Fun

C is a computer language invented by Dennis Ritchie in 1972, while he was working for AT&T at Bell Labs. He called it "C" because it came after "B", which was an earlier language developed by a colleague.

Earlier chapters of The Secret Guide to Computers explained how to program in BASIC and PASCAL. C resembles those languages but has two advantages: C runs faster and consumes less RAM.

C has become the most popular language for creating advanced programs. The worldís largest software companies have switched to C from assembly language:

Created using assembly lang. Created using C

DBASE 2 DBASE 3, 3+, 4, and 5

Word Perfect 4.0, 4.1, and 4.2 Word Perfect 5, 5.1, 6, 6.1, and 7

1-2-3 release 1, 1A, 2, 2.01, 2.2, 2.3, 2.4 1-2-3 release 3, 3.1, 3.2, 3.3, and 3.4

If you become an expert C programmer, you can help run those rich software companies and get rich yourself!

Before studying C, study BASIC and PASCAL, which are similar but easier.

Unlike BASIC and PASCAL, C lets you easily create a pointer, which is a note about which part of RAM to use. If you create the pointer incorrectly, C will use the wrong part of RAM ó and erase whatever information had been there before. For example, C might erase the part of RAM used by DOS, so that DOS becomes confused and accidentally erases your disks!

A faulty pointer (which points to the wrong part of RAM) is called a runaway pointer, and itís a C programmerís greatest fear. Even if your innocent-looking program doesnít seem to mention pointers, a small error in your program might make C create a pointer that wrecks your computer. Thatís why many C programmers look thin and haggard and bite their nails. To keep your nails looking pretty, make backup copies of your floppy and hard disks before trying to program in C.

C is like a sports car with no brakes: itís fast, fun, slim, sleek, and dangerous. If you program in C, your friends will admire you and even whistle at you as you zoom along the freeway of computer heaven; but if youíre not careful, your programs and disks will crash, and so will your career!

Versions of C and C++

This chapter explains two famous versions of C: Microsoftís Quick C (version 2) and Borlandís Turbo C (version 2).

Newer versions Recently, newer versions of C have been invented. Unfortunately, the newer versions are more demanding. For example, they demand that you put this line at the top of every program ó

#include <stdio.h>

and they demand that you say "void main" instead of just "main".

C++ Recently, programmers have been switching to a variant of C called C++. Most commands are the same in C++ as in C, but these commands are different:

Old-fashioned C command How to say that in C++

#include <stdio.h> #include <iostream.h>

main(){ void main(){

printf("we love"); cout<<"we love";

printf("we love\n"); cout<<"we love\n";

printf("eat %d nuts", 5+2); cout<<"eat "<<5+2<<" nuts"

puts("we love"); cout<<"we love\n";

putchar(grade) cout<<grade;

scanf("%d",&age); cin>>age;

gets(name); cin>>grade;

/* we love */ // we love

char *name; name=(char *)malloc(41); char name[41];

Besides the commands in that list, C++ handles several fancy commands differently. Also, C++ lets you use an advanced technique called object-oriented programming (OOP), in which you define "objects" and give those objects "properties".

Combo boxes Now when you try to buy C from Microsoft or Borland, you get a combo box that includes C and also C++.

Microsoftís combo box is called Visual C++; it requires Windows and costs $90. Borlandís combo box is called Turbo C++; choose either the DOS version ($92) or the Windows version ($73). Those prices are from a discount dealer (Computer Discount Warehouse at 800-726-4239).

When using one those combo boxes, your programís top line tells the computer whether you want to use C or C++:

for C, make your programís top line say: #include <stdio.h>

for C++, make your programís top line say: #include <iostream.h>

What you should do Future editions of The Secret Guide to Computers will contain more details about C++ and the combo boxes. To get an announcement about future editions, use the coupon on this bookís back page or phone me at 617-666-2666.

In the meantime, have fun reading this explanation of the famous classic C versions: Microsoft Quick C version 2 and Borlandís Turbo C version 2.

Copy to the hard disk

Turbo C and Quick C come on floppy disks, which you should copy to your hard disk. Hereís how.

