Monty Monty - 24 days ago 8
C Question

Finding the position of a substring in a larger string

I have created a function that should find the numerical position of the first character of a substring in a larger string. I am having some problems with the output and I am not too sure why. These problems include -1 being returned every single time instead of the integer position of the substring. I have debugged and cannot trace where the function goes wrong.

This is how the function should perform: If my string is "The dog was fast" and I am searching for the substring "dog", the function should return 4. Thanks to chqrlie for help with the loop.

Here is the function:

int findSubString(char original[], char toFind[]) {

size_t i, j;
int originalLength = 0;
int toFindLength = 0;

originalLength = strlen(original) + 1;
toFindLength = strlen(toFind) + 1;

for (i = 0; i < toFindLength + 1; i++) {
for (j = 0; j < originalLength + 1; j++) {
if (toFind[j] == '\0') {
return i;
}
if (original[i + j] != toFind[j]) {
break;
}
}
if (original[i] == '\0') {
return -1;
}
}
}


The function parameters cannot be modified, this is a requirement. Any help appreciated!

Answer

These statements inside the loops

       if (toFind[j] == '\0') {
            return i;
        }

results in undefined behavior because the string toFind can be shorter than the string orifinal.

The same is valid for this loop

        if (original[i + j] != toFind[j]) {
            break;
        }

because i + j can be greater than the length of the string original.

And there is no need to scan all characters of the string original if you are going to find a substring inside it.

Also you should check whether the length of the string original is not less than the length of the string toFind.

If you want to find only the first character of the string toFind in the string original it is enough to use standard C function strchr. If you want to find the whole string toFind in the string original then you could use another C standard function strstr.

If yo want to write the function yourself to find a string in other string then it can look for example the following way

I declared the function like

long long int findSubString( const char original[], const char toFind[] );

however you can write its declaration as you like for example like

int findSubString( char original[], char toFind[] );

But in this case you should declare function local variable success like

int success = -1;

and output the result using format specifier "%d" instead of "%lld".

Here you are.

#include <stdio.h>
#include <string.h>
#include <stddef.h>

long long int findSubString( const char original[], const char toFind[] )
{
    size_t n = strlen( original );
    size_t m = strlen( toFind );

    long long int success = -1;

    if ( !( n < m ) )
    {
        n = n - m + 1;

        for ( size_t i = 0; success == -1 && i < n; i++ )
        {
            size_t j = 0;
            while ( j < m && original[i+j] == toFind[j] ) j++;

            if ( j == m ) success = i;
        }
    }

    return success;
}

int main(void) 
{
    printf( "%lld\n", findSubString( "The dog was fast", "dog" ) );

    return 0;
}

Its output is

4