Finding the position of a substring in a larger string

Question

I have created a function that should find the numerical position of the first character of a substring in a larger string. I am having some problems with the output and I a

Answer 1

Your loop counter tests are incorrect: wrong upper limit and the limits are off by one. Note that the tests are actually not necessary as you exit both loops when hitting the '\0' terminators.

Here is a simpler version:

int findSubString(const char *original, const char *toFind) {
    for (size_t i = 0;; i++) {
        for (size_t j = 0;; j++) {
            if (toFind[j] == '\0') {
                return i;
            }
            if (original[i + j] != toFind[j]) {
                break;
            }
        }
        if (original[i] == '\0') {
            return -1;
        }
    }
}

There is a small advantage at computing the string lengths to reduce the number of comparisons in pathological cases such as findSubString("aaaaaaaaaaa", "aaaaaaaaaaaa");

int findSubString(const char *original, const char *toFind) {
    size_t originalLength = strlen(original);
    size_t toFindLength = strlen(toFind);

    if (toFindLength <= originalLength) {
        for (size_t i = 0; i <= originalLength - toFindLength; i++) {
            for (size_t j = 0;; j++) {
                if (toFind[j] == '\0') {
                    return i;
                }
                if (original[i + j] != toFind[j]) {
                    break;
                }
            }
        }
    }
    return -1;
}