Turn on the computer without any floppy in drive A. As I explained in the MS-DOS chapter, make sure your hard disk contains a CONFIG.SYS file saying "files=20" (or a bigger number) and "buffers=10" (or a bigger number).

Hereís what to do next.

Turbo C Turbo C comes on six 5ľ-inch floppy disks. When you see the C prompt, put the Turbo C Install/Help Disk in drive A and type "a:". The computer displays an A prompt.

Type "install". The computer says "Turbo C Installation Utility".

Press ENTER three times. The computer says "Turbo C Directory". Press the F9 key.

When the computer tells you, put the other Turbo C disks in drive A and press ENTER.

The computer says "Turbo C is now installed on your system." Ignore the computerís comments about PATH. Press ENTER.

Turn off the computer, so you can start fresh.

Quick C Quick C comes on ten 5ľ-inch floppy disks. Put the Quick C Setup Disk in drive A and type "a:". The computer displays an A prompt.

Type "setup". The computer says "Microsoft Quick C Setup Program". Press ENTER ten times.

The computer asks, "Include in combined libraries?" The computer is asking whether youíre planning to write programs that include graphics. Most C programs do not use graphics; most C courses do not discuss graphics; this chapter does not discuss graphics. Chapter 12 of the Quick C instruction manual does discuss graphics. If you plan to study and use graphics, press Y and ENTER, then Y again and ENTER (which makes the computer copy graphics commands to the hard disk); otherwise, press ENTER twice (so the computer doesnít bother copying graphics commands to the hard disk, and your hard disk wonít be so full).

The computer asks, "Do you want to change any of the above options?" Press N and ENTER.

The computer asks, "Install Microsoft Mouse?" If you have a Microsoft Mouse (or clone), press ENTER; if you have no mouse or a different brand, press N then ENTER.

Press ENTER four more times. The computer asks again, "Do you want to change any of the above options?" Press N and ENTER.

Press ENTER eight more times. The computer asks again, "Do you want to change any of the above options?" Press N and ENTER.

Press ENTER again. When the computer tells you, put other Quick C disks in drive A press ENTER.

The computer says, "Press any key to start building combined libraries." Press ENTER twice. Ignore the computerís comments about "creating sample configuration files".

Youíll see an A prompt. Type "copy con c:\qc2\qc2.bat" (and press ENTER at the end of that command). Underneath that command, type this:

set lib=\qc2\lib

set include=\qc2\include

cd bin

qc

cd \

Underneath all that, press the F6 key then ENTER.

Turn off the computer, so you can start fresh.

Start C

To start using C, turn on the computer without any floppy in drive A.

To do Turbo C, type "do tc". To do Quick C, type "do qc2".

(That "do" method works if you put the DO.BAT file onto your hard disk as I recommended in the MS-DOS chapter. If you have not put DO.BAT onto your hard disk, do Turbo C by typing "cd tc" and then "tc"; do Quick C by typing "cd qc2" and then "qc2".)

If youíre using Turbo C, then press F10.

Type your program

For example, type this C program:

main(){

puts("make your nose");

puts("touch your toes");

}

The program begins by saying "main()", which means: here comes the main program.

Do not capitalize the word "main". If you type "MAIN" instead of "main", the computer will gripe. In C, you must type all commands by using lower-case letters, not capitals.

After the "main()", the rest of the program is enclosed in braces: {}. Like PASCALís "BEGIN" and "END", they mark the beginning and end of the program.

The lines between the braces are indented. To indent, tap the TAB key. The indentation is optional but helps other programmers understand your program.

Once youíve indented a line, the computer automatically indents all the lines underneath it. (If the computer automatically indents a line that you donít want to indent, press the BACKSPACE key.)

The indented lines tell the computer to print two strings: "make your nose" and "touch your toes".

Notice that C says "puts" instead of "print". (BASIC says PRINT; PASCAL says WRITELN; C says "puts" instead.) The command "puts" means: put the string onto your screen.

After "puts", you must put parentheses. (PASCALís WRITELN requires parentheses also.)

Like BASIC, C makes you put each string in quotation marks.

Like PASCAL, C requires a semicolon at the end of each typical line. Hereís the rule: each line of C should end with a semicolon or brace.

