WHAT'S A VARIABLE? A letter can stand for a number. For example, X can stand for the number 47, as in this program: 10 X=47 20 PRINT X+2 Line 10 says X stands for the number 47. In other words, X is a name for the number 47. Line 20 says to print X+2. Since X is 47, 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. Line 10 is called an assignment statement, because it assigns 47 to X. More examples Here's another example: 10 Y=38 20 PRINT Y-2 Line 10 says Y is a numeric variable that stands for the number 38. Line 20 says to print Y-2. Since Y is 38, Y-2 is 36; so the computer will print 36. Example: 10 B=8 20 PRINT B*3 Line 10 says B is 8. Line 20 says to print B*3, which is 8*3, which is 24; so the computer will print 24. One variable can define another: 10 M=6 20 P=M+1 30 PRINT M*P Line 10 says M is 6. Line 20 says P is M+1, which is 6+1, which is 7; so P is 7. Line 30 says to print M*P, which is 6*7, which is 42; so the computer will print 42. A value can change: 10 F=4 20 F=9 30 PRINT F*2 Line 10 says F's value is 4. Line 20 changes F's value to 9, so line 30 prints 18. Hassles On the left side of the equal sign, you must have one variable: Allowed Not allowed Not allowed P=M+1 P-M=1 1=P-M     one variable two variables not a variable The variable on the left side of the equation is the only one that changes. For example, the statement P=M+1 changes the value of P but not M. The statement A=B changes the value of A but not B: 10 A=1 20 B=7 30 A=B 40 PRINT A+B Line 30 changes A, to make it equal B; so A becomes 7. Since both A and B are now 7, line 40 prints 14. ``A=B'' versus ``B=A'' Saying ``A=B'' has a different effect from ``B=A''. That's because ``A=B'' changes the value of A (but not B); ``B=A'' changes the value of B (but not A). Compare these programs: 10 A=1 10 A=1 20 B=7 20 B=7 30 A=B 30 B=A 40 PRINT A+B 40 PRINT A+B In the left program (which you saw before), line 30 changes A to 7, so both A and B are 7. Line 40 prints 14. In the right program, line 30 changes B to 1, so both A and B are 1. Line 40 prints 2. A variable is a box If variables ever confuse you, think of a variable as being a box. Here's how. . . . The computer's random-access memory (RAM) consists of electronic boxes. This program puts a number into a box: 10 X=47 20 PRINT X+2 Line 10 puts 47 into box X, like this: 旼컴컴컴컴컴컴� Box X � 47 � 읕컴컴컴컴컴컴� Line 20 says to print what's in box X, plus 2. So the computer will print 49. You can change what's in a box: 10 F=4 20 F=9 30 PRINT F*2 Line 10 puts 4 into box F: 旼컴컴컴컴컴컴� Box F � 4 � 읕컴컴컴컴컴컴� Line 20 puts 9 into box F; the 9 replaces the 4: 旼컴컴컴컴컴컴� Box F � 9 � 읕컴컴컴컴컴컴� Line 30 prints 18. To see why ``A=B'' has a different effect from ``B=A'', compare these programs again: 10 A=1 10 A=1 20 B=7 20 B=7 30 A=B 30 B=A 40 PRINT A+B 40 PRINT A+B In both programs, lines 10 and 20 do this: 旼컴컴컴컴컴컴� Box A � 1 � 읕컴컴컴컴컴컴� 旼컴컴컴컴컴컴� Box B � 7 � 읕컴컴컴컴컴컴� In the left program, line 30 makes the number in box A become 7 (so both boxes contain 7, and line 40 prints 14). In the right program, line 30 makes the number in box B become 1 (so both boxes contain 1, and line 40 prints 2). 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: multiply $1297.43 by 2 multiply $1297.43 by .05 add $1297.43 to $483.19 divide $1297.43 by 37 subtract $1297.43 from $8598.61 multiply $1297.43 by 28.7 To do those six calculations, you could run this program: 10 PRINT 1297.43*2; 1297.43*.05; 1297.43+483.19; 1297.43/37; 8598.61-1297.43 20 PRINT 1297.43*28.7 But that program's silly, since it contains the number 1297.43 six times. This program's briefer, because it uses a variable: 10 C=1297.43 20 PRINT C*2; C*.05; C+483.19; C/37; 8598.61-C; C*28.7 So whenever you need to use a number several times, turn the number into a variable, which will make your program briefer. String variables A string is any collection of characters, such as ``I LOVE YOU''. Each string must be in quotation marks. A letter can stand for a string ___ if you put a dollar sign after the letter, like this: 10 G$="DOWN" 20 PRINT G$ Line 10 says G$ stands for the string ``DOWN''. Line 20 prints: DOWN 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. Line 10 is pronounced, ``G String is DOWN''. If you're paranoid, you'll love this program: 10 L$="THEY'RE LAUGHING AT YOU" 20 PRINT L$ 30 PRINT L$ 40 PRINT L$ Line 10 says L$ stands for the string ``THEY'RE LAUGHING AT YOU''. Lines 20, 30, and 40 make the computer print: THEY'RE LAUGHING AT YOU THEY'RE LAUGHING AT YOU THEY'RE LAUGHING AT YOU Nursery rhymes The computer can recite nursery rhymes: 10 P$="PEAS PORRIDGE" 20 PRINT P$;" HOT" 30 PRINT P$;" COLD" 40 PRINT P$;" IN THE POT" 50 PRINT "NINE DAYS OLD" Line 10 says P$ stands for ``PEAS PORRIDGE''. Lines 20-50 make the computer print: PEAS PORRIDGE HOT PEAS PORRIDGE COLD PEAS PORRIDGE IN THE POT NINE DAYS OLD This program prints a fancier rhyme: 10 H$="HICKORY, DICKORY, DOCK!" 20 M$="THE MOUSE (SQUEAK! SQUEAK!)" 30 C$="THE CLOCK (TICK! TOCK!)" 40 PRINT H$ 50 PRINT M$;" RAN UP ";C$ 60 PRINT C$;" STRUCK ONE" 70 PRINT M$;" RAN DOWN" 80 PRINT H$ Lines 10-30 define H$, M$, and C$. Lines 40-80 make the computer print: HICKORY, DICKORY, DOCK! THE MOUSE (SQUEAK! SQUEAK!) RAN UP THE CLOCK (TICK! TOCK!) THE CLOCK (TICK! TOCK!) STRUCK ONE THE MOUSE (SQUEAK! SQUEAK!) RAN DOWN HICKORY, DICKORY, DOCK! Undefined variables If you don't define a numeric variable, the computer assumes it's zero: 10 PRINT R Since R hasn't been defined, line 10 prints zero. The computer doesn't look ahead: 10 PRINT J 20 J=5 When the computer encounters line 10, it doesn't look ahead to find out what J is. As of line 10, J is still undefined, so the computer prints zero. If you don't define a string variable, the computer assumes it's blank: 10 PRINT F$ Since F$ hasn't been defined, line 10 makes the computer print a line that says nothing; the line the computer prints is blank. Long variable names A numeric variable's name can be a letter (such as X) or a longer combination of characters, such as: PROFIT.IN.1989.BEFORE.NOVEMBER.PROMOTION For example, you can type: 10 PROFIT.IN.1989.BEFORE.NOVEMBER.PROMOTION = 3497.18 20 PROFIT.IN.1989 = PROFIT.IN.1989.BEFORE.NOVEMBER.PROMOTION + 6214.27 30 PRINT PROFIT.IN.1989 The computer will print: 9711.45 The variable's name can be quite long: up to 40 characters! The first character in the name must be a letter. The remaining characters can be letters, digits, or periods. The name must not be a word that has a special meaning to the computer. For example, the name cannot be PRINT. If the variable stands for a string, the name can have up to 40 characters, followed by a dollar sign, making a total of 41 characters, like this: MY.JOB.IN.1989.BEFORE.NOVEMBER.PROMOTION$ Although modern computers permit long variable names, primitive computers don't. To find out whether your computer permits long variable names, check the ``Versions of BASIC'' appendix. Professional programmers use long variable names, because long names make the programs easier to understand. I'll avoid long names since some computers don't permit them; but if your computer permits them, use them! INPUT 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: 10 INPUT "WHAT IS YOUR NAME";N$ 20 PRINT "I ADORE ANYONE WHOSE NAME IS ";N$ When the computer sees line 10, the computer asks ``WHAT IS YOUR NAME?'' and then waits for you to answer the question. Your answer will be called N$. For example, if you answer MARIA, then N$ is MARIA. Line 20 makes the computer print: I ADORE ANYONE WHOSE NAME IS MARIA Here's the whole conversation; I've underlined the parts typed by you. . . . You tell the computer to run:RUN The computer asks for your name:WHAT IS YOUR NAME? MARIA The computer praises your name:I ADORE ANYONE WHOSE NAME IS MARIA Try that example. Be careful! When you type line 10, which says INPUT, 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. . . . You tell the computer to run:RUN The computer asks for your name:WHAT IS YOUR NAME? BUD The computer praises your name:I ADORE ANYONE WHOSE NAME IS BUD When the computer asks for your name, if you say something weird, the computer will give you a weird reply. . . . Make the computer run:RUN Computer asks your name:WHAT IS YOUR NAME? NONE OF YOUR BUSINESS!!! The computer replies:I ADORE ANYONE WHOSE NAME IS NONE OF YOUR BUSINESS!!! College admissions This program prints a letter, admitting you to the college of your choice: 10 INPUT "WHAT COLLEGE WOULD YOU LIKE TO ENTER";C$ 20 PRINT "CONGRATULATIONS!" 30 PRINT "YOU HAVE JUST BEEN ADMITTED TO ";C$ 40 PRINT "IT FITS YOUR PERSONALITY" 50 PRINT "I HOPE YOU GO TO ";C$ 60 PRINT " RESPECTFULLY YOURS" 70 PRINT " THE DEAN OF ADMISSIONS" When the computer sees line 10, 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 tell the computer to run:RUN The computer asks:WHAT COLLEGE WOULD YOU LIKE TO ENTER? HARVARD The computer admits you:CONGRATULATIONS! YOU HAVE JUST BEEN ADMITTED TO HARVARD IT FITS YOUR PERSONALITY I HOPE YOU GO TO HARVARD RESPECTFULLY YOURS THE DEAN OF ADMISSIONS You can choose any college you wish: RUN WHAT COLLEGE WOULD YOU LIKE TO ENTER? HELL CONGRATULATIONS! YOU HAVE JUST BEEN ADMITTED TO HELL IT FITS YOUR PERSONALITY I HOPE YOU GO TO HELL RESPECTFULLY YOURS THE DEAN OF ADMISSIONS That program consists of three parts: 1. The computer begins by asking you a question (``What college would you like to enter?''). The computer's question is called the prompt, because it prompts you to answer. 