Question

You've had your first taste of how C does loops, but the boolean expression i < argc might have not been clear to you. Let me explain something about it before we see how a while-loop works.

In C, there's not really a "boolean" type, and instead any integer that's 0 is "false" and otherwise it's "true". In the last exercise the expression i < argc actually resulted in 1 or 0, not an explicit True or False like in Python. This is another example of C being closer to how a computer works, because to a computer truth values are just integers.

Now you'll take and implement the same program from the last exercise but use a while-loop instead. This will let you compare the two so you can see how one is related to another.

#include <stdio.h>

int main(int argc, char *argv[])
{
    // go through each string in argv

    int i = 0;
    while(i < argc) {
        printf("arg %d: %s\n", i, argv[i]);
        i++;
    }

    // let's make our own array of strings
    char *states[] = {
        "California", "Oregon",
        "Washington", "Texas"
    };

    int num_states = 4;
    i = 0;  // watch for this
    while(i < num_states) {
        printf("state %d: %s\n", i, states[i]);
        i++;
    }

    return 0;
}

You can see from this that a while-loop is simpler:

while(TEST) {
    CODE;
}

It simply runs the CODE as long as TEST is true (1). This means that to replicate how the for-loop works we need to do our own initializing and incrementing of i .

What You Should See

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

$ make ex11
cc -Wall -g    ex11.c   -o ex11
$ ./ex11
arg 0: ./ex11
state 0: California
state 1: Oregon
state 2: Washington
state 3: Texas
$
$ ./ex11 test it
arg 0: ./ex11
arg 1: test
arg 2: it
state 0: California
state 1: Oregon
state 2: Washington
state 3: Texas
$

How To Break It

In your own code you should favor for-loop constructs over while-loop because a for-loop is harder to break. Here's a few common ways:

• Forget to initialize the first int i; so have it loop wrong.
• 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 get a "forever loop", one of the dreaded problems of the first decade or two of programming.

Extra Credit

• Make these loops count backward by using i-- to start at argc and count down to 0. You may have to do some math to make the array indexes work right.
• Use a while loop to copy the values from argv into states .
• Make this copy loop never fail such that if there's too many argv elements it won't put them all into states .
• Research if you've really copied these strings. The answer may surprise and confuse you though.