C For Loop Not Working?

Question

I'm working with strings in C as character arrays, and I'm trying to ensure that I can dynamically pass values into my for loops.

The following code works, no problem:

for (int i = -6; i < 11; i++) {
    int test = OverlapStrength(fragments[1], fragments[2], i, 0);
    printf("%d\n", test);
}

In fact, this code works, too:

for (int i = -strlen(fragments[2]) + 1; i < 11; i++) {
    int test = OverlapStrength(fragments[1], fragments[2], i, 0);
    printf("%d\n", test);
}

But for some reason, this code doesn't print ANYTHING:

for (int i = -strlen(fragments[2]) + 1; i < strlen(fragments[1]); i++) {
    int test = OverlapStrength(fragments[1], fragments[2], i, 0);
    printf("%d\n", test);
}

I have checked the values for both -strlen(fragments[2]) + 1 and strlen(fragments[1]) just before the loop and they check out to -6 and 11 respectively. Clearly the loop works when I place those values directly into their places, but when I replace the second one with the strlen calculations, it breaks and I can't figure out why for the life of me. Help?

Edit

OverlapStrength takes its arguments as constants so I can't change them, so I'm pretty sure I'm not changing the fragments as I go. Here's the method declaration:

int OverlapStrength(const char one[], const char two[], int startOne, int startTwo)

The contents of the fragments shouldn't be important, but they're simply strings that I'm trying to piece back together from overlapping fragments. I have already checked that my fragments are all coming out properly and that their lengths are computed properly when done outside of declaring this loop.

Answer 1

strlen returns value of type size_t, which is probably a typedef for unsigned int for your case. Then you are comparing a signed int (i) and unsigned int (strlen(...)). C decides then to cast your signed value to an unsigned type (because of default type promotions). -6 converted to unsigned int is 4294967290, therefore your comparison is false, so the loop ends.

To fix this, you can for example cast strlen to a signed value, e.g.:

i < (int) strlen(fragments[1])

Answer 2

In a for-loop, the codition (the i < strlen(fragments[1]) part) gets evaluated on every iteration. If OverlapStrength changes the value of fragments[1] to something less than i, the loop will abort.

To fix this, use a constant:

int loopUntil = strlen(fragments[1]);
for (int i = -strlen(fragments[2]) + 1; i < loopUntil; i++) {
    int test = OverlapStrength(fragments[1], fragments[2], i, 0);
    printf("%d\n", test);
}