Using a 'for' loop iterator after the loop exits in C

Question

For years, I've gotten in to the habit of not using the value of a for loop iterator after the loop exits. I could have sworn that I did this, because it used to produce a compiler warning, but after I was challenged in a recent code review, I was proven wrong.

For example, I always did this (NOTE: our code standards prohibit the use of the "break" keyword):

int i, result;
bool done = false;
for (i=0; i<10 && !done; i++) {
    if (some_condition) {
        result = i;
        done = true;
    }
}
// Value of i may be undefined here

Now, obviously the result variable could be removed, if I can rely on the value of i. I thought that because of compiler optimization, you could not rely on the value of the loop iterator. Am I just remembering a phantom teaching? Or is this the standard (specifically regarding GNU C)?

Answer 1

There is nothing wrong in C89, C99, or C11 to access the iteration variable after the for statement.

 int i;

 for (i = 0; i < 10; i++) {
     /* Some code */
 }

 printf("%d\n", i);  // No magic, the value is 10

From C99, you can use also a declaration as the first clause of the for statement, and in that case of course the declared variable cannot be used after the for statement.

Answer 2

Different languages have different rules. In Pascal, the compiler is allowed to optimize away storing the loop index after the final increment, so it might be the first loop-terminating value or it might be the last valid value.

Answer 3

There are plenty of usage cases where the for loop is used for nothing else but advancing the iterator. This can be seen in some implementations of strlen (though admittedly there are other ways to do strlen), and other sorts of functions whose goal it is to find a certain limit:

/*find the index of the first element which is odd*/
for (ii = 0; ii < nelem && arry[ii] % 2 == 0; ii++);

As mentioned, the point of confusion may come from constructs where the iterator itself is defined within the for statement.

In general for statements are very very powerful, and it's unfortunate that they're usually never utilized to their full potential.

For example, a different version of the same loop can be written as follows (though it wouldn't demonstrate the safety of using the iterator):

#include <stdio.h>
int main(void)
{
    int cur, ii = 0, nelem, arry [] = { 1, 2, 4, 6, 8, 8, 3, 42, 45, 67 };
    int sum = 0;

    nelem = sizeof(arry) / sizeof(int);
    /* Look mom! no curly braces! */

    for (
            ii = 0;
            ii < nelem && ((cur = arry[ii]) %2 == 0 ||
                                ((printf("Found odd number: %d\n", cur)||1)));
            ii++, sum += cur
        );
    printf("Sum of all numbers is %d\n", sum);
    return 0;
}

In this particular case, it seems like a lot of work for this specific problem, but it can be very handy for some things.