CODEWITHSUNDEEP
arrayscstring
Pung
Pung

Reputation: 25

Search for a string variable within another string variable

Search for a string variable within another string variable while avoiding the usage of any function in the <string.h> library. Im not sure why my code isnt working. Any suggestions or alternative methods are very appreciated. edit: it's ok to use strlen but not other functions

#include <stdlib.h>
#include<string.h>
#include <stdio.h>
 int strsearch(char *B, char *C);
int main()
{
    char B[]= "thisatest";
    char C[]= "tes";
    strsearch(B, C);
    return 0;
}
 int strsearch(char  *B, char *C ){
     int i, b, c, k;
     k= -1;
     b= strlen(B);
     c= strlen(C);
     for (i=0; i<b; i++){
         if (B[i+c]== C)
         return i;
         return -1;
     }
     }

Upvotes: 1

Views: 71

Answers (1)

Ted Lyngmo
Ted Lyngmo

Reputation: 117962

Your return -1; is within the loop. It'll return if the first if (B[i+c]== C) comparison you do fails. Move it out of the loop.

You compare the wrong things. B[i+c] is a char and C is a char*. If you want to compare strings, use strcmp, strncmp or memcmp depending on the situation and supply the correct arguments.

  • strcmp for arbitrary strings that you want to compare until the null terminator is found in both.
  • strncmp if you want to limit the comparison to n number of characters but still be stopping at a null terminator. Seems like a good fit for a substring comparison.
  • memcmp if you are sure that the length you supply will not make it pass a null terminator since it doesn't check for a null terminator. It just compare bytes in memory where you tell it to, for as long as you tell it to.

I'm using memcmp in this example:

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

#define SS_NOTFOUND ((size_t)-1)
size_t strsearch(const char *B, const char *C) {
    size_t Blen = strlen(B);
    size_t Clen = strlen(C);

    // If Blen < Clen, there's no way C can
    // can be a substring of B:
    if(Blen < Clen) return SS_NOTFOUND;

    // Subtract the length of C from the length of B. There is no need to
    // search in B beyond the point where C can't fit:
    Blen -= Clen;

    for (size_t i = 0; i <= Blen; i++) {
        // memcmp is fine here since no null terminators will be passed
        // due to the check (Blen < Clen) at the start and the subtraction
        // of Clen from Blen above.

        if (memcmp(B + i,   C,  Clen) == 0) return i;
        //         char*  char*
    }
    return SS_NOTFOUND;
}

int main() {
    char B[] = "thisatest";
    char C[] = "tes";
    
    size_t pos = strsearch(B, C);

    if(pos == SS_NOTFOUND) puts("sorry, not found");
    else printf("Found at index %zu\n", pos);
}

Output:

Found at index 5

If you for some reason can't use memcmp, create your own and use that instead in the example above:

int MemCmp(const void *s1, const void *s2, size_t len) {
    const unsigned char* a = s1;
    const unsigned char* b = s2;
    
    for(;len; --len, ++a, ++b) {
        if(*a < *b) return -1;
        else if(*a > *b) return 1;
    }
    return 0;
}

Upvotes: 1

Related Questions