Question

I was going through some Amazon interview questions on CareerCup.com, and I came across this interesting question which I haven’t been able to figure out how to do. I have been thinking on this since 2 days. Either I am taking a way off approach, or its a genuinely hard function to write.

Question is as follows:

Write a function in C that can find if a string is a sub-string of another. Note that a mismatch of one character
should be ignored.

A mismatch can be an extra character: ’dog’ matches ‘xxxdoogyyyy’  
A mismatch can be a missing character: ’dog’ matches ‘xxxdgyyyy’ 
A mismatch can be a different character: ’dog’ matches ‘xxxdigyyyy’

The return value wasn’t mentioned in the question, so I assume the signature of the function can be something like this:

char * MatchWithTolerance(const char * str, const char * substr);

If there is a match with the given rules, return the pointer to the beginning of matched substring within the string. Else return null.

Bonus

If someone can also figure out a generic way of making the tolerance to n instead of 1, then that would be just brilliant.
In that case the signature would be:

char * MatchWithTolerance(const char * str, const char * substr, unsigned int tolerance = 1);

Answer 1

This seems to work, let me know if you find any errors and I’ll try to fix them:

int findHelper(const char *str, const char *substr, int mustMatch = 0)
{
    if ( *substr == '\0' )
        return 1;

    if ( *str == '\0' )
        return 0;

    if ( *str == *substr )
        return findHelper(str + 1, substr + 1, mustMatch);
    else
    {
        if ( mustMatch )
            return 0;

        if ( *(str + 1) == *substr )
            return findHelper(str + 1, substr, 1);
        else if ( *str == *(substr + 1) )
            return findHelper(str, substr + 1, 1);
        else if ( *(str + 1) == *(substr + 1) )
            return findHelper(str + 1, substr + 1, 1);
        else if ( *(substr + 1) == '\0' )
            return 1;
        else
            return 0;
    }
}

int find(const char *str, const char *substr)
{
    int ok = 0;
    while ( *str != '\0' )
        ok |= findHelper(str++, substr, 0);

    return ok;
}


int main()
{
    printf("%d\n", find("xxxdoogyyyy", "dog"));
    printf("%d\n", find("xxxdgyyyy", "dog"));
    printf("%d\n", find("xxxdigyyyy", "dog"));
}

Basically, I make sure only one character can differ, and run the function that does this for every suffix of the haystack.