Substring Extraction problem

Question

Given 2 strings, write a function that returns the position of String B that matches String A if String A is a substring of String B. Otherwise return -1. Example:

strA = "ello"
strB = "Hello_World"
Your function should return 1.

strA = "blah"
strB = "blha"
Your function should return -1.

Answer 1

Is this homework? Anyway, look the string.h documentation and you should find what you need without much trouble. You will need to write a very thin wrapper over one of the functions. Or, of course, you can just write it all yourself.

EDIT: Well, someone else gave a answer, so here's my attempt.

#include <string.h>

ssize_t str_index(const char *strA, const char *strB)
{
    const char *result;
    return (result = strstr(strB, strA)) ? (result - strB) : -1;  
}

The only tricks are that the parameter order is reversed from strstr, you're returning an ssize_t instead of char *, and the failure code is thus -1.

Answer 2

Brute force version:

int strpos(char* a, char* b) {
  int n1, n2, i, j;
  n1 = strlen(a);
  n2 = strlen(b);
  for (i = 0; i < n1-n2; i++) {
    for (j = 0; j < n2; j++) {
      if (a[i+j] != b[j]) break;
      else if (j+1 == n2) return i;
    }
  }
  return -1;
}

More effective algorithms: Wikipedia: String searching

Answer 3

This sounds almost exactly like a homework problem. In the strange case that it is not a homework problem, the C library function strstr returns a pointer to the first occurrence of a string inside another (or null if it is not there). From there, it is trivial to determine the index using pointer arithmetic.

Answer 4

#include <string.h>

int search(char* a, char* b) {
    char* pos;
    pos = strstr(b, a);
    if(pos == 0) return -1;
    return (int)(pos-b);
}

Answer 5

Trying learning about strstr. Its available in string.h There is NEVER a reason to code something already available in the standard library. Unless ofcourse, its a homework question