Run the program

When you finish typing the program, tell the computer to run it. Hereís how: for Turbo C, press F10 then type RR; for Quick C, press F5 and if the computer asks "Rebuild?" press ENTER.

If you typed the program wrong, the computer will say ERRORS. (If youíre using Turbo C, then press ENTER and F6.) Correct the error, then tell the computer to run the program again.

When the computer runs the program correctly, the computer prints:

make your nose

touch your toes

(If youíre using Turbo C, that printing flashes on the screen too briefly for you to read; after the flash, press Alt with F5, which makes the printing reappear.)

After the computerís run the program successfully and youíve read what the computer printed, press ENTER.

Manipulate your program

Hereís how to manipulate your program.

Turbo C Turbo C lets you manipulate your program in the same way as Turbo PASCAL 5.5. For details, read the explanation of Turbo PASCAL 5.5 on pages 467-468. On those pages, read the sections entitled "Advanced editing", "Save your program", "Switch programs", and "Exit".

Quick C Quick C lets you manipulate your program in the same way as Quick PASCAL. For details, read the explanation of Quick PASCAL on pages 467-468. On those pages, read just the sections entitled "Advanced editing" and "Save your program".

Hereís how to do further manipulations in Quick C.Ö

Save your program. To copy your program to the hard disk, press the Alt key then say "File Save" (by typing FS). If the computer says "File Name", type a name for the program (such as NOSE) and press ENTER. If you called the program "NOSE", the computer will put the program onto the hard disk as "NOSE.C". (It will be in the BIN subdirectory of the QC2 subdirectory.)

Erase the screen. To erase the screen so you can start writing a new program, press the Alt key then say "File New" (by typing FN). If the computer asks "Do you want to save?", press N.

Retrieve a program. To retrieve a saved program, press the Alt key then say "File Open" (by typing FO). Type the programís name (such as NOSE) and press ENTER. If the computer asks "Do you want to save?", press N.

Exit. When you finish using Quick C, hereís how to return to the DOS prompt. Press the Alt key. Say "File eXit" (by typing FX). If the computer asks "Do you want to save?", press N.

Many versions

Suppose you write a program, save it as "NOSE", and then run it. Your hard disk will contain several versions of NOSE.

The first version, NOSE.C, is the program you typed. Turbo C puts NOSE.C in the TC subdirectory; Quick C puts NOSE.C in the BIN subdirectory of the QC2 subdirectory.

When you say to run, the computer automatically creates and runs NOSE.EXE, which is the Executable machine-language version of your program. After you quit C, you can run NOSE.EXE by typing "cd \tc" (for Turbo C) or "cd \qc2\bin" (for Quick C) and then typing "nose".

You can erase the version called NOSE.OBJ. The computer creates it just to help create NOSE.EXE.

Quick C creates three extra files (NOSE.MDT, NOSE.ILK, and NOSE.SYM), which you can erase.

\n

Hereís a short cut. Instead of typing ó

main(){

puts("make your nose");

puts("touch your toes");

}

you can type:

main(){

puts("make your nose\ntouch your toes");

}

The symbol \n means: new line. It tells the computer to press the ENTER key. So puts("make your nose\ntouch your toes") tells the computer to print "make your nose", then press the ENTER key, then print "touch your toes". The computer will print:

make your nose

touch your toes

When you type the symbol \n, make sure you type a backslash: \. Do not type a division sign: /.

Printf

Instead of saying "puts", you can say "printf", like this:

main(){

printf("I love her so");

printf("up in the morning");

}

The command "printf" means: perform the print function. That program makes the computer print "I love her so" and "up in the morning" on the same line as each other, so the computer will print:

I love her soup in the morning

To force printf to press the ENTER key after "I love her so", say \n, like this:

main(){

printf("I love her so\n");

printf("up in the morning");

}

That makes the computer print:

I love her so

up in the morning

"Printf" differs from "puts" in two ways:

Printf doesnít make the computer press the ENTER key (unless you say \n).

Puts handles just strings. Printf is fancier: it can handle numbers also.

Since printf is fancier, it requires more RAM than puts.