2. Your answer (the college's name) is called your input, because it's information that you're putting into the computer. 3. The computer's reply (the admission letter) is called the computer's output, because it's the final answer that the computer puts out. 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: ONCE UPON A TIME, THERE WAS A YOUNGSTER NAMED _____________ your name WHO HAD A FRIEND NAMED _________________ friend's name _____________ WANTED TO ________________________ _________________ your name verb (such as "pat") friend's name BUT _________________ DIDN'T WANT TO ________________________ _____________ friend's name verb (such as "pat") your name WILL _____________ _______________________ _________________ your name verb (such as "pat") friend's name WILL _________________ ________________________ _____________ friend's name verb (such as "pat") your name TO FIND OUT, COME BACK AND SEE THE NEXT EXCITING EPISODE OF _____________ AND _________________ your name friend's name 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: 10 INPUT "WHAT IS YOUR NAME";Y$ 20 INPUT "WHAT'S YOUR FRIEND'S NAME";F$ 30 INPUT "IN 1 WORD, SAY SOMETHING YOU CAN DO TO YOUR FRIEND";V$ Then make the computer print the story: 40 PRINT "HERE'S MY STORY...." 50 PRINT "ONCE UPON A TIME, THERE WAS A YOUNGSTER NAMED ";Y$ 60 PRINT "WHO HAD A FRIEND NAMED ";F$ 70 PRINT Y$;" WANTED TO ";V$;" ";F$ 80 PRINT "BUT ";F$;" DIDN'T WANT TO ";V$;" ";Y$ 90 PRINT "WILL ";Y$;" ";V$;" ";F$ 100 PRINT "WILL ";F$;" ";V$;" ";Y$ 110 PRINT "TO FIND OUT, COME BACK AND SEE THE NEXT EXCITING EPISODE" 120 PRINT "OF ";Y$;" AND ";F$ Here's a sample run: WHAT'S YOUR NAME? DRACULA WHAT'S YOUR FRIEND'S NAME? MARILYN MONROE IN 1 WORD, SAY SOMETHING YOU CAN DO TO YOUR FRIEND? BITE HERE'S MY STORY.... ONCE UPON A TIME, THERE WAS A YOUNGSTER NAMED DRACULA WHO HAD A FRIEND NAMED MARILYN MONROE DRACULA WANTED TO BITE MARILYN MONROE BUT MARILYN MONROE DIDN'T WANT TO BITE DRACULA WILL DRACULA BITE MARILYN MONROE WILL MARILYN MONROE BITE DRACULA TO FIND OUT, COME BACK AND SEE THE NEXT EXCITING EPISODE OF DRACULA AND MARILYN MONROE Here's another run: WHAT'S YOUR NAME? SUPERMAN WHAT'S YOUR FRIEND'S NAME? KING KONG IN 1 WORD, SAY SOMETHING YOU CAN DO TO YOUR FRIEND? TICKLE HERE'S MY STORY.... ONCE UPON A TIME, THERE WAS A YOUNGSTER NAMED SUPERMAN WHO HAD A FRIEND NAMED KING KONG SUPERMAN WANTED TO TICKLE KING KONG BUT KING KONG DIDN'T WANT TO TICKLE SUPERMAN WILL SUPERMAN TICKLE KING KONG WILL KING KONG TICKLE SUPERMAN TO FIND OUT, COME BACK AND SEE THE NEXT EXCITING EPISODE OF SUPERMAN AND KING KONG Try it: put in your own name, the name of your friend, and something you'd like to do to your friend. Contest This program prints a certificate saying you won a contest: 10 INPUT "WHAT'S YOUR NAME";Y$ 20 INPUT "WHAT'S YOUR FRIEND'S NAME";F$ 30 INPUT "WHAT'S THE NAME OF ANOTHER FRIEND";A$ 40 INPUT "NAME A COLOR";C$ 50 INPUT "NAME A PLACE";P$ 60 INPUT "NAME A FOOD";D$ 70 INPUT "NAME AN OBJECT";J$ 80 INPUT "NAME A PART OF THE BODY";B$ 90 INPUT "NAME A STYLE OF COOKING (SUCH AS BAKED OR FRIED)";S$ 100 PRINT 110 PRINT "CONGRATULATIONS ";Y$ 120 PRINT "YOU'VE WON THE BEAUTY CONTEST, BECAUSE OF YOUR GORGEOUS ";B$ 130 PRINT "YOUR PRIZE IS A ";C$;" ";J$ 140 PRINT "PLUS A TRIP TO ";P$;" WITH YOUR FRIEND ";F$ 150 PRINT "PLUS...AND THIS IS THE BEST PART OF ALL..." 160 PRINT "DINNER FOR THE TWO OF YOU AT ";A$;"'S NEW RESTAURANT" 170 PRINT "WHERE ";A$;" WILL GIVE YOU ALL THE ";S$;" ";D$;" YOU CAN EAT" 180 PRINT "CONGRATULATIONS ";Y$;"...TODAY'S YOUR LUCKY DAY..." 190 PRINT "NOW EVERYBODY WANTS TO KISS YOUR AWARD-WINNING ";B$ Here's a sample run: WHAT'S YOUR NAME? LONG JOHN SILVER WHAT'S YOUR FRIEND'S NAME? THE PARROT WHAT'S THE NAME OF ANOTHER FRIEND? JIM NAME A COLOR? GOLD NAME A PLACE? TREASURE ISLAND NAME A FOOD? RUM-SOAKED COCONUTS NAME AN OBJECT? CHEST OF JEWELS NAME A PART OF THE BODY? MISSING LEG NAME A STYLE OF COOKING (SUCH AS BAKED OR FRIED)? BARBECUED CONGRATULATIONS LONG JOHN SILVER YOU'VE WON THE BEAUTY CONTEST, BECAUSE OF YOUR GORGEOUS MISSING LEG YOUR PRIZE IS A GOLD CHEST OF JEWELS PLUS A TRIP TO TREASURE ISLAND WITH YOUR FRIEND THE PARROT PLUS...AND THIS IS THE BEST PART OF ALL... DINNER FOR THE TWO OF YOU AT JIM'S NEW RESTAURANT WHERE JIM WILL GIVE YOU ALL THE BARBECUED RUM-SOAKED COCONUTS YOU CAN EAT CONGRATULATIONS LONG JOHN SILVER...TODAY'S YOUR LUCKY DAY... NOW EVERYBODY WANTS TO KISS YOUR AWARD-WINNING MISSING LEG This run describes the contest that brought Ronald Reagan to the White House: WHAT'S YOUR NAME? RONNIE REAGAN WHAT'S YOUR FRIEND'S NAME? NANCY WHAT'S THE NAME OF ANOTHER FRIEND? ALICE NAME A COLOR? RED-WHITE-AND-BLUE NAME A PLACE? THE WHITE HOUSE NAME A FOOD? JELLY BEANS NAME AN OBJECT? COWBOY HAT NAME A PART OF THE BODY? CHEEKS NAME A STYLE OF COOKING (SUCH AS BAKED OR FRIED)? STEAMED CONGRATULATIONS RONNIE REAGAN YOU'VE WON THE BEAUTY CONTEST, BECAUSE OF YOUR GORGEOUS CHEEKS YOUR PRIZE IS A RED-WHITE-AND-BLUE COWBOY HAT PLUS A TRIP TO THE WHITE HOUSE WITH YOUR FRIEND NANCY PLUS...AND THIS IS THE BEST PART OF ALL... DINNER FOR THE TWO OF YOU AT ALICE'S NEW RESTAURANT WHERE ALICE WILL GIVE YOU ALL THE STEAMED JELLY BEANS YOU CAN EAT CONGRATULATIONS RONNIE REAGAN...TODAY'S YOUR LUCKY DAY... NOW EVERYBODY WANTS TO KISS YOUR AWARD-WINNING CHEEKS Bills If you're a nasty bill collector, you'll love this program: 10 INPUT "WHAT IS THE CUSTOMER'S FIRST NAME";F$ 20 INPUT "LAST NAME";L$ 30 INPUT "STREET ADDRESS";A$ 40 INPUT "CITY";C$ 50 INPUT "STATE";S$ 60 INPUT "ZIP CODE";Z$ 70 PRINT 80 PRINT F$;" ";L$ 90 PRINT A$ 100 PRINT C$;" ";S$;" ";Z$ 110 PRINT 120 PRINT "DEAR ";F$;"," 130 PRINT " YOU STILL HAVEN'T PAID THE BILL." 140 PRINT "IF YOU DON'T PAY IT SOON, ";F$;"," 150 PRINT "I'LL COME VISIT YOU IN ";C$ 160 PRINT "AND PERSONALLY SHOOT YOU." 170 PRINT " YOURS TRULY," 180 PRINT " SURE-AS-SHOOTIN'" 190 PRINT " YOUR CRAZY CREDITOR" Can you figure out what that program does? Numeric input This program makes the computer predict your future: 10 PRINT "I PREDICT WHAT'LL HAPPEN TO YOU IN THE YEAR 2000!" 20 INPUT "IN WHAT YEAR WERE YOU BORN";Y 30 PRINT "IN THE YEAR 2000, YOU'LL TURN";2000-Y;"YEARS OLD." Here's a sample run: I PREDICT WHAT'LL HAPPEN TO YOU IN THE YEAR 2000! IN WHAT YEAR WERE YOU BORN? 1962 IN THE YEAR 2000, YOU'LL TURN 38 YEARS OLD. 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: 10 INPUT "HOW MANY TICKETS";T 20 PRINT "THE TOTAL PRICE IS $";T*2.79 This program tells you how much the ``energy crisis'' costs you, when you drive your car: 10 INPUT "HOW MANY MILES DO YOU WANT TO DRIVE";M 20 INPUT "HOW MANY PENNIES DOES A GALLON OF GAS COST";P 30 INPUT "HOW MANY MILES-PER-GALLON DOES YOUR CAR GET";R 40 PRINT "THE GAS FOR YOUR TRIP WILL COST YOU $";M*P/(R*100) Here's a sample run: HOW MANY MILES DO YOU WANT TO DRIVE? 400 HOW MANY PENNIES DOES A GALLON OF GAS COST? 95.9 HOW MANY MILES-PER-GALLON DOES YOUR CAR GET? 31 THE GAS FOR YOUR TRIP WILL COST YOU $ 12.3742 Conversion This program converts feet to inches: 10 INPUT "HOW MANY FEET";F 20 PRINT F;"FEET =";F*12;"INCHES" Here's a sample run: HOW MANY FEET? 3 3 FEET = 36 INCHES Trying to convert to the metric system? This program converts inches to centimeters: 10 INPUT "HOW MANY INCHES";I 20 PRINT I;"INCHES =";I*2.54;"CENTIMETERS" Nice day today, isn't it? This program converts the temperature from Celsius to Fahrenheit: 10 INPUT "HOW MANY DEGREES CELSIUS";C 20 PRINT C;"DEGREES CELSIUS =";C*1.8+32;"DEGREES FAHRENHEIT" Here's a sample run: HOW MANY DEGREES CELSIUS? 