Question

For a homework assignment, I need to implement a function which takes a char *s and a char c and return the index of c if found, and -1 otherwise.

Here’s my first try:

int IndexOf(const char *s, char c) {
    for (int i = 0; *s != '\0'; ++i, ++s) {
        if (*s == c) {
            return i;
        }
    }

    return -1;
}

Is that an okay implementation, or are there things to improve?

EDIT Sry, didn’t mention that I only should use pointer-arithmetic/dereferencing, not something like s[i]. Besides, no use of the Standard Library is allowed.

Answer 1

Yours is perfectly fine, as far as it goes. You should also write a simple test program that tests for the first char, last char, and a missing char.

Piling on to the ‘other ways to do it’ group, here is one with no break, a single return, and showing off pointer arithmetic. But, beware: if I were grading your homework, I would grade yours higher than mine. Yours is clear and maintainable, mine needlessly uses ?: and pointer subtraction.

#include <stdio.h>

int IndexOf(const char *s, const char c)
{
    const char * const p = s;
    while(*s && *s != c) s++;
    return (*s) ? s-p : -1;
}

#ifdef TEST
int main()
{
    printf("hello, h: %d\n", IndexOf("hello", 'h'));
    printf("hello, g: %d\n", IndexOf("hello", 'g'));
    printf("hello, o: %d\n", IndexOf("hello", 'o'));
    printf("hello, 0: %d\n", IndexOf("hello", 0));
}
#endif

The output of this program is:

hello, h: 0
hello, g: -1
hello, o: 4
hello, 0: -1