 

Math

If you have 750 apples and buy 12 more, how many apples will you have altogether? This program prints the answer:

main(){

printf("you will have %d apples",750+12);

}

In the second line, the %d means: a bunch of digits. The computer will print "you will have", then a bunch of digits, then "apples". The "bunch of digits" will be the answer to 750+12. Altogether, the computer will print:

you will have 762 apples

In C, each printf line begins with a string (such as "you will have %d apples") and typically ends with a computation (such as 750+12). The computer automatically inserts the computationís answer into the string.

If you omit the words "you will have" and "apples", like
this ó

main(){

printf("%d",750+12);

}

the computer will print just the number 762. In that program, if you forget the "%d" or forget to say printf instead of puts, the computer will print wrong answers.

To print two answers on the same line, say %d twice:

main(){

printf("%d %d", 21+4, 68+1);

}

That program makes the computer print both answers:

25 69

The computer leaves a space between the answers because of the space between the %dís. If you omit the space between the %dís and say "%d%d" instead, the computer will print:

2569

Like BASIC and PASCAL, C lets you use the symbols +, -, *, /, parentheses, decimal points, and e notation. But if youíre not careful, the computer will print wrong answers. Hereís why.Ö

Integers versus double precision

C handles two types of numbers well.

One type of number is called an integer (or int). An int contains no decimal point and no e. For example, -27 and 30000 are ints.

The other type of number that C handles well is called a double-precision number (or a double). A double contains a decimal point or an e. For example, -27.0 and 3e4 are doubles. You can abbreviate: instead of writing "-27.0", you can write "-27.", and instead of writing "0.37" you can write ".37".

To print an int, say "%d" in the printf statement. To print a double, say "%.15g" instead. (The .15 makes the computer print 15 significant digits, and the g makes it use a general method of printing.)

Largest and tiniest numbers

The largest permissible int is 32767. The lowest is
-32768.

If you feed the computer an int thatís too large, the computer wonít complain. Instead, the computer will print a wrong answer!

The largest permissible double is approximately 1.7e308. The tiniest is approximately 1.7e-308.

Tricky arithmetic

If you combine ints, the answer is an int. For example, 2+3 is this int: 5.

11/4 is this int: 2. (11/4 is not 2.75.)

If you combine doubles, the answer is a double. If you combine an int with a double, the answer is a double.

How much is 200*300? Theoretically, the answer should be this int: 60000. But since 60000 is too large to be an int, the computer will print a wrong answer. To make the computer multiply 200 by 300 correctly, ask for 200.0*300.0, like this:

main(){

printf("%.15g",200.0*300.0);

}

That program makes the computer get the correct answer, 60000.0. The computer wonít bother printing the ".0"; it will print:

60000

Advanced math

The computer can do advanced math. For example, it can compute square roots. This program makes the computer print the square root of 9:

#include <math.h>

main(){

printf("%.15g",sqrt(9.0));

}

The computer will print 3.

Say sqrt(9.0) rather than sqrt(9), because the number you find the square root of should be double-precision, not an integer. If you make the mistake of saying sqrt(9), Turbo C and Quick C will print the correct answer but slowly; older versions of C will print a wrong answer.

That programís top line tells the computer to include a math heading. Begin that line by typing the symbol #, and end with the symbol > instead of a semicolon or brace. That line makes the computer use a file called MATH.H, which is in a subdirectory called INCLUDE, which is part of your C subdirectory. That MATH.H file tells the computer to make advanced-math answers be double-precision.

If you forget to say #include <math.h>, the computer will ignore MATH.H, think advanced-math answers are integers, and print wrong answers.

Besides sqrt, you can use other advanced math functions. All advanced-math functions require that you use double-precision numbers and say #include <math.h>. Hereís a list of those advanced-math functions.

To handle exponents, you can use sqrt (square root), exp (exponential power of e), log (logarithm base e), and log10 (logarithm base 10). You can also use pow: for example, pow(3.0,2.0) is 3.0 raised to the 2.0 power.

For trigonometry, you can use sin (sine), cos (cosine), tan (tangent), asin (arcsin), acos (arccosine), atan (arctangent), sinh (sine hyperbolic), cosh (cosine hyperbolic), and tanh (tangent hyperbolic). You can also use atan2: for example, atan2(y,x) is the arctangent of y divided by x.

