Exercise 2-5

Write the function any(s1,s2), which returns the first location in the string s1 where any character from the string s2 occurs, or -1 if s1 contains no characters from s2. (The standard library function strpbrk does the same job but returns a pointer to the location.)

For every character of s1, we need to compare it with every character of s2. This seems to call for a pair of nested for-loops! If at any point a character in s1 equals a character in s2, we return the index of the character in s1. However, if we reach the end of the loop—which can only occur if we had not already returned from the function—we return -1. 

    /* any:  return location of first occurence of any character from s2 in s1 */
    int any(char s1[],  char s2[])
    {
        int i, j;
    
        for (i = 0; s1[i] != '\0'; ++i)
            for (j = 0; s2[j] != '\0'; ++j)
                if (s1[i] == s2[j])
                    return i;
        return -1;
    }