Exercise 4.1- strindex which returns rightmost occurance¶

Code¶

Write the function strindex(s,t) which returns the position of the rightmost occurrence of t in s, or -1 if there is none.

/* strindex which returns rightmost occurance */

#include<stdio.h>

int mstrindex(char source[],char searchfor[]);

int main(void)
{
    char line[] = "abcdedfabcde";
    char pattern[] = "abc";

    int found;

    /* It should match the a the 7th position. */

    found = mstrindex(line, pattern);

    printf("Found the right index: %d\n", found);

}

int mstrindex(char s[],char t[])
{
    int i,j,k, result;

    result = -1;

    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')
            result = i;
    }
    return result;
}

Explanation¶

We find the rightmost of index of our substring in this program. If we ask to look for pattern “abc” in the line “abcdedfabcde”, the program should correctly identify the rightmost occurance which happens at position 7.

This is done by our mstringindex function and in this loop.

result = -1;

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')
        result = i;
}

The outer loop goes over each character in string s and in the inner we check if we find a substring t matching in the outer loop. If we find a substring match, we dont break the loop, but record the position i and proceed further. Thus our right most match is noted. If no search is found, then the result, -1 is returned.

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