For absolute value, use fabs (floating absolute). For example, fabs(-2.3) is 2.3.

To round, use floor (which rounds down) or ceil (which stands for "ceiling" and rounds up). For example, floor(26.319) is 26.000, and ceil(26.319) is 27.000.

Numeric variables

Like BASIC, C lets you use variables. For example, you can say:

n=3;

A variableís name can be short (such as n) or long (such as town_population_in_1988). The name must begin with a letter. The name can contain letters, digits, and underlines, but not blank spaces. The computer looks at just the first eight characters and ignores the rest, so the computer considers town_population_in_1988 to be the same as town_pop.

At the top of your program, say what type of number the variable stands for. For example, if n and town_population_in_1988 will stand for numbers that are ints and mortgage_rate will stand for a double, begin your program by saying:

main(){

int n,town_population_in_1988;

double mortgage_rate;

Hereís a short cut. Instead of beginning your program by saying ó

main(){

int n;

n=3;

just say:

main(){

int n=3;

If youíre writing a program in which n starts at 3 and population_in_1988 starts at 21000, begin your program by saying:

main(){

int n=3, population_in_1988=21000;

Increasing & decreasing

The symbol ++ means "increase". For example, ++n means "increase n". If you say ó

main(){

int n=3;

++n;

printf("%d",n);

}

the n starts at 3 and increases to 4, so the computer prints 4.

Saying ++n gives the same answer as n=n+1, but the computer handles ++n faster.

The symbol ++ increases the number by 1, even if the number is a decimal. For example, if x is 17.4 and you say ++x, the x will become 18.4.

The opposite of ++ is --. The symbol -- means "decrease". For example, --n means "decrease n". Saying --n gives the same answer as n=n-1 but faster.

Strange short cuts

If you use the following short cuts, your programs will be briefer and run faster.

Instead of saying n=n+2, say n+=2, which means "nís increase is 2". Similarly, instead of saying n=n*3, say n*=3, which means "nís multiplier is 3".

Instead of saying ++n and then giving another command, say ++n in the middle of the other command. For example, instead of saying ó

++n;

j=7*n;

say:

j=7*++n;

Thatís pronounced: "j is 7 times an increased n". So if n was 2, saying j=7*++n makes n become 3 and j become 21.

Notice that when you say j=7*++n, the computer increases n before computing j. If you say j=7*n++ instead, the computer increases n after computing j; so j=7*n++ has the same effect as saying:

j=7*n;

++n;

How to input

This program predicts how old youíll be ten years from now:

Program Meaning

main(){

int age; The age is an integer.

printf("How old are you? "); Ask "How old are you? ".

scanf("%d",&age); Wait for the human to input an age.

printf("Ten years from now, you'll be %d years old.",age+10); Print the result.

}

Notice that in the scanf statement, you must say &age instead of age. (If you forget the symbol &, youíll have a runaway pointer ó and a disaster!) Hereís a sample run:

How old are you? 27

Ten years from now, you'll be 37 years old.

The next program converts feet to inches. It even handles decimals: it can convert 1.5 feet to 18.0 inches.

Program Meaning

main(){

double feet; The number of feet is double-precision.

printf("How many feet? "); Ask "How many feet? ".

scanf("%lf",&feet); Wait for the human to input how many feet.

printf("That makes %.15g inches.",feet*12.0); Print the result.

}

Notice that to input a double-precision number, the scanf statement must say "%lf", which means "long floating-point", which is a fancy way of saying "double precision".

Arrays

Like BASIC and PASCAL, C lets you create arrays. For example, if you want x to be a list of 3 double-precision numbers, begin your program by saying:

double x[3];

That says x will be a list of 3 double-precision numbers, called x[0], x[1], and x[2]. Notice that C starts counting at 0. (PASCAL starts counting at 1 instead; PASCAL would call those numbers x[1], x[2], and x[3].)

Hereís a complete C program using that array:

main(){

double x[3];

x[0]=10.6;

x[1]=3.2;

x[2]=1.1;

printf("%.15g",x[0]+x[1]+x[2]);

}

The computer will print the sum, 14.9.