20 20 DEGREES CELSIUS = 68 DEGREES FAHRENHEIT 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. IF . . . THEN The computer understands the words IF and THEN. Therapist Let's turn your computer into a therapist. To make the computer ask the patient, ``HOW ARE YOU?'', begin the program like this: 10 INPUT "HOW ARE YOU";A$ That line makes the computer ask HOW ARE YOU and wait for the patient's answer, which is called A$. If the patient feels FINE, let's make the computer say THAT'S GOOD. Here's how: 20 IF A$="FINE" THEN PRINT "THAT'S GOOD" That line says that if the patient's answer (A$) is FINE, the computer will print THAT'S GOOD. If the patient feels LOUSY instead, let's make the computer say TOO BAD. Here's how: 30 IF A$="LOUSY" THEN PRINT "TOO BAD" Here's the entire program: 10 INPUT "HOW ARE YOU";A$ 20 IF A$="FINE" THEN PRINT "THAT'S GOOD" 30 IF A$="LOUSY" THEN PRINT "TOO BAD" When the patient types RUN, line 10 makes the computer ask HOW ARE YOU and wait for the patient's answer, which is called A$. If the patient's answer is FINE, line 20 makes the computer reply THAT'S GOOD. If the patient's answer is LOUSY, line 30 makes the computer reply TOO BAD. Try running that program! Here's a sample run: RUN HOW ARE YOU? FINE THAT'S GOOD Here's another run: RUN HOW ARE YOU? LOUSY TOO BAD Underlying theory Whenever you say IF, you should also say THEN. For example, line 20 (which says IF) also says THEN. Similarly, line 30 (which says IF) says THEN. Do not put a comma before THEN. What comes between IF and THEN is called the condition. In line 20, the condition is: A$="FINE" If that condition is true (if A$ really does equal ``FINE''), the computer does what comes after the word THEN, which is called the consequence, which is: PRINT "THAT'S GOOD" Increase the computer's vocabulary In that program, what happens if the patient says some word other than FINE or LOUSY? For example, what happens if the patient says TERRIBLE? Since the program doesn't tell the computer how to react to TERRIBLE, the computer won't print any reaction at all. The computer will do just what it always does at the end of a program: it will say OK (or READY or a similar word). Let's enhance the program, so that if the patient says TERRIBLE, the computer will say TOUGH TURKEY. Add the shaded line: 10 INPUT "HOW ARE YOU";A$ 20 IF A$="FINE" THEN PRINT "THAT'S GOOD" 30 IF A$="LOUSY" THEN PRINT "TOO BAD" 40 IF A$="TERRIBLE" THEN PRINT "TOUGH TURKEY" I feel the same way Let's make the computer end the conversation by saying I FEEL THE SAME WAY. Add the shaded line: 10 INPUT "HOW ARE YOU";A$ 20 IF A$="FINE" THEN PRINT "THAT'S GOOD" 30 IF A$="LOUSY" THEN PRINT "TOO BAD" 40 IF A$="TERRIBLE" THEN PRINT "TOUGH TURKEY" 50 PRINT "I FEEL THE SAME WAY" Since line 50 does not contain the word IF, the computer will always print I FEEL THE SAME WAY at the end of the conversation, regardless of what the patient says. Here's a sample run: RUN HOW ARE YOU? FINE THAT'S GOOD I FEEL THE SAME WAY Here's another: RUN HOW ARE YOU? TERRIBLE TOUGH TURKEY I FEEL THE SAME WAY Another: RUN HOW ARE YOU? LONELY I FEEL THE SAME WAY Another: RUN HOW ARE YOU? I DON'T WANT TO TALK WITH YOU I FEEL THE SAME WAY Avoid monotony Having the computer always say I FEEL THE SAME WAY is monotonous. To make the program more interesting, let's make the computer say I FEEL THE SAME WAY only if the patient doesn't say FINE, LOUSY, or TERRIBLE. So if the patient says FINE, LOUSY, or TERRIBLE, let's prevent the computer from saying I FEEL THE SAME WAY. Whenever you want to make the computer skip saying I FEEL THE SAME WAY, type the word END, like this: 10 INPUT "HOW ARE YOU";A$ 20 IF A$="FINE" THEN PRINT "THAT'S GOOD": END 30 IF A$="LOUSY" THEN PRINT "TOO BAD": END 40 IF A$="TERRIBLE" THEN PRINT "TOUGH TURKEY": END 50 PRINT "I FEEL THE SAME WAY" In that program, if the patient types FINE, LOUSY, or TERRIBLE, lines 20-40 make the computer print a two-word message and then END, without printing I FEEL THE SAME WAY. The only way the computer can reach line 50 (which prints I FEEL THE SAME WAY) is if the patient avoids FINE, LOUSY, and TERRIBLE. Here's a sample run: RUN HOW ARE YOU? FINE THAT'S GOOD Here's another: RUN HOW ARE YOU? LONELY I FEEL THE SAME WAY Charge $50 After the computer's given the patient therapy, let's make the computer charge $50. Just add this line: 60 PRINT "I HOPE YOU ENJOYED YOUR THERAPY--NOW YOU OWE $50" To make sure the computer always goes to that line and collects the $50, regardless of what the patient says, replace each END by GO TO 60. Altogether, the program looks like this: 10 INPUT "HOW ARE YOU";A$ 20 IF A$="FINE" THEN PRINT "THAT'S GOOD": GO TO 60 30 IF A$="LOUSY" THEN PRINT "TOO BAD": GO TO 60 40 IF A$="TERRIBLE" THEN PRINT "TOUGH TURKEY": GO TO 60 50 PRINT "I FEEL THE SAME WAY" 60 PRINT "I HOPE YOU ENJOYED YOUR THERAPY--NOW YOU OWE $50" Here's a sample run: RUN HOW ARE YOU? FINE THAT'S GOOD I HOPE YOU ENJOYED YOUR THERAPY--NOW YOU OWE $50 Here's another: RUN HOW ARE YOU? LONELY I FEEL THE SAME WAY I HOPE YOU ENJOYED YOUR THERAPY--NOW YOU OWE $50 In that program, try changing the strings to make the computer print smarter remarks, become a better therapist, and charge even more money. Keywords In that program, the only words the computer understands are INPUT, IF, THEN, PRINT, GO, and TO. Those words are called keywords. Using just those keywords, you can write any program you wish! Here's why. . . . Can a computer become President? To become President of the United States, you need four basic skills. First, you must be a good talker, so you can give effective speeches saying ``Vote for me!'', express your views, and make folks do what you want. But even if you're a good talker, you're useless unless you're also a good listener. You must be able to listen to people's needs and ask, ``What can I do to make you happy and get you to vote for me?'' But even if you're a good talker and listener, you're still useless unless you can make decisions. Should you give more money to poor people? Should you bomb the enemy? Which actions should you take, and under what conditions? But even if you're a good talker and listener and decision maker, you still need one more trait to become President: you must be able to take the daily grind of politics. You must, again and again, shake hands, make compromises, and raise funds. You must have the patience to put up with the repetitive monotony of those chores. 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 four qualities the computer has! The word PRINT turns the computer into a good speech-maker: by using the word PRINT, you can make the computer write whatever speech you wish. The word INPUT turns the computer into a good listener: by using the word INPUT, you can make the computer ask humans lots of questions, to find out who the humans are and what they want. The words IF and THEN turn the computer into a decision maker: the computer can analyze the IF condition, determine whether that condition is true, and act accordingly. Finally, the words GO and TO enable the computer to perform loops, which the computer will repeat patiently. So by using the words PRINT, INPUT, IF THEN, and GO TO, you can make the computer imitate any intellectual human activity. Those four magic phrases ___ PRINT, INPUT, IF THEN, and GO TO ___ are the only phrases 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 just those four magic phrases. The only question is: how? The Secret Guide to Computers 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 four phrases: PRINT, INPUT, IF THEN, and GO TO. Programming is the ultimate act of scientific reductionism: programmers reduce all of life scientifically to just four phrases. In addition to those keywords (PRINT, INPUT, IF, THEN, GO, and TO), the computer understands extra keywords also, such as END. Those extra keywords aren't strictly necessary: if they hadn't been invented, you could still write programs without them. But they make programming easier. A 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 THEN, GO TO, and END. The mysteries of life Let's dig deeper into the mysteries of PRINT, INPUT, IF THEN, GO TO, 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 that you do be explained in terms of PRINT, INPUT, IF THEN, and GO TO? By the time you finish The Secret Guide to Computers, you'll know! Mary Poppins meets Frankenstein To make the computer interrogate a human, have the computer ask: ARE YOU MALE OR FEMALE? If the human answers MALE, let's make the computer say: SO IS FRANKENSTEIN If the human answers FEMALE, let's make the computer say: SO IS MARY POPPINS Here's the program: 10 INPUT "ARE YOU MALE OR FEMALE";A$ 20 IF A$="MALE" THEN PRINT "SO IS FRANKENSTEIN" 30 IF A$="FEMALE" THEN PRINT "SO IS MARY POPPINS" Here's a sample run: RUN ARE YOU MALE OR FEMALE? MALE SO IS FRANKENSTEIN Here's another: RUN ARE YOU MALE OR FEMALE? FEMALE SO IS MARY POPPINS Neither MALE nor FEMALE? What does that program do if the human says neither MALE nor FEMALE? What if the human says SUPER-MALE or MACHO or NOT SURE or BOTH or YES? In those cases, the program doesn't tell the computer how to reply, so the computer will make no reply at all. Let's improve the program, so that if the human says neither MALE nor FEMALE the computer will reply ___ PLEASE SAY MALE OR FEMALE ARE YOU MALE OR FEMALE? and force the human to answer the question correctly. To do that, add this line ___ 40 PRINT "PLEASE SAY MALE OR FEMALE": GO TO 10 and put END at the end of lines 20 and 30, so the program looks like this: 10 INPUT "ARE YOU MALE OR FEMALE";A$ 20 IF A$="MALE" THEN PRINT "SO IS FRANKENSTEIN": END 30 IF A$="FEMALE" THEN PRINT "SO IS MARY POPPINS": END 40 PRINT "PLEASE SAY MALE OR FEMALE": GO TO 10 Line 10 makes the computer ask ``ARE YOU MALE OR FEMALE?'' and wait for the human's answer, which is called A$. If the human's answer is MALE, line 20 makes the computer print SO IS FRANKENSTEIN and then end. If the human's answer is FEMALE, line 30 makes the computer print SO IS MARY POPPINS and then END. If the human's answer is neither MALE nor FEMALE, the computer skips over lines 20 and 30, so it comes to line 40, which makes it print PLEASE SAY MALE OR FEMALE and then go back to line 10, which forces the human to answer the question again. Here's a sample run: RUN ARE YOU MALE OR FEMALE? MALE SO IS FRANKENSTEIN