Exercise 9: While-Loop and Boolean Expressions

The first looping construct I'll show you is the while-loop, and it's the simplest, useful loop you could possibly use in C. Here's this exercise's code for discussion:

#include <stdio.h>

int main(int argc, char *argv[])
{
    int i = 0;
    while (i < 25) {
        printf("%d", i);
        i++;
    }

    // need this to add a final newline
    printf("\n");

    return 0;
}

From this code, and from your memorization of the basic syntax, you can see that a while-loop is simply this:

while(TEST) {
    CODE;
}

It simply runs the CODE as long as TEST is true (1). So to replicate how the for-loop works, we need to do our own initializing and incrementing of i. Remember that i++ increments i with the post-increment operator. Refer back to your list of tokens if you didn't recognize that.

What You Should See

The output is basically the same, so I just did it a little differently so that you can see it run another way.

$ make ex9
cc -Wall -g    ex9.c   -o ex9
$ ./ex9
0123456789101112131415161718192021222324 
$

How to Break It

There are several ways to get a while-loop wrong, so I don't recommend you use it unless you must. Here are a few easy ways to break it:

• Forget to initialize the first int i;. Depending on what i starts with, the loop might not run at all, or run for an extremely long time.

• Forget to initialize the second loop's i so that it retains the value from the end of the first loop. Now your second loop might or might not run.

• Forget to do a i++ increment at the end of the loop and you'll get a forever loop, one of the dreaded problems common in the first decade or two of programming.

Extra Credit

• Make the loop count backward by using i-- to start at 25 and go to 0.

• Write a few more complex while-loops using what you know so far.