Notice that if you say double x[3], you can refer to x[0], x[1], and x[2], but not x[3]. If you accidentally refer to x[3], youíll be creating a runaway pointer.

If you want x to be a table having 2 rows and 3 columns of double-precision numbers, begin your program by saying:

main(){

double x[2][3];

Notice that C says x[2][3]. (PASCAL says x[2,3] instead.) In C, if you accidentally say x[2,3] instead of x[2][3], youíll have a runaway pointer.

Since C always starts counting at 0 (not 1), the number in the tableís upper-left corner is called x[0][0].

 

 

Character variables

A variable can stand for a character. For example, suppose youíre in school, take a test, and get an A on it. To proclaim your grade, write a program containing this line:

grade='A';

Hereís the complete program:

Program Meaning

#include <stdio.h> INCLUDE the STanDard I/O Headers.

main(){

char grade; The grade is a character.

grade='A'; The grade is ĎAí.

putchar(grade); Print the character thatís the grade.

}

The computer will print:

A

The usual way to print the grade is to say putchar(grade). To teach the computer what putchar means, say #include <stdio.h>.

Another way to print the grade is to say printf("%c",grade); the "%c" means "character".

To input a grade, say scanf("%c",&grade) or grade=getchar(). If you say scanf("%c",&grade), the user must press the ENTER key after entering the grade. If you say grade=getchar() instead, the user doesnít have to press ENTER. Like putchar, getchar requires you to say #include <stdio.h>.

Strings of characters

A variable can stand for a whole string of characters:

Program Meaning

main(){

char *torture; Torture is a whole string of characters.

torture="slice off your head"; Hereís torture.

puts(torture); Print the string thatís torture.

}

The computer will print:

slice off your head

That program begins by saying char *torture. The * means "string", so that char * means "character string". If you omit the *, torture will be just one character instead of a string of characters.

Put each string (such as "slice off your head") in double-quotes ("). Put a single character (such as ĎAí) in single-quotes (').

To print a string, say puts. To print a single character, say putchar.

The usual way to print the torture string is to say puts(torture). Another way is to say printf("%s",torture); the "%s" means "string".

Gets Hereís how to input a string:

main(){

char *name; name=(char *)malloc(41);

printf("What is your name? ");

gets(name);

printf("I like the name %s",name);

}

Hereís a sample run of that program:

What is your name? Maria Gonzales

I like the name Maria Gonzales

In that program, the most important line is "gets(name)". That tells the computer to get a string from the user; it lets the user input a name. Notice that to input a string instead of a number, you say "gets" instead of "scanf".

Malloc In that program, the top line says "name=(char *)malloc(41)". That warns the computer that the personís name might consume up to 41 characters in RAM (40 bytes for the name itself, plus 1 byte for the end-of-string marker). It makes the computer reserve 41 bytes of RAM for the name. The "malloc" means "memory allocation". If you forget to say "name=(char *)malloc(41)", youíll have a runaway pointer.

To be extra safe, in case the person types an extra-long name, you might be better off saying malloc(51) or even malloc(81). Since most strings in most programs are less than 81 characters long, hereís a rule of thumb: to be safe, say malloc(81) for each string.

In the program that says torture="slice off your head", you donít have to say torture=(char *)malloc(20), because when the computer sees "slice off your head" in your program, the computer automatically reserves 20 bytes of RAM to hold it. You need malloc just for variables that are input or that are built up by combining other variables.

Substrings If torture is "slice off your head", torture+1 means "torture without the first character"; it is "lice off your head". Similarly, torture+2 is "torture without the first 2 characters"; it is "ice off your head".

(Technically, torture+2 means "the string that begins 2 characters past the beginning of torture"; itís "the string whose starting memory address is 2 higher than tortureís".)

For example:

Program Meaning

main(){

char *torture; Torture is a whole string of characters.

torture="slice off your head"; Hereís torture.

printf("%s",torture+2); Print "ice off your head".