Here's another: RUN ARE YOU MALE OR FEMALE? FEMALE SO IS MARY POPPINS Another: RUN ARE YOU MALE OR FEMALE? MACHO PLEASE SAY MALE OR FEMALE ARE YOU MALE OR FEMALE? MALE SO IS FRANKENSTEIN Another: RUN ARE YOU MALE OR FEMALE? SUPER-MALE PLEASE SAY MALE OR FEMALE ARE YOU MALE OR FEMALE? NONE OF YOUR BUSINESS PLEASE SAY MALE OR FEMALE ARE YOU MALE OR FEMALE? MAIL PLEASE SAY MALE OR FEMALE ARE YOU MALE OR FEMALE? MALE SO IS FRANKENSTEIN In that program, if the human makes a typing error and answers neither MALE nor FEMALE, the computer arrives at line 40, which makes the computer gripe and tell the human to try again. So the purpose of line 40 is to react to errors. Line 40 is called an error-handling routine or an error trap. That's because an error's like a vicious monster, and line 40's purpose is to trap it. Do you like Mary Poppins? Let's extend the conversation. If the human says FEMALE, let's make the computer say SO IS MARY POPPINS and then ask ``DO YOU LIKE HER?'' If the human says YES, let's make the computer say: I LIKE HER TOO--SHE IS MY MOTHER If the human says NO, let's make the computer say: NEITHER DO I--SHE STILL OWES ME A DIME If the human says neither YES nor NO, let's make the computer say: PLEASE SAY YES OR NO DO YOU LIKE HER? Here's the program: 10 INPUT "ARE YOU MALE OR FEMALE";A$ 20 IF A$="MALE" THEN PRINT "SO IS FRANKENSTEIN": END 30 IF A$="FEMALE" THEN PRINT "SO IS MARY POPPINS": GO TO 100 40 PRINT "PLEASE SAY MALE OR FEMALE": GO TO 10 100 INPUT "DO YOU LIKE HER";B$ 110 IF B$="YES" THEN PRINT "I LIKE HER TOO--SHE IS MY MOTHER ": END 120 IF B$="NO" THEN PRINT "NEITHER DO I--SHE STILL OWES ME A DIME": END 130 PRINT "PLEASE SAY YES OR NO": GO TO 100 Line 30 says: if the human's answer is FEMALE, print SO IS MARY POPPINS and then go to line 100, which asks ``DO YOU LIKE HER?'' Lines 110 and 120 make the computer react to the human's opinion of Mary Poppins. Line 130 is like line 40: it's an error trap. Weird programs The computer's abilities are limited only by your own imagination ___ and your weirdness. Here are some weird programs from weird minds. . . . Friends Like a human, the computer wants to meet new friends. This program makes the computer show its true feelings: 10 INPUT "ARE YOU MY FRIEND";A$ 20 IF A$="YES" THEN PRINT "THAT'S SWELL": END 30 IF A$="NO" THEN PRINT "GO JUMP IN A LAKE": END 40 PRINT "PLEASE SAY YES OR NO": GO TO 10 When you type RUN, 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. Watch TV The most inventive programmers are kids. This program was written by a girl in the sixth grade: 10 INPUT "CAN I COME OVER TO YOUR HOUSE TO WATCH T.V.";A$ 20 IF A$="YES" THEN PRINT "THANKS. I'LL BE THERE AT 5 P.M.": END 30 IF A$="NO" THEN PRINT "HUMPH! YOUR FEET SMELL, ANYWAY.": END 40 PRINT "PLEASE SAY YES OR NO": GO TO 10 When you type RUN, the computer asks to watch your TV. If you say YES, the computer promises to come to your house at 5. If you refuse, the computer insults your feet. Honesty Another sixth-grade girl wrote this program, to test your honesty: 10 PRINT "FKGJDFGKJ*#K$JSLF*/#$()$&(IKJNHBGD52:?./KSDJK$E(EF$#/JI K(*" 20 PRINT "FASDFJKL:JFRFVFJUNJI*&()JNE$#SKI#(!SERF HHW NNWAZ MAME !!!" 30 PRINT "ZBB%%%%%##)))))FESDFJK DSFE N.D.JJUJASD EHWLKD******" 40 INPUT "DO YOU UNDERSTAND WHAT I SAID";A$ 50 IF A$="NO" THEN PRINT "SORRY TO HAVE BOTHERED YOU": END 60 IF A$="YES" THEN GO TO 100 70 PRINT "PLEASE SAY YES OR NO": GO TO 10 100 PRINT "SSFJSLFKDJFL++++4567345677839XSDWFEGF/#$&**()---==!!ZZ XX" 110 PRINT "###EDFHTG NVFDF MKJKF ==+--*$&% #RHFS SESD DOPEKKKNJGFD DSBS" 120 INPUT "OKAY, WHAT DID I SAY";B$ 130 PRINT "YOU ARE A LIAR, A LIAR, A BIG FAT LIAR!" When you type RUN, lines 10-30 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. Daddy's always right A Daddy wrote a program for his five-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: 10 INPUT "WHAT'S YOUR NAME";N$ 20 INPUT "WHAT'S 2 AND 2";A 30 IF N$="DADDY" THEN PRINT "YES, DADDY IS ALWAYS RIGHT": END 40 IF A=4 THEN PRINT "NO, 2 AND 2 IS 22": END 50 PRINT "NO, 2 AND 2 IS 4" Fancy relations You can make the IF clause very fancy: IF clauseMeaning IF A$="MALE"If A$ is ``MALE'' IF A=4 If A is 4 IF A<4 If A is less than 4 IF A>4 If A is greater than 4 IF A<=4 If A is less than or equal to 4 IF A>=4 If A is greater than or equal to 4 IF A<>4 If A is not 4 IF A$<"MALE"If A is a word that comes before ``MALE'' in the dictionary IF A$>"MALE"If A is a word that comes after ``MALE'' in the dictionary In the IF statement, the symbols =, <, >, <=, >=, and <> are called relations. When you write a relation, put the equal sign last: RightWrong <= =< >= => 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'': IF X=7 OR X=8 THEN PRINT "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. Right:IF X=7 OR X=8 THEN PRINT "WONDERFUL"(``X=7'' and ``X=8'' are conditions.) Wrong:IF X=7 OR 8 THEN PRINT "WONDERFUL"(``8'' is not a condition.) 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'': IF P>5 AND P<10 THEN PRINT "TUNA FISH" Here's how to say, ``If S is at least 60 and less than 65, print YOU ALMOST FAILED'': IF S>=60 AND S<65 THEN PRINT "YOU ALMOST FAILED" Here's how to say, ``If N is a number from 1 to 10, print THAT'S GOOD'': IF N>=1 AND N<=10 THEN PRINT "THAT'S GOOD" ELSE Here's how to say, ``If A is less than 18, print MINOR; but if A is not less than 18, print ADULT'': IF A<18 THEN PRINT "MINOR" ELSE PRINT "ADULT" That line says to either PRINT ``MINOR'' or ELSE PRINT ``ADULT''. If A is less than 18, the computer will PRINT ``MINOR''; otherwise, the computer will PRINT ``ADULT''. Here's how to say, ``If A is less than 18, print MINOR and print YOUNG; if A is not less than 18, print ADULT and print OLD'': IF A<18 THEN PRINT "MINOR": PRINT "YOUNG" ELSE PRINT "ADULT": PRINT "OLD" Primitive computers don't understand the word ELSE. To find out whether your computer is primitive, check the ``Versions of BASIC'' appendix. DATA . . . READ Let's make the computer print this message: I LOVE MEAT I LOVE POTATOES I LOVE LETTUCE I LOVE TOMATOES I LOVE BUTTER I LOVE CHEESE I LOVE ONIONS I LOVE PEAS That message concerns this list of food: MEAT, POTATOES, LETTUCE, TOMATOES, BUTTER, 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, BUTTER, CHEESE, ONIONS, PEAS. Whenever a problem involves data, put the data at the top of the program, like this: 10 DATA MEAT,POTATOES,LETTUCE,TOMATOES,BUTTER,CHEESE,ONIONS,PEAS You must tell the computer to READ the DATA: 20 READ A$ Line 20 makes the computer read the first datum (MEAT) and call it A$. So A$ is MEAT. Since A$ is MEAT, this line makes the computer print I LOVE MEAT: 30 PRINT "I LOVE ";A$ 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 the rest of the data: tell the computer to GO back to the READ statement. Since the READ statement is line 20, tell the computer to GO TO 20, like this: 40 GO TO 20 Line 40 makes the computer GO back TO line 20, which reads the next datum (POTATOES). Altogether, the program looks like this: 10 DATA MEAT,POTATOES,LETTUCE,TOMATOES,BUTTER,CHEESE,ONIONS,PEAS 20 READ A$ 30 PRINT "I LOVE ";A$ 40 GO TO 20 Lines 20-40 form a loop. Like most loops, the loop's bottom line says GO TO. Since the loop's top line says READ, the loop is called a READ loop. The computer goes round and round the loop. Each time the computer comes to line 20, it reads another datum. The first time it comes to line 20, it reads MEAT; the next time, it reads POTATOES; the next time, it reads LETTUCE; the next time, it reads TOMATOES; etc. Line 30 makes the computer PRINT what it's read. Altogether, the computer will print: I LOVE MEAT I LOVE POTATOES I LOVE LETTUCE I LOVE TOMATOES I LOVE BUTTER I LOVE CHEESE I LOVE ONIONS I LOVE PEAS After the computer prints I LOVE PEAS, it comes to line 40 again, which makes it go back to line 20, so the computer tries to read even more data; but no more data remains! So the computer says: OUT OF DATA Then the computer stops. Most practical computer programs involve data (a list that doesn't change). As a programmer, your job is to notice what the data is and begin your program by saying DATA. After you type the data, write the program's next line, which should say READ. Farther down in your program, you should say GO TO so that you create READ loop. Your program consists of two parts: the DATA and the READ loop. In the DATA statement, you must put quotation marks around any string that contains a comma or colon. For example, if one of the foods is ``HOT, JUICY, BIG, THICK, STEAKS'' (which contains commas), you must put quotation marks around it. Avoiding OUT OF DATA When you run that sample program about food, the last three lines the computer prints are: I LOVE ONIONS I LOVE PEAS OUT OF DATA Instead of saying OUT OF DATA, let's make the computer say ``I LOVE ALL THOSE FOODS'', so that the last three lines look like this: I LOVE ONIONS I LOVE PEAS I LOVE ALL THOSE FOODS Here's how. . . . Underneath the data, say DATA END: 15 DATA END When the computer reads the DATA END, make the computer say I LOVE ALL THOSE FOODS and end: 20 READ A$: IF A$="END" THEN PRINT "I LOVE ALL THOSE FOODS": END The DATA