Improving function that compares two strings

Question

I'm learning C and have made this very simple function to compare two strings. I would like to know how it can be improved:

int match(char* string1, char* string2)
{
 size_t i = 0;
 while (string1[i] != '0円' || string2[i] != '0円') {
 if (string1[i] != string2[i]) {
 return 0;
 }
 ++i;
 }
 return 1;
}

I figured it will only do 2 comparisons per iteration, and 3 in the last iteration. Also, I believe it won't go beyond the end of any array because it will return before iterating beyond the null terminator.

Answer 1

Your parameters should be const and your loop need only to check one string for the terminator. For example in the following loop, if *s1 is 0 the loop stops because of the first condition. If *s1 is not 0 but *s2 is 0, the loop stops because of the 2nd condition.

int match(const char *s1, const char *s2)
{
 while (*s1 && (*s1++ == *s2++)) {
 // nothing
 }
 return *s1 - *s2;
}

Answer 2

In your while loop, you check whether string1[i] or string2[i] are not null. If they are of unequal lengths, they will have their null terminators at different values of i; because at least one is non-null at each point, your while loop would then continue forever. To fix this, all you have to do is change || to &&, because you need to make sure that both of your characters are valid.

Another improvement that I would consider would be changing the return value to be zero if they are equal; this would match the built-in strcmp function. Then, you could return i as the place at which the two strings diverge, also matching strcmp. This provides the programmer with more useful information from one function call without a loss in value, because 0 is false and everything else is true.

• 200_success: Don't worry, the termination works fine. If they are unequal length, the if-comparison inside the while-loop will be triggered, and return 0.
• acrucker: Right, I didn't consider that. In any case, it would be more readable for a && operator. Also, if OP cares about performance, he should know that && is short-circuiting: It will only evaluate the right operand if the left operand is true, providing some (small) performance gain.
• 200_success: Once you change || to &&, it no longer works correctly. When one string begins with the other, it would report them as being equal.