}

Hereís how to make cool be torture+2, which is "ice off your head":

Program Meaning

main(){

char *torture,*cool; Torture and cool are strings.

torture="slice off your head"; Hereís torture.

cool=torture+2; Cool is "ice off your head".

printf("%s",cool); Print "ice off your head".

}

Since cool uses the same part of RAM as torture, cool doesnít have to be mallocíed.

Extracting a character from a string If torture is "slice off your head", *torture indicates the first character in torture; it is Ďsí. This program makes the computer print the Ďsí:

Program Meaning

#include <stdio.h> Include the definition of putchar, etc.

main(){

char *torture,hiss; Tortureís a string. Hiss is a character.

torture="slice off your head"; Hereís torture.

hiss=*torture; Hiss is tortureís first character, Ďsí.

putchar(hiss); Print Ďsí.

}

If you want to print just tortureís third character (which is the i), ask for *(torture+2), which means "the character in torture beyond the first 2"; or just ask for torture[2].

Suppose tortureís a string, and you say cool=torture. If you change coolís third character to an x (by saying cool[2]=Ďxí), youíll also be changing the third character of torture, since cool and torture share characters with each other and share the same part of RAM.

Logic

Like most computer languages, C lets you say "if", "while", "for", and "goto" and create comments and subroutines. Hereís how.Ö

If

If a personís age is less than 18, letís make the computer say "You are still a minor." Hereís the fundamental line:

if (age<18) puts ("You are still a minor.");

Notice you must put parentheses after the word "if".

If a personís age is less than 18, letís make the computer say "You are still a minor." and also say "Ah, the joys of youth!" and "I wish I could be as young as you!" Hereís how to say all that:

if (age<18){

puts("You are still a minor.");

puts("Ah, the joys of youth!");

puts("I wish I could be as young as you!");

}

Letís put that structure into a complete program:

main(){

int age;

printf("how old are you? ");

scanf("%d",&age);

if (age<18){

puts("You are still a minor.");

puts("Ah, the joys of youth!");

puts("I wish I could be as young as you!");

}

else{

puts("You are an adult.");

puts("Now we can have some adult fun!");

}

puts("Glad to have met you.");

}

If the personís age is less than 18, the computer will print "You are still a minor." and "Ah, the joys of youth!" and "I wish I could be as young as you!" If the personís age is not less than 18, the computer will print "You are an adult." and "Now we can have some adult fun!" Regardless of the personís age, the computer will end the conversation by saying "Glad to have met you."

The "if" statement uses this notation:

Notation Meaning

if (age<18){ if age is less than 18

if (age<=18){ if age is less than or equal to 18

if (age==18){ if age is equal to 18

if (age!=18){ if age is not equal to 18

if (age<18 && weight>200){ if age<18 and weight>200

if (age<18 || weight>200){ if age<18 or weight>200

Look at that table carefully! Notice that in the "if" statement, you must use double symbols: you must say "==" instead of "=", say "&&" instead of "&", and say "||" instead of "|". If you accidentally use single symbols instead of double, the computer will print wrong answers.

Strings To put strings in an "if" statement, you must say "strcmp", which warns the computer to do a "string comparison".

For example, suppose x and y are strings, and you want to test whether theyíre equal. Do not say "if (x==y)". Instead, say "if (strcmp(x,y)==0)", which means "if string comparison between x and y shows 0 difference between them".

To test whether xís string comes before yís in the dictionary, do not say "if (x<y)". Instead, say "if (strcmp(x,y)<0)".

While

Letís make the computer print the word "love" repeatedly, like this:

love love love love love love love love love love love etc. love love love love love love love love love love love etc.

love love love love love love love love love love love etc.

etc.

This program does it:

main(){

while (1) printf("love ");

}

In that program, the "while (1)" means: do repeatedly. The computer will do printf("love") repeatedly, looping forever ó or until you abort the program.

Letís make the computer start at 20 and keep counting, so the computer will print:

20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 etc.

This program does it:

Program Meaning

main(){

int i=20; Start the integer i at 20.

while (1){ Repeat these lines forever:

printf("%d ",i); print i

++i; increase i

}

}

In that program, if you say "while (i<30)" instead of "while (1)", the computer will do the loop only while i remains less than 30; the computer will print just:

20 21 22 23 24 25 26 27 28 29

Instead of saying "while (i<30)", you can say "while (i<=29)".