END underneath the data is called the end mark, because it marks the data's end. The routine that says ___ IF A$="END" THEN PRINT "I LOVE ALL THOSE FOODS": END is called the end routine, because the computer does that routine at the end. RESTORE That program prints one copy of the computer's favorite foods. If you want the computer to print many copies, change line 20 to this: 20 READ A$: IF A$="END" THEN PRINT "I LOVE ALL THOSE FOODS": RESTORE: GO TO 20 The word RESTORE tells the computer to go back to the beginning of the data. The computer will print: I LOVE MEAT I LOVE POTATOES I LOVE LETTUCE I LOVE TOMATOES I LOVE BUTTER I LOVE CHEESE I LOVE ONIONS I LOVE PEAS I LOVE ALL THOSE FOODS I LOVE MEAT I LOVE POTATOES I LOVE LETTUCE I LOVE TOMATOES I LOVE BUTTER I LOVE CHEESE I LOVE ONIONS I LOVE PEAS I LOVE ALL THOSE FOODS I LOVE MEAT I LOVE POTATOES etc. The computer will print copies of what it likes again and again, forever, unless you abort the program. Henry the Eighth Let's make the computer print this nursery rhyme: I love ice cream I love red I love ocean I love bed I love tall grass I love to wed I love candles I love divorce I love my kingdom I love my horse I love you Of course, of course, For I am Henry the Eighth! If you own a jump rope, have fun: try to recite that poem while skipping rope! This program makes the computer recite the poem repeatedly: 10 DATA ICE CREAM,RED,OCEAN,BED,TALL GRASS,TO WED 11 DATA CANDLES,DIVORCE,MY KINGDOM,MY HORSE,YOU 15 DATA END 20 READ A$: IF A$="END" THEN PRINT "OF COURSE, OF COURSE,": PRINT "FOR I AM HENR Y THE EIGHTH!": PRINT: RESTORE: GO TO 20 30 PRINT "I LOVE ";A$ 35 IF A$="TO WED" THEN PRINT 40 GO TO 20 Since the data's too long to fit on a single line, I've put part of the data in line 10 and the rest in line 11. Each line of data must begin with the word DATA. In each line, put commas between the items. Do not put a comma at the end of the line. The program resembles the previous one. The only new line is 35, which makes the computer leave a blank line underneath ``TO WED'', to mark the bottom of the first verse. Party Let's throw a party! To make the party yummy, let's ask each guest to bring a kind of food that resembles the guest's name. For example, let's have Sal bring salad, Russ bring Russian dressing, Sue bring soup, Tom bring turkey, Winnie bring wine, Kay bring cake, and Al bring Alka-Seltzer. Let's send all those people invitations, in this form: DEAR _____________ person's name WE'RE THROWING A PARTY AT THE SECRET CLUBHOUSE TOMORROW AT NOON! PLEASE BRING ____ food To make the computer print all the invitations, begin by feeding the computer the DATA: 10 DATA SAL,SALAD,RUSS,RUSSIAN DRESSING,SUE,SOUP,TOM,TURKEY 20 DATA WINNIE,WINE,KAY,CAKE,AL,ALKA-SELTZER The data comes in pairs; the first pair consists of SAL and SALAD. Tell the computer to READ each pair of DATA: 30 READ P$,F$ Line 30 makes the computer read the first pair of data; so P$ is the first person (SAL), and F$ is his food (SALAD). These lines print the letter: 40 PRINT "DEAR ";P$ 50 PRINT " WE'RE THROWING A PARTY AT THE SECRET CLUBHOUSE TOMORROW AT NOON!" 60 PRINT "PLEASE BRING ";F$ At the end of the letter, leave two blank lines, to make room for your signature: 70 PRINT 80 PRINT Then complete the READ loop, by making the computer go back to the beginning of the loop, to read the next pair: 90 GO TO 30 The computer will print a letter to each person, like this: DEAR SAL WE'RE THROWING A PARTY AT THE SECRET CLUBHOUSE TOMORROW AT NOON! PLEASE BRING SALAD DEAR RUSS WE'RE THROWING A PARTY AT THE SECRET CLUBHOUSE TOMORROW AT NOON! PLEASE BRING RUSSIAN DRESSING etc. After printing all the letters, the computer will say: OUT OF DATA Instead of saying OUT OF DATA, let's make the computer say: I'VE FINISHED WRITING THE LETTERS To do that, put END at the end of the data, twice ___ 25 DATA END,END and say what to do when the computer reaches the END: 30 READ P$,F$: IF P$="END" THEN PRINT "I'VE FINISHED WRITING THE LETTERS": END You need two ENDs at the end of the data, because the READ statement says to read two strings (P$ and F$). Debts Suppose these people owe you things: PersonWhat the person owes Bob $537.29 Mike a dime Sue 2 golf balls Harry a steak dinner at Mario's Mommy a kiss Let's remind those people of their debt, by writing them letters, in this form: DEAR _____________ person's name I JUST WANT TO REMIND YOU... THAT YOU STILL OWE ME ____ debt Begin with the DATA: 10 DATA BOB,$537.29,MIKE,A DIME,SUE,2 GOLF BALLS 20 DATA HARRY,A STEAK DINNER AT MARIO'S,MOMMY,A KISS The data comes in pairs; the first pair consists of BOB and $537.29. Tell the computer to READ each pair of DATA: 30 READ P$,D$ Line 30 makes the computer read the first pair of data; so P$ is the first person (BOB), and D$ is his debt ($537.29). Here's the rest of the program: 40 PRINT "DEAR ";P$ 50 PRINT " I JUST WANT TO REMIND YOU..." 50 PRINT "THAT YOU STILL OWE ME ";D$ 70 PRINT 80 PRINT 90 GO TO 30 The computer will print a letter to each person, like this: DEAR BOB I JUST WANT TO REMIND YOU... THAT YOU STILL OWE ME $537.29 DEAR MIKE I JUST WANT TO REMIND YOU... THAT YOU STILL OWE ME A DIME etc. After printing all the letters, the computer will say: OUT OF DATA To prevent the computer from saying OUT OF DATA, add line 25 and retype line 30, as follows: 25 DATA END,END 30 READ P$,D$: IF P$="END" THEN PRINT "I'VE FINISHED WRITING THE LETTERS": END Diets Suppose you're running a diet clinic and get these results: Person Weight beforeWeight after Joe 273 pounds219 pounds Mary 412 pounds371 pounds Bill 241 pounds173 pounds Sam 309 pounds198 pounds Here's how to make the computer print a nice report. . . . Begin by feeding it the DATA: 10 DATA JOE,273,219,MARY,412,371,BILL,241,173,SAM,309,198 The DATA comes in triplets: the first triplet consists of JOE, 273, and 219. Tell the computer to READ each triplet of DATA: 20 READ N$,B,A That line makes the computer read the first triplet of data; so N$ is the first person's name (JOE), B is his weight before (273), and A is his weight after (219). Since B and A stand for numbers instead of strings, they don't have any dollar signs. These lines print the report about him: 30 PRINT N$;" WEIGHED";B;"POUNDS BEFORE ATTENDING THE DIET C LINIC" 40 PRINT "BUT WEIGHED JUST";A;"POUNDS AFTERWARDS" 50 PRINT "THAT'S A LOSS OF";B-A;"POUNDS" At the end of that report about him, leave a blank line: 60 PRINT Then complete the READ loop, by making the computer go back to the loop's beginning: 70 GO TO 20 The computer will print: JOE WEIGHED 273 POUNDS BEFORE ATTENDING THE DIET CLINIC BUT WEIGHED JUST 219 POUNDS AFTERWARDS THAT'S A LOSS OF 54 POUNDS MARY WEIGHED 412 POUNDS BEFORE ATTENDING THE DIET CLINIC BUT WEIGHED JUST 371 POUNDS AFTERWARDS THAT'S A LOSS OF 41 POUNDS etc. At the end the computer will say OUT OF DATA. Instead, let's make it say: COME TO OUR DIET CLINIC! To do that, put END and two zeros at the end of the data ___ 15 DATA END,0,0 and say what to do when the computer reaches the END: 20 READ N$,B,A: IF N$="END" THEN PRINT "COME TO THE DIET CLI NIC!": END You need the two zeros after the END, because the READ statement says to read two numbers (B and A) after the string N$. If you omit the zeros, the computer will say OUT OF DATA. If you hate zeros, you can use other numbers instead; but most programmers prefer zeros. French colors 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: ROUGE Altogether, a run will look like this: RUN WHICH COLOR INTERESTS YOU? RED IN FRENCH, IT'S ROUGE The program begins simply: 10 INPUT "WHICH COLOR INTERESTS YOU";C$ Next, we must make the computer translate the color into French. To do so, feed the computer this English-French dictionary: EnglishFrench white blanc yellowjaune orangeorange red rouge green vert blue bleu brown brun black noir That dictionary becomes the data: 20 DATA WHITE,BLANC,YELLOW,JAUNE,ORANGE,ORANGE,RED,ROUGE 30 DATA GREEN,VERT,BLUE,BLEU,BROWN,BRUN,BLACK,NOIR The data comes in pairs; the first pair consists of WHITE and BLANC. Tell the computer to READ each pair of DATA: 40 READ E$,F$ That line makes the computer read the first pair of data; so E$ is the first English color (WHITE), and F$ is its French equivalent (BLANC). But that pair of data might be the wrong pair. For example, if the human requested RED, the human does not want the pair of WHITE and BLANC; instead, the human wants the pair of RED and ROUGE. Tell the computer that if the human's input (RED) doesn't match the English in the pair, go read another pair: 50 IF E$<>C$ THEN GO TO 40 That line says: if E$ (which is the English in the data) is not C$ (the color the human requested), go to 40 (which reads another pair). Lines 40 and 50 form a loop. The computer goes round and round the loop, until it finds the pair of data that matches the human's request. Since the loop's purpose is to search for matching data, it's called a search loop. After the computer's found the correct English-French pair, make the computer print the French: 60 PRINT "IN FRENCH, IT'S ";F$ Altogether, the program looks like this. . . . Ask the human: 10 