Finding the position of a substring in a larger string

Question

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 a

Answer 1

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 original.

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 you 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 
#include 
#include 

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