For

Hereís a more natural way to get that output of numbers from 20 to 29:

main(){

int i;

for (i=20; i<=29; ++i) printf("%d ",i);

}

In that program, the "for (i=20; i<=29; ++i)" means "Do repeatedly. Start with i=20, and keep repeating as long as i<=29. At the end of each repetition, do ++i."

In that "for" statement, if you change the ++i to i+=3, the computer will increase i by 3 instead of by 1, so that the computer will print:

20 23 26 29

The "for" statement is quite flexible. You can even say "for (i=20; i<100; i*=2)", which makes i start at 20 and keep doubling, so the computer prints:

20 40 80

Like "if" and "while", the "for" statement can sit atop a group of indented lines that are in braces.

Goto

You can say "goto". For example, if you say "goto yummy", the computer will go to the line whose name is yummy:

main(){

puts("my dog");

goto yummy;

puts("never");

yummy: puts("drinks whiskey");

}

The computer will print:

my dog

drinks whiskey

Comments

To put a comment in your program, begin the comment with the symbol /* and end it with */. Hereís an example:

/* The following program

is fishy */

main(){

puts("Our funny God"); /* notice religious motif */

puts("invented cod"); /* said by nasty flounder */

}

The computer will print just:

Our funny God

invented cod

Subroutines

Like PASCAL, C lets you invent subroutines and give them names. For example, hereís how to invent a subroutine called "insult" and use it in the main routine:

Program Meaning

main(){ Hereís main routine:

puts("We all know..."); print "We all know..."

insult(); do the insult

puts("...but we love you"); print the ending

}

insult(){ How to insult:

puts("you are stupid"); print "you are stupid"

puts("you are ugly"); print "you are ugly"

}

The computer will print:

We all know...

you are stupid

you are ugly

...but we love you

Like BASIC, C wants you to type the main routine first, then the definition of the subroutine (called "insult").

Whenever you write a subroutineís name, you must put parentheses afterwards, like this: insult(). Those parentheses tell the computer: insultís a subroutine, not a variable.

Hereís another example:

Program Meaning

main(){ The main routine says

laugh(); to laugh.

}

laugh(){ Hereís how to laugh:

int i; print "ha" 100 times.

for (i=1; i<=100; ++i) printf("ha ");

}

The main routine says to laugh. The subroutine defines "laugh" to mean: print "ha " a hundred times. Notice that the "int i" is in the subroutine, not the main routine.

Letís create a more flexible subroutine, so that whenever the main routine says laugh(2), the computer will print "ha ha"; whenever the main routine says laugh(5), the computer will print "ha ha ha ha ha"; and so on. Hereís how:

main(){

puts("Here is a short laugh");

laugh(2);

puts("\nHere is a longer laugh");

laugh(5);

}

laugh(int n){ Hereís how to laugh(n):

int i; print "ha", n times.

for (i=1; i<=n; ++i) printf("ha ");

}

Average Letís define the "average" of a pair of integers, so that "average(3,7)" means the average of 3 and 7 (which is 5), and so a main routine saying "i=average(3,7)" makes i be 5.

This subroutine defines the "average" of all pairs of integers:

average(int a, int b){

return ((a+b)/2);

}

The top line says, "Hereís how to find the average of any two integers, a and b." The next line says, "Return to the main routine, with this answer: (a+b)/2."

Notice that the word "return" must be followed by parentheses.

Double-precision average Letís revise the subroutine, to make it handle double-precision numbers instead of integers, so a main routine saying "x=average(3.0,7.0)" makes x be 5.0.

Hereís how:

double average(double a, double b){

return ((a+b)/2.0);

}

The subroutine begins by saying "double average". That says the average will be a double-precision number. If you omit the word "double" and say just "average", the computer will make the average be an integer instead, because the computer assumes all subroutine answers are integers, unless you specifically say "double" or "char" or some other alternative.

So to get a double-precision answer, you must begin the subroutine by saying "double". You must also say "double" in the main routine:

main(){

double x,average();

x=average(3.0,7.0);

printf("%.15g",x);

}