INPUT "WHICH COLOR INTERESTS YOU";C$ Use this dictionary: 20 DATA WHITE,BLANC,YELLOW,JAUNE,ORANGE,ORANGE,RED,ROUGE 30 DATA GREEN,VERT,BLUE,BLEU,BROWN,BRUN,BLACK,NOIR Look at the dictionary: 40 READ E$,F$ If not found, try again: 50 IF E$<>C$ THEN GO TO 40 Print the French: 60 PRINT "IN FRENCH, IT'S ";F$ Here's a sample run: RUN WHICH COLOR INTERESTS YOU? RED IN FRENCH, IT'S ROUGE Here's another: RUN WHICH COLOR INTERESTS YOU? BROWN IN FRENCH, IT'S BRUN Here's another: RUN WHICH COLOR INTERESTS YOU? PINK OUT OF DATA The computer says OUT OF DATA because it can't find PINK in the data. Instead of saying OUT OF DATA, let's make the computer say I WASN'T TAUGHT THAT COLOR. To do that, put END at the end of the data; and when the computer reaches the END, make the computer say I WASN'T TAUGHT THAT COLOR: 10 INPUT "WHICH COLOR INTERESTS YOU";C$ 20 DATA WHITE,BLANC,YELLOW,JAUNE,ORANGE,ORANGE,RED,ROUGE 30 DATA GREEN,VERT,BLUE,BLEU,BROWN,BRUN,BLACK,NOIR 35 DATA END,END 40 READ E$,F$: IF E$="END" THEN PRINT "I WASN'T TAUGHT THAT COLOR": END 50 IF E$<>C$ THEN GO TO 40 60 PRINT "IN FRENCH, IT'S ";F$ After line 60, the program just ends. Instead of letting the computer end, let's make it automatically rerun the program and translate another color. To do that, say GO TO and RESTORE: 10 INPUT "WHICH COLOR INTERESTS YOU";C$ 20 DATA WHITE,BLANC,YELLOW,JAUNE,ORANGE,ORANGE,RED,ROUGE 30 DATA GREEN,VERT,BLUE,BLEU,BROWN,BRUN,BLACK,NOIR,END,END 35 DATA END,END 40 READ E$,F$: IF E$="END" THEN PRINT "I WASN'T TAUGHT THAT COLOR": GO TO 70 50 IF E$<>C$ THEN GO TO 40 60 PRINT "IN FRENCH, IT'S ";F$ 70 RESTORE 80 GO TO 10 FOR . . . NEXT Let's make the computer print every number from 1 to 100, like this: 1 2 3 4 5 6 7 etc. 100 To do that, type this line ___ 20 PRINT X and also say that you want X to be every number from 1 to 100, like this: 10 FOR X = 1 TO 100 20 PRINT X Whenever you write a program that contains the word FOR, you must say NEXT. So your program should look like this: 10 FOR X = 1 TO 100 20 PRINT X 30 NEXT X That program works; it makes the computer print every number from 1 to 100. Here's how it works. . . . The computer begins at line 10, which says that you want X to be every number from 1 to 100. So X starts at 1. Then the computer comes to line 20, which says to print X; so the computer prints: 1 Then the computer comes to line 30, which says to do the same thing for the next X, and for the next X, and for the next X; so the computer prints 2, and 3, and 4, and so on, all the way up to 100. The computer prints many numbers, because the computer does line 20 many times (once for each X). The computer does line 20 many times, because line 20 is between the words FOR and NEXT: it's underneath FOR, and above NEXT. The computer repeats anything that's between the words FOR and NEXT. Most programmers get in the habit of indenting everything that comes between the words FOR and NEXT; so in that program, I indented the statement that says PRINT X. To make the indentation, you can hit the SPACE bar repeatedly. In line 30, if you're too lazy to type the X, you can omit it and just type: 30 NEXT When men meet women Let's make the computer print these lyrics: I SAW 2 MEN MEET 2 WOMEN TRA-LA-LA! I SAW 3 MEN MEET 3 WOMEN TRA-LA-LA! I SAW 4 MEN MEET 4 WOMEN TRA-LA-LA! I SAW 5 MEN MEET 5 WOMEN TRA-LA-LA! THEY ALL HAD A PARTY! HA-HA-HA! To do that, type these lines ___ The first line of each verse: 20 PRINT "I SAW";X;"MEN" The second line of each verse: 30 PRINT "MEET";X;"WOMEN" The third line of each verse: 40 PRINT "TRA-LA-LA!" Blank line under each verse: 50 PRINT and make X be every number from 2 up to 5: 10 FOR X = 2 TO 5 20 PRINT "I SAW";X;"MEN" 30 PRINT "MEET";X;"WOMEN" 40 PRINT "TRA-LA-LA!" 50 PRINT 60 NEXT At the end of the song, print the closing couplet: 10 FOR X = 2 TO 5 20 PRINT "I SAW";X;"MEN" 30 PRINT "MEET";X;"WOMEN" 40 PRINT "TRA-LA-LA!" 50 PRINT 60 NEXT 70 PRINT "THEY ALL HAD A PARTY!" 80 PRINT "HA-HA-HA!" That program makes the computer print the entire song. Here's an analysis: 10 FOR X = 2 TO 5 The computer will do the 20 PRINT "I SAW";X;"MEN" indented lines repeatedly, 30 PRINT "MEET";X;"WOMEN" for X=2, X=3, X=4, and X=5. 40 PRINT "TRA-LA-LA!" 50 PRINT 60 NEXT Then the computer will 70 PRINT "THEY ALL HAD A PARTY!" print this couplet once. 80 PRINT "HA-HA-HA!" Since the computer does lines 20-50 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: 10 FOR I = 2 TO 5 20 PRINT "I SAW";I;"MEN" 30 PRINT "MEET";I;"WOMEN" 40 PRINT "TRA-LA-LA!" 50 PRINT 60 NEXT 70 PRINT "THEY ALL HAD A PARTY!" 80 PRINT "HA-HA-HA!" 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. 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 100, like this: THE SQUARE OF 3 IS 9 THE SQUARE OF 4 IS 16 THE SQUARE OF 5 IS 25 THE SQUARE OF 6 IS 36 THE SQUARE OF 7 IS 49 etc. THE SQUARE OF 100 IS 10000 To do that, type this line ___ 20 PRINT "THE SQUARE OF";I;"IS";I*I and make I be every number from 3 up to 100, like this: 10 FOR I = 3 TO 100 20 PRINT "THE SQUARE OF";I;"IS";I*I 30 NEXT Secret meeting This program prints 12 copies of the same message: 10 FOR I = 1 TO 12 20 PRINT "HUSH, HUSH!" 30 PRINT " WE'RE HAVING A SECRET MEETING..." 40 PRINT " IN THE COMPUTER ROOM..." 50 PRINT " TONIGHT..." 60 PRINT " AT 2 A.M." 70 PRINT " WEAR A FUNNY HAT." 80 PRINT 90 PRINT 100 NEXT Lines 80 and 90 leave blank lines at the end of each copy for your signature. Midnight This program makes the computer count to midnight: 10 FOR I = 1 TO 11 20 PRINT I 30 NEXT 40 PRINT "MIDNIGHT" The computer will print: 1 2 3 4 5 6 7 8 9 10 11 MIDNIGHT Let's put a semicolon at the end of line 20: 10 FOR I = 1 TO 11 20 PRINT I; 30 NEXT 40 PRINT "MIDNIGHT" The semicolon makes the computer print each item on the same line, like this: 1 2 3 4 5 6 7 8 9 10 11 MIDNIGHT If you want the computer to press the ENTER key before MIDNIGHT, insert a PRINT line: 10 FOR I = 1 TO 11 20 PRINT I; 30 NEXT 35 PRINT 40 PRINT "MIDNIGHT" Line 35 makes the computer press the ENTER key just before MIDNIGHT, so the computer will print MIDNIGHT on a separate line, like this: 1 2 3 4 5 6 7 8 9 10 11 MIDNIGHT In line 20, the semicolon means: do not press the ENTER key after I. Line 35 means: do press the ENTER key. So line 35 undoes line 20 and makes the computer press the ENTER key before MIDNIGHT. Let's make the computer count to midnight 3 times, like this: 1 2 3 4 5 6 7 8 9 10 11 MIDNIGHT 1 2 3 4 5 6 7 8 9 10 11 MIDNIGHT 1 2 3 4 5 6 7 8 9 10 11 MIDNIGHT To do that, put the entire program between the words FOR and NEXT: 5 FOR A = 1 TO 3 10 FOR I = 1 TO 11 20 PRINT I; 30 NEXT 35 PRINT 40 PRINT "MIDNIGHT" 50 NEXT That version contains a loop inside a loop: the loop that says ``FOR I'' is inside the loop that says ``FOR A''. The A 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 A 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. Favorite color This program plays a guessing game: 10 PRINT "I'LL GIVE YOU FIVE GUESSES...." 20 FOR I = 1 TO 5 30 INPUT "WHAT'S MY FAVORITE COLOR";G$ 40 IF G$="PINK" THEN GO TO 100 50 PRINT "NO." 60 NEXT 70 PRINT "SORRY, YOUR FIVE GUESSES ARE UP! YOU LOSE." 80 END 100 PRINT "CONGRATULATIONS! YOU DISCOVERED MY FAVORITE COLOR." 110 PRINT "IT TOOK YOU";I;"GUESSES". Line 10 warns the human that only five guesses are allowed. Line 20 makes the computer count from 1 to 5; to begin, I is 1. Line 30 asks the human to guess the computer's favorite color; the guess is called G$. If the guess is PINK, the computer jumps from line 40 to line 100, prints CONGRATULATIONS, and tells how many guesses the human took. But if the guess is not PINK, the computer proceeds from line 40 to line 50, prints NO, and goes on to the next guess. If the human guesses five times without success, the computer proceeds from line 60 to line 70 and prints SORRY . . . YOU LOSE. For example, if the human's third guess is PINK, the computer prints: CONGRATULATIONS! YOU DISCOVERED MY FAVORITE COLOR. IT TOOK YOU 3 GUESSES. If the human's very first guess is PINK, the computer prints: CONGRATULATIONS! YOU DISCOVERED MY FAVORITE COLOR. IT TOOK YOU 1 GUESSES. Saying ``1 GUESSES'' is bad grammar but understandable. Lines 20-60 form a loop. Line 20 says the loop will normally be done five times. The line after the loop, line 70, is the loop's normal exit. But if the human happens to input PINK, the computer jumps out of the loop early, to line 100, which is the loop's abnormal exit. Finite pause Have you ever met someone who acts romantic, then suddenly says something cruel that breaks the romance? Let's make the computer act that way. For example, let's make the computer begin by crooning a romantic message: 10 PRINT "YOUR LUSCIOUS LIPS SMELL LIKE THE FINEST WINE" Then let's make the computer pause, to give the human a chance to admire that romantic message and make the human wait in suspense for the next outburst of computer emotion. Here's how to make the computer pause: 20 FOR I = 1 TO 7000: NEXT That line makes the computer pause, while it counts up to 7000. That line makes the computer mumble to itself, ``I is 1, I is 2, I is 3, I is 4, . . . '' without printing anything on