Exercise 4-1

We can start by basing our function off of strindex.

    /* strindex:  return index of t in s, -1 if none */
    int strindex(char s[], char t[])
    {
        int i, j, k;
    
        for (i = 0; s[i] != '\0'; i++) {
            for (j=i, k=0; t[k]!='\0' && s[j]==t[k]; j++, k++)
                ;
            if (k > 0 && t[k] == '\0')
                return i;
        }
        return -1;
    }

In strindex, i starts as zero and increments if t is not found starting from i, which means it returns the leftmost occurence. In order to return the rightmost, we need to set i to equal the index of the last character of s, and decrement it after every iteration. Additionally, we need to make sure that we also exit the loop if s[j] == '\0' since that is now also a possible occurrence as i starts off as the length instead.

    /* strindex:  return index of t in s, -1 if none */
    int strindex(char s[], char t[])
    /* strrindex:  return rightmost index of t in s, -1 if none */
    int strrindex(char s[], char t[])
    {
        int i, j, k;

        for (i = 0; s[i] != '\0'; ++i)
            ;
        while (--i >= 0) {
            for (j=i, k=0; s[j]!='\0' && t[k]!='\0' && s[j]==t[k]; j++, k++)
                ;
            if (k > 0 && t[k] == '\0')
                return i;
        }
        return -1;
    }

Note: inside the while-loop, the prefix decrement operator is used because initially, i is equal to the index of \0 in s.