Chapter 16: C the Loop, C the Loop++ 209

for(count=10;count>0;count--)

Save the source code file back to disk and then recompile it.

The program runs the same, but your C programming buddies will nod their heads in amazement at your deft use of the decrementation operator.

More Incrementation Madness

Incrementation and looping go hand in hand like (this week’s) Hollywood’s hottest couple.

Looping is an important part of programming — doing things over and over, like a famous actor rehearses his lines. (Make that famous stage actor.) In C, that can be done only with the for loop if you increment (or decrement) a variable’s value.

Tied in with looping are the ++ and -- operators, which you can also use independently from looping to increase or decrease a variable’s value — like some actresses increase their bust size through various surgical techniques, and their age via bald-faced lying.

Given that, you should still keep in mind that incrementing and decrementing don’t have to be done one tick at a time. For example, the LARDO.C program boosts the w variable’s value by 8 by using the following statement in Line 18:

w=w+8;

The value of the variable w is increased by 8. The following statement decreases w by 3:

w=w-3;

This is still a form of incrementing and decrementing, though values larger than 1 are used. It’s perfectly legit. And — as a bonus — you can use these types of incrementing or decrementing in loops. An example is coming forthwith.

Although you can increment a variable by a value larger than 1, the ++ operator increases a variable’s value by only 1. The same holds true for --, which always decreases a variable’s value by 1.

Fortunately, the C language lacks a +++ or --- operator. Forget about it!

210 Part III: Giving Your Programs the Ability to Run Amok

Leaping loops!

After losing or wining the game, the kids get together as a team and chant:

2, 4, 6, 8, who do we appreciate?

This chant is followed by the other team’s name. It’s a nice, polite, all-American athletic chant that means either “You sad sacks were easy to beat and, if we were unsupervised, we would be vandalizing your bicycles by now” or “You defeated us through treachery and deceit and, if we were unsupervised, we would be pummeling your heads with our aluminum bats.”

Anyway, the following program uses a for loop to generate the “2, 4, 6, 8” part of the chant. It’s a for loop that, yes, skips a bit as it counts. It’s what I call a leaping loop:

#include <stdio.h>

int main()

{

int i;

for(i=2;i<10;i=i+2) printf(“%d “,i);

printf(“who do we appreciate? GNU!\n”); return(0);

}

Choose New in your editor and type the preceding source code. In the printf() statement in the for loop, note the space after the %d. It goes “double quote, percent sign, little d, space, double quote.” Save the file to disk as CHANT.C.

Compile and run the program. Here’s what your output should look like:

2 4 6 8 who do we appreciate? GNU!

The loop starts at 2 and increments up to 10 by using the i=i+2 formula. The loop reads like this: “Start with i equal to 2, and while the value of i is less than 10, repeat the following, adding 2 to variable i each time you loop.”

You can change Line 7 of the program to have the loop count by even numbers to any value. For example:

for(i=2;i<1000;i=i+2)

This modification makes the computer count by twos from 2 to 998. It doesn’t do much for the chant, but it works. (Indeed, it would take for­ ever to get to The Pizza Place if that were the case.)

Chapter 16: C the Loop, C the Loop++ 211

Counting to 1,000 by fives

The following program is an update to the old 100.C program, from Chapter 15. In this case, the program counts to 1,000 by fives — a task that would literally take days without a computer:

#include <stdio.h>

int main()

{

int i;

for(i=5;i<=1000;i=i+5)

printf(“%d\t”,i);

return(0);

}

Start off with a new, clean slate in your editor. Type the preceding source code. It’s nothing fancy. Indeed, it’s just a take-off from the old 100.C program. Save the file to disk as 1000.C.

Compile 1000.C and run the result. Your screen fills with values from 5 to 1000, all lined up in rows and columns.

This leaping loop counts by fives because of the i=i+5 part of the for statement. The i=i+5 operation keeps increasing the value of the i vari­ able by 5.

The loop begins counting at 5 because of the i=5 part of the for loop. It stops counting at 1,000 because of the i<=1000 part of the loop. That’s “less than or equal to 1000,” which is how you get to 1,000.

Cryptic C operator symbols, Volume III: The madness continues

C is full of shortcuts, and mathematical operations are where you find most of them clustered like bees over a stray Zagnut bar. I feel that the two most cryp­ tic shortcuts are for changing a variable’s value by 1: ++ to increment and -- to decrement. But there are more!

To add 5 to a variable’s value, for example, such as in the 1000.C program, you use the following:

i=i+5

212 Part III: Giving Your Programs the Ability to Run Amok

The cryptic C language shortcut for this operation is

i+=5

This line means “Increase the value of variable i by five.” Unfortunately, it just doesn’t look like that’s what it means.

Although I can swallow ++ to increment and -- to decrement, the += thing seri­ ously looks like a typo. Sad news: It’s not. Even sadder: There are more of them, one each for adding, subtracting, multiplying, or dividing a variable’s value by a certain amount (or by another variable). Table 16-2 lists the lot of them.

Table 16-2	Cryptic Shortcuts for Common Math Operations
Long, Boring Way	Cryptic Shortcut
var=var+5	var+=5
x=x+y	x+=y
var=var-5	var-=5
x=x-y	x-=y
var=var*5	var*=5
x=x*y	x*=y
var=var/5	var/=5
x=x/y	x/=y

In Table 16-2, you see two examples for each cryptic shortcut. The first one uses the variable var, which is modified by a constant value, 5. The second uses two variables; the first one, x, is modified by another variable, y.

Yes, the shortcuts for incrementing, decrementing, or changing a variable are cryptic. You don’t have to use them. You suffer no penalty for forgetting about them. I refer to them here for two reasons: It can be done, and C gurus love tossing this stuff into programs; so don’t let the shortcuts scare you when you see them.

On your own: Modify the preceding two programs, CHANT.C and 1000.C.

Replace the long math condition in the for loop with a shortcut version.

Answers are provided at the end of this chapter.