the screen. While the computer is secretly and silently mumbling to itself up to 7000, the human has a chance to read and admire the romantic message printed by line 10. Finally, let's make the computer analyze the meaning of lips smelling like wine: 30 PRINT "YOU MUST BE DRUNK" So altogether, that program makes the computer print YOUR LUSCIOUS LIPS SMELL LIKE THE FINEST WINE, then pause, then say YOU MUST BE DRUNK. In that program, line 20 makes the computer pause, by making the computer silently mumble up to 7000. The typical computer mumbles 1000 times per second. Since line 20 says to mumble 7000 times, that line takes 7 seconds altogether, and so it makes the computer pause for 7 seconds. If you want the computer to pause for 12 seconds instead of 7, retype line 20 so it says to mumble 12000 times instead of 7000. Although the typical computer mumbles 1000 times per second, your computer might mumble slightly faster or slower than that. Try the program on your computer, and notice how many seconds your computer pauses when doing 7000 mumbles. If your computer's typical, it pauses 7 seconds; if your computer mumbles quickly, it pauses less than 7 seconds; if your computer mumbles slowly, it pauses more than 7 seconds. Warning: your computer mumbles more slowly in long programs than in short ones. This program makes the computer print a famous joke: 10 PRINT "YOUR TEETH ARE LIKE STARS" 20 FOR I = 1 TO 7000: NEXT 30 PRINT "THEY COME OUT AT NIGHT" Line 30 is the punch line. Line 20 makes the computer pause 7 seconds, before giving the punch line. Experiment: invent your own joke, and make the computer pause before printing the punch line. Eye test This program makes the computer test how fast you can read: 10 CLS 20 PRINT "IF YOU CAN READ THIS, YOU READ QUICKLY" 30 FOR I = 1 TO 250: NEXT 40 CLS Line 10 makes the computer clear the screen, so that the entire screen becomes blank. Line 20 makes the computer print: IF YOU CAN READ THIS, YOU READ QUICKLY Line 30 makes the computer pause for a quarter of a second (since 250 is a quarter of 1000). While the computer pauses, the message stays on the screen. Line 40 erases the screen, so the message stayed on the screen for just a quarter of a second. If you could read the message in a quarter of a second, you have very quick eyes! I can't read that quickly. Can you? Try it! To make the eye test easier, allow yourself longer than a quarter of a second to read the message, by changing the 250 to a larger number. Get together with your friends, and see how quickly they can read. Flash different messages on the screen by changing line 20. For example, try changing line 20 to this: 20 PRINT "MUMBLING MORONS MAKE MY MOM MISS MURDER MYSTERIES MONDAY MORNING" If your friends can read all that in a quarter of a second, they probably belong in the Guinness Book of World Records. To prevent your friends from cheating, make them close their eyes while you're typing the program. When you've finished typing the program, press the CLEAR key, so that the program becomes invisible. Then tell your friends to open their eyes and type RUN. STEP The FOR statement can be varied: Statement Meaning FOR I = 5 TO 17 STEP .1I will go from 5 to 17, counting by tenths. So I will be 5, then 5.1, then 5.2, etc., up to 17. FOR I = 5 TO 17 STEP 3I will be every third number from 5 to 17. So I will be 5, then 8, then 11, then 14, then 17. FOR I = 17 TO 5 STEP -3I will be every third number from 17 down to 5. So I will be 17, then 14, then 11, then 8, then 5. To count down, you must use the word STEP. To count from 17 down to 5, give this instruction: FOR I = 17 TO 5 STEP -1 This program prints a rocket countdown: 10 FOR I = 10 TO 1 STEP -1 20 PRINT I 30 NEXT 40 PRINT "BLAST OFF!" The computer will print: 10 9 8 7 6 5 4 3 2 1 BLAST OFF! This statement is tricky: FOR I = 5 TO 16 STEP 3 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. 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. VARIABLES & CONSTANTS A numeric constant is a simple number, such as: 0 1 2 8 43.7 -524.6 .003 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 contain any arithmetic. For example, since 7+1 contains 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: "I LOVE YOU" "76 TROMBONES" "GO AWAY!!!" "XYPW EXR///746" A constant is a numeric constant or a string constant: 0 8 -524.6 1.3E5 "I LOVE YOU" "XYPW EXR///746" 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: A$ B$ Y$ Z$ If the variable stands for a number, it's called a numeric variable and lacks a dollar sign, like this: A B Y Z So all these are variables: A$ B$ Y$ Z$ A B Y Z Expressions A numeric expression is a numeric constant (such as 8) or a numeric variable (such as A) 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$) or a combination. An expression is a numeric expression or a string expression. Statements At the end of a GO TO statement, the line number must be a numeric constant. Right:50 GO TO 100 (100 is a numeric constant.) Wrong:50 GO TO N (N is not a numeric constant.) The INPUT statement's prompt must be a string constant: Right:10 INPUT "WHAT IS YOUR NAME";N$(``WHAT IS YOUR NAME'' is a constant.) Wrong:10 INPUT Q$;N$ (Q$ is not a constant.) In a DATA statement, you must have constants. Right:10 DATA 8, 1.3E5 (8 and 1.3E5 are constants.) Wrong:10 DATA 7+1, 1.3E5(7+1 is not a constant.) In the DATA statement, if the constant is a string, you can omit the quotation marks (unless the string contains a comma or a colon). Right:10 DATA "JOE","MARY" Also right:10 DATA JOE,MARY Here are the forms of the most popular BASIC statements: General form Example PRINT list of expressions PRINT "THE TEMPERATURE IS";4+25;"DEGREES" GO TO numeric constant GO TO 10 END END STOP STOP variable = expression X = 47+2 INPUT string constant ; variable INPUT "WHAT IS YOUR NAME";N$ IF condition THEN list of statements IF A>=18 THEN PRINT "YOU": PRINT "VOTE" DATA list of constants DATA JOE,273,219,MARY,412,371 READ list of variables READ N$,B,A RESTORE RESTORE FOR numeric variable = FOR I = 59+1 TO 100+N STEP 2+3 numeric expression TO numeric expression STEP numeric expression NEXT numeric variable NEXT I LOOP TECHNIQUES Here's a strange program: 10 A=5 20 A=3+A 30 PRINT A Line 20 means: the new A is 3 plus the old A. So the new A is 3+5, which is 8. Line 30 prints: 8 Let's look at that program more closely. Line 10 puts 5 into box A: When the computer sees line 20, it examines the equation's right side and sees the 3+A. Since A is 5, the 3+A is 3+5, which is 8. So line 20 says: A=8. The computer puts 8 into box A: Line 30 prints 8. Here's another weirdo: 10 B=6 20 B=B+1 30 PRINT B*2 Line 20 says the new B is ``the old B plus 1''. So the new B is 6+1, which is 7. Line 30 prints: 14 In that program, line 10 says B is 6; but line 20 increases B, by adding 1 to B; so B becomes 7. Programmers say that B has been increased or incremented. In line 20, the ``1'' is called the increase or the increment. The opposite of ``increment'' is decrement: 10 J=500 20 J=J-1 30 PRINT J Line 10 says J starts at 500. But line 20 says the new J is ``the old J minus 1'', so the new J is 500-1, which is 499. Line 30 prints: 499 In that program, J was decreased (or decremented). In line 20, the ``1'' is called the decrease (or decrement). Counting Suppose you want the computer to count, starting at 3, like this: 3 4 5 6 7 8 etc. This program does it, by a special technique: 10 C=3 20 PRINT C 30 C=C+1 40 GO TO 20 In that program, C is called the counter, because it helps the computer count. Line 10 says C starts at 3. Line 20 makes the computer print C, so the computer prints: 3 Line 30 increases C by adding 1 to it, so C becomes 4. Line 40 sends the computer back to line 20, which prints the new value of C: 4 Then the computer comes to line 30 again, which increases C again so C becomes 5. Line 40 sends the computer back to line 20 again, which prints: 5 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. General procedure Here's the general procedure for making the computer count: 1. Start C at some value (such as 3). 2. Use C. (For example, tell the computer to PRINT C.) 3. Increase C, by saying C=C+1. 4. GO back TO step 2. Variations To read the printing more easily, put a semicolon at the end of the PRINT statement: 10 C=3 20 PRINT C; 30 C=C+1 40 GO TO 20 The semicolon makes the computer print horizontally: 3 4 5 6 7 8 etc. This program makes the computer count, starting at 1: 10 C=1 20 PRINT C; 30 C=C+1 40 GO TO 20 The computer will print 1, 2, 3, 4, etc. This program makes the computer count, starting at 0: 10 C=0 20 PRINT C; 30 C=C+1 40 GO TO 20 The computer will print 0, 1, 2, 3, 4, etc. Quiz Let's make the computer give this quiz: What's the capital of Nevada? What's the chemical symbol for iron? What word means 'brother or sister'? What was Beethoven's first name? How many cups are in a quart? To make the computer score the quiz, we must tell it the correct answers, which are: Carson City Fe sibling Ludwig 4 So the program contains this data: 10 DATA WHAT'S THE CAPITAL OF NEVADA,CARSON CITY 20 DATA WHAT'S THE CHEMICAL SYMBOL FOR IRON,FE 30 DATA WHAT WORD MEANS 'BROTHER OR SISTER',SIBLING 40 DATA WHAT WAS BEETHOVEN'S FIRST NAME,LUDWIG 50 DATA HOW MANY CUPS ARE IN A QUART,4 Tell the computer to READ the data: 100 READ Q$,A$ That line reads a pair of data: it reads a question (Q$) and the correct answer (A$). Make the computer ask the question and wait for the human's response: 110 PRINT Q$; 120 INPUT "??";H$ Line 110 prints the question. Line 120 prints question marks after the question, and waits for the human to respond; the human's response is called H$. Finally, evaluate the human's response. If the human's response (H$) is the correct answer (A$), make the computer say ``CORRECT'' and GO TO the next question: 130 IF H$=A$ THEN PRINT "CORRECT": GO TO 100 But if the human's response is wrong, make the computer say ``NO'' and reveal the correct answer: 140 PRINT "NO, THE ANSWER IS: ";A$: GO TO 100 Here's a sample run: RUN WHAT'S THE CAPITAL OF NEVADA??? LAS VEGAS NO, THE ANSWER IS: CARSON CITY WHAT'S THE CHEMICAL SYMBOL FOR IRON??? FE CORRECT WHAT WORD MEANS 'BROTHER OR SISTER'??? I GIVE UP NO, THE ANSWER IS: SIBLING WHAT WAS BEETHOVEN'S FIRST NAME??? LUDVIG NO, THE ANSWER IS: LUDWIG HOW MANY CUPS ARE IN A QUART??? 4 CORRECT OUT OF DATA To give a quiz about different topics, change the data in lines 10-50. Avoid OUT OF DATA Instead of making the computer say OUT OF DATA, let's make it say: I HOPE YOU ENJOYED THE QUIZ To do that, write an end mark and an end routine: 60 DATA END,END 100 READ Q$,A$: IF Q$="END" THEN PRINT "I HOPE YOU ENJOYED T HE QUIZ": END Count the correct answers 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: 5 C=0 10 DATA WHAT'S THE CAPITAL OF NEVADA,CARSON CITY 20 DATA WHAT'S THE CHEMICAL SYMBOL FOR IRON,FE 30 DATA WHAT WORD MEANS 'BROTHER OR SISTER',SIBLING 40 DATA WHAT WAS BEETHOVEN'S FIRST NAME,LUDWIG 50 DATA HOW MANY CUPS ARE IN A QUART,4 60 DATA END,END 100 READ Q$,A$: IF Q$="END" THEN PRINT "YOU ANSWERED";C;"OF THE QUESTIONS CORRECTLY": PRINT "I HOPE YOU ENJOYED THE QUIZ ": END 110 PRINT Q$; 120 INPUT "??";H$ 130 IF H$=A$ THEN PRINT "CORRECT": C=C+1: GO TO 100 140 PRINT "NO, THE ANSWER IS: ";A$: GO TO 100 At the beginning of the program, the human hasn't answered any questions correctly yet, so line 5 begins the counter at 0. Each time the human answers a question correctly, line 130 increases the counter. When the program ends, line 100 prints the counter, by printing a message such as: YOU ANSWERED 2 OF THE QUESTIONS CORRECTLY It would be nicer to print ___ YOU ANSWERED 2 OF THE 5 QUESTIONS CORRECTLY YOUR SCORE IS 40 % or, if the quiz were changed to include 8 questions: YOU ANSWERED 2 OF THE 8 QUESTIONS CORRECTLY YOUR SCORE IS 25 % To make the computer print such a message, we must make the computer count how many questions were asked. So we need another counter. Since we already used C to count the number of correct answers, let's use Q to count the number of questions asked. Like C, Q must start at 0; and we must increase Q, by adding 1 each time another question is asked: 5 C=0: Q=0 10 DATA WHAT'S THE CAPITAL OF NEVADA,CARSON CITY 20 DATA WHAT'S THE CHEMICAL SYMBOL FOR IRON,FE 30 DATA WHAT WORD MEANS 'BROTHER OR SISTER',SIBLING 40 DATA WHAT WAS BEETHOVEN'S FIRST NAME,LUDWIG 50 DATA HOW MANY CUPS ARE IN A QUART,4 60 DATA END,END 100 READ Q$,A$: IF Q$="END" THEN PRINT "YOU ANSWERED";C;"OF THE";Q;"QUESTIONS CORRECTLY": PRINT "YOUR SCORE IS";C/Q*100; "%": PRINT "I HOPE YOU ENJOYED THE QUIZ": END 110 PRINT Q$; 115 Q=Q+1 120 INPUT "??";H$ 130 IF H$=A$ THEN PRINT "CORRECT": C=C+1: GO TO 100 140 PRINT "NO, THE ANSWER IS: ";A$: GO TO 100 Summing Let's make the computer imitate an adding machine, so a run looks like this: RUN NOW THE SUM IS 0 WHAT NUMBER DO YOU WANT TO ADD TO THE SUM? 5 NOW THE SUM IS 5 WHAT NUMBER DO YOU WANT TO ADD TO THE SUM? 3 NOW THE SUM IS 8 WHAT NUMBER DO YOU WANT TO ADD TO THE SUM? 6.1 NOW THE SUM IS 14.1 WHAT NUMBER DO YOU WANT TO ADD TO THE SUM? -10 NOW THE SUM IS 4.1 etc. Here's the program: 10 S=0 20 PRINT "NOW THE SUM IS";S 30 INPUT "WHAT NUMBER DO YOU WANT TO ADD TO THE SUM";X 40 S=S+X 50 GO TO 20 Line 10 starts the sum at 0. Line 20 prints the sum. Line 30 asks the human what number to add to the sum; the human's number is called X. Line 40 adds X to the sum, so the sum changes. Line 50 makes the computer go to line 20, which prints the new sum. Lines 20-50 form an infinite loop, which you must abort. Here's the general procedure for making the computer find a sum: 1. Start S at 0. 2. Use S. (For example, tell the computer to PRINT S.) 3. Find out what number to add to S. (For example, let the human input an X.) 4. Increase S, by saying S = S + the number to be added. 5. GO back TO step 2. Checking account If your bank's nasty, it charges you 10� to process each good check that you write, and a $5 penalty for each check that bounces; and it pays no interest on the money you've deposited. This program makes the computer imitate such a bank. . . . Start the sum at 0:10 S=0 Chat with the human:20 PRINT "YOUR CHECKING ACCOUNT CONTAINS";S 30 INPUT "DEPOSIT OR WITHDRAW";A$ 40 IF A$="DEPOSIT" THEN GO TO 100 50 IF A$="WITHDRAW" THEN GO TO 200 60 PRINT "PLEASE SAY DEPOSIT OR WITHDRAW": GO TO 30 Deposit some money:100 INPUT "HOW MUCH DO YOU WANT TO DEPOSIT";D 110 S=S+D 120 GO TO 20 Withdraw some money:200 INPUT "HOW MUCH DO YOU WANT TO WITHDRAW";W 210 W=W+.10 220 IF W<=S THEN PRINT "OKAY": S=S-W: GO TO 20 230 PRINT "THAT CHECK BOUNCED": S=S-5: GO TO 20 In that program, the total amount of money in the checking account is called the sum. Line 10 starts that sum at 0. Line 20 prints the sum. Line 30 asks whether the human wants to deposit or withdraw. If the human says DEPOSIT, the computer goes from line 40 to line 100 (which asks how much to deposit), then to line 110 (which adds the deposit to the sum in the account), then back to line 20 (for the next transaction). But if the human says WITHDRAW instead of DEPOSIT, the computer goes from line 50 to line 200 (which asks how much to withdraw), then to line 210 (which adds the 10� service charge to the withdrawal amount). Then the computer reaches line 220, which checks whether the sum S in the account is large enough to cover the withdrawal (W). If W<=S, the computer says OKAY and processes the check, by subtracting W from the sum in the account. If W>S instead, the computer says THAT CHECK BOUNCED and decreases the sum in the account by the $5 penalty. How the program is nasty That program is nasty to customers. For example, suppose you have $1 in your account, and you try to write a check for 95�. Since 95� + the 10� service charge = $1.05, which is more than you have in your account, your check will bounce, and you'll be penalized $5. That makes your balance will become negative $4, and the bank will demand that you pay the bank $4 ___ just because you wrote a check for 95�! Another nuisance is when you leave town permanently and want to close your account. If your account contains $1, you can't get your dollar back! The most you can withdraw is 90�, because 90� + the 10� service charge = $1. That nasty program makes customers hate the bank ___ and hate the computer! How to stop the nastiness The bank should make the program friendlier. Here's how. To stop accusing the customer of owing money, the bank should change any negative sum to 0: 15 IF S<0 THEN S=0 To make sure the computer goes to that line, the bank's program should say GO TO 15 instead of GO TO 20; so the bank should change line 230 to this: 230 PRINT "THAT CHECK BOUNCED": S=S-5: GO TO 15 Also, to be friendly, the bank should ignore the 10� service charge when deciding whether a check will clear. So the bank should eliminate line 210. On the other hand, if the check does clear, the bank should impose the 10� service charge afterwards, like this: 220 IF W<=S THEN PRINT "OKAY": S=S-W-.10: GO TO 15 So if the bank is kind, it will insert line 15, use the new version of line 230, eliminate line 210, and use the new version of line 220. 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; $5 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. Start the Sum at 0: 10 S=0 Make I go from 7 to 100:20 FOR I = 7 TO 100 Increase the Sum, by adding each I to it:30 S=S+I 40 NEXT Print the final Sum (which is 5029):50 PRINT S 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: 10 S=0 20 FOR I = 7 TO 100 30 S=S+I*I 40 NEXT 50 PRINT S It's the same as the previous program, except that line 30 says to add I*I instead of I. Line 50 prints the final sum, which is 338259. Data sums This program adds together the numbers in the data: 10 S=0 20 DATA 5, 3, 6.1, etc. 30 DATA etc. 40 DATA etc. 50 DATA 0 60 READ X: IF X=0 THEN PRINT S: END 70 S=S+X 80 GO TO 60 Line 10 starts the sum at 0. Lines 20-40 contain the numbers to be added. The zero in line 50 is an end mark. Line 60 reads an X from the data. If X=0, the end of the data's been reached, so we want the computer to print the sum (S) and end. But if the X it reads is not zero, the computer proceeds from line 60 to line 70, adds X to the sum, and goes from line 80 to line 60, which reads another X.