look at what other programmers have done already, your programming effort may turn out to be a mere exercise, useless to the rest of the world.

Simplify Too often, programmers embark on huge projects and never get them done. Once you have an idea of what’s been done before and how hard your project is, simplify it.

Instead of making the computer play a perfect game of chess, how about settling for a game in which the computer plays unremarkably but at least doesn’t cheat? Instead of translating every English sentence into French, how about translating just English colors? (We wrote that program already.)

So pick a less ambitious, more realistic goal, which if achieved will please you and be a steppingstone to your ultimate goal.

Finding a bug in a program is like finding a needle in a haystack: removing the needle is easier if the haystack is small than if you wait until more hay’s been piled on.

Specify the I/O Make your new, simple goal more precise. That’s called specification. One way to be specific is to draw a picture, showing what your screen will look like if your program’s running successfully.

In that picture, find the lines typed by the computer. They become your program’s “print” statements. Find the lines typed by the human: they become the “input” statements. Now you can start writing your program: write the “print” and “input” statements on paper, with a pencil, and leave blank lines between them. You’ll fill in the blanks later.

Suppose you want the computer to find the average of two numbers. Your picture will look like this:

Your program at this stage will be:

All you have left to do is figure out what the “etc.” is. Here’s the general method.…

Choose your statements Suppose you didn’t have a computer. Then how would you get the answer?

Would you have to use a mathematical formula? If so, put the formula into your program, but remember that the equation’s left side must have just one variable. For example, if you’re trying to solve a problem about right triangles, you might have to use the Pythagorean formula a²+b²=c²; but the left side of the equation must have just one variable, so your program must say a=sqr(c^2-b^2), or b=sqr(c^2-a^2), or c=sqr(a^2+b^2), depending on whether you’re trying to compute a, b, or c.

Would you have to use a memorized list, such as an English-French dictionary or the population of each state or the weight of each chemical element? If so, that list becomes your data, and you must read it. If it would be helpful to have the data numbered — so the first piece of data is called x(1), the next piece of data is called x(2), etc. — use the “dim” statement.

Subscripts are particularly useful if one long list of information will be referred to several times in the program.

Does your reasoning repeat? That means your program should have a loop. If you know how many times to repeat, say “for…next”. If you’re not sure how often, say “repeat...until”. If the thing to be repeated isn’t repeated immediately, but just after several other things have happened, make the repeated part be a subprocedure.

At some point in your reasoning, do you have to make a decision? Do you have to choose among several alternatives? To choose between 2 alternatives, say “if”. To choose among 3 or more alternatives, say “case”. If you want the computer to make the choice arbitrarily, “by chance” instead of for a reason, say “if rnd”.

Do you have to compare two things? The way to say “compare x with y” is: “if x=y”.

Write pseudocode Some English teachers say that before you write a paper, you should make an outline. Some computer teachers give similar advice about writing programs.

The “outline” can look like a program in which some of the lines are written in plain English instead of computerese. For example, one statement in your outline might be:

Such a statement, written in English instead of in computerese, is called pseudocode. Later, when you fill in the details, expand that pseudocode to this:

Organize yourself Keep the program’s over-all organization simple. That will make it easier for you to expand the program and find bugs. Here’s some folklore, handed down from generation to generation of programmers, that will simplify your organization.…

Use top-down programming. That means write a one-sentence description of your program; then ejxpand that sentence to several sentences; then expand each of those sentences to several more sentences; and so on, until you can’t expand any more. Then turn each of those new sentences into lines of program. Then your program will be in the same order as the English sentences, therefore organized the same way as an English-speaking mind.

A variation is to use subprocedures. That means writing the essence of the program as a very short main procedure; instead of filling in the grubby details immediately, replace each piece of grubbiness by a subprocedure. Your program will be like a good book: your main procedure will move swiftly, and the annoying details will be relegated to the appendices at the back; the appendices are the subprocedures. Make each procedure brief — no more than 30 lines — so the entire procedure can fit on the screen; if it starts getting longer and grubbier, replace each piece of grubbiness by another subprocedure.

Avoid “goto”. It’s hard for a human to understand a program that’s a morass of “goto” statements. It’s like trying to read a book where each paragraph says to turn to a different page! When you must say “goto”, try to go forward instead of backwards and not go too far.

Use variables After you’ve written some lines of your program, you may notice that your reasoning “almost repeats”: several lines bear a strong resemblance to each other. You can’t use “repeat…until” or “for...next” unless the lines repeat exactly. To make the repetition complete, use a variable to represent the parts that are different.

