Comparing substring of a character array with another character array in C

Question

I have two characters arrays called arraypi and arraye containing numbers that I read from a file. Each have 1,000,000 characters. I need to start from the first character in arraye (In this case, 7) and search for it in arraypi. If 7 exists in arraypi then I have to search for the next substring of arraye(in this case, 71). Then search for 718, 7182 and so on until the substring does not exist in arraypi. Then I have to simply put the length of the biggest substring in a integer variable and print it.

Worth mentioning that arraypi contains a newline every 50 characters whereas arraye contains a newline every 80 although I don't think that will be problem right?

I tried thinking about a way to accomplish this but so far I haven't thought of something.

Answer 1

I am not absolutely sure if I got this right. I have something like this on my mind:

• Assume that we have the whole arraypi is in a browser
• You use the key combination ctrl+f for find
• Start typing the contents of arraye letter by letter until you see the red no match
• You want the number of characters you were able to type until then

If that's right, then an algorithm like the following should do the trick:

#include <stdio.h>
#define iswhitespace(X) ((X) == '\n' || (X) == ' ' || (X) == '\t')

int main( ) {

    char e[1000] = "somet\n\nhing";
    char pi[1000] = "some other t\nhing\t som\neth\n\ning";

    int longestlen = 0;
    int longestx = 0;
    int pix = 0;
    int ex = 0;
    int piwhitespace = 0;       // <-- added
    int ewhitespace = 0;        // <-- these

    while ( pix + ex + piwhitespace < 1000 ) {

        // added the following 4 lines to make it whitespace insensitive
        while ( iswhitespace(e[ex + ewhitespace]) )
            ewhitespace++;
        while ( iswhitespace(pi[pix + ex + piwhitespace]) )
            piwhitespace++;

        if ( e[ex + ewhitespace] != '\0' && pi[pix + ex + piwhitespace] != '\0' && pi[pix + ex + piwhitespace] == e[ex + ewhitespace] ) {
            // the following 4 lines are for obtaining correct longestx value
            if ( ex == 0 ) {
                pix += piwhitespace;
                piwhitespace = 0;
            }
            ex++;
        }
        else {
            if ( ex > longestlen ) {
                longestlen = ex;
                longestx = pix;
            }
            pix += piwhitespace + 1;
            piwhitespace = 0;
            // the two lines above could be replaced with
            // pix++;
            // and it would work just fine, the injection is unnecessary here
            ex = 0;
            ewhitespace = 0;
        }
    }

    printf( "Longest sqn is %d chars long starting at %d", longestlen, longestx + 1 );

    putchar( 10 );
    return 0;
}

What's happening there is, the loop searches for a starting point for match first. Until it finds a match, it increments the index for the array being examined. When it finds a starting point, it then starts incrementing the index for the array containing the search term, keeping the other index constant.

Until a next mismatch, which is when a record-check is made, search term index is reset and examinee index starts getting incremented once again.

I hope this helps, somehow, hopefully more than resolving this single-time struggle.

Edit:

Changed the code to disregard white space characters.