For example, suppose your program contains these lines:

Each of those lines says to print 29.3428+9.87627*sqr(a number). The number keeps changing, so call it x. All those “print” lines can be replaced by this loop:

Here’s a harder example to fix:

Again, let’s use x. All those “print” lines can be combined like this:

This one’s even tougher:

Let’s assume a, b, c, d, e, f, g, h, and i have been computed earlier in the program. The trick to shortening those lines is to change the names of the variables. Throughout the program, say x(1) instead of a, say x(2) instead of b, say x(3) instead of c, etc. Say dim x(9) at the beginning of your program. Then replace all those “print” lines by this loop:

Make it efficient

Your program should be efficient. That means it should use as little of the computer’s time and memory as possible.

To use less of the computer’s memory, make your dimensions as small as possible. Try writing the program without any arrays at all; if that turns out to be terribly inconvenient, use the smallest and fewest arrays possible.

To use less of the computer’s time, avoid having the computer do the same thing more than once.

These lines force the computer to compute sqr(8.2*n+7) three times:

You should change them to:

These lines force the computer to compute x^9+2 a hundred times:

You should change them to:

These lines force the computer to count to 100 twice:

You should combine the two “for...next” loops into a single “for...next” loop, so the computer counts to 100 just once. Here’s how:

Instead of exponents, use multiplication, which is faster:

Test it

When you’ve written a program, test it: run it and see whether it works.

If the computer does not gripe, your tendency will be to say “Whoopee!” Don’t cheer too loudly. The answers the computer prints might be wrong. Even if its answers look reasonable, don’t assume they’re right: the computer’s errors can be subtle. Check some of its answers by computing them with a pencil.

Even if the answers the computer prints are correct, don’t cheer. Maybe you were just lucky. Type different input, and see whether your program still works. Probably you can input something that will make your program go crazy or print a wrong answer. Your mission: to find input that will reveal the existence of a bug.

Try 6 kinds of input.…

Try simple input Type in simple integers, like 2 and 10, so the computation is simple, and you can check the computer’s answers easily.

Try input that increases See how the computer’s answer changes when the input changes from 2 to 1000.

Does the change in the computer’s answer look reasonable? Does the computer’s answer go up when it should go up, and down when it should go down?… and by a reasonable amount?

Try input testing each “if” For a program that says —

input an x less than 7 (to see whether line 10 works), then an x greater than 7 (to see whether the line underneath the “if” line works), then an x equal to 7 (to see whether you really want “<” instead of “<=”), then an x very close to 7, to check round-off error.

For a program that says —

input an x, y, and z that make x^2+y less than z. Then try inputs that make x^2+y very close to z.

Try extreme input What happens if you input:

Find out.

If the input is supposed to be a string, what happens if you input aaaaa or zzzzz? What happens if you capitalize the input? If there are supposed to be two inputs, what happens if you input the same thing for each?

Try input making a line act strange If your program contains division, try input that will make the divisor be zero or a tiny decimal close to zero. If your program contains the square root of a quantity, try input that will make the quantity be negative. If your program says “for i=x to y”, try input that will make y be less than x, then equal to x. If your program mentions x(i), try input that will make i be zero or negative or greater than the dim.

Try input that causes round-off error: for a program that says “x-y” or says “if x=y”, try input that will make x almost equal y.

Try garbage Computers often print wrong answers. A computer can print a wrong answer because its circuitry is broken or because a program has a bug. But the main reason why computers print wrong answers is incorrect input. Incorrect input is called garbage and has several causes.…

The user’s finger slips. Instead of 400, he inputs 4000. Instead of 27, he inputs 72. Trying to type .753, he leaves out the decimal point.

The user got wrong info. He tries to input the temperature, but his thermometer is leaking. He tries to input the results of a questionnaire, but everybody who filled out his questionnaire lied.

The instructions aren’t clear, so the user isn’t sure what to input.

The user tries to joke or sabotage. Instead of inputting his name, he types an obscene comment. When asked how many brothers and sisters he has, he says 275.

Responsibility! As a programmer, it’s your duty to include clear directions for using your program, and you must make the program reject ridiculous input.

For example, if your program is supposed to print weekly paychecks, it should refuse to print checks for more than $10000. Your program should contain these lines:

That “if” line is called an error trap (or error-handling routine). Your program should contain several, to prevent printing checks that are too small (2¢?) or negative or otherwise ridiculous ($200.73145?)

To see how your program reacts to input that’s either garbage or unusual, ask a friend to run your program. That person might input something you never thought of.

Document it

Write an explanation that helps other people understand your program.

An explanation is called documentation. When you write an explanation, you’re documenting the program.

You can write the documentation on a separate sheet of paper, or you can make the computer print the documentation when the user runs or lists the program.

A popular device is to begin the program by making the computer ask the user:

You need two kinds of documentation: how to use the program, and how the program was written.

How to use the program Your explanation of using the program should include:

How the program was written An explanation of how you wrote the program will help other programmers borrow your ideas, and help them expand your program to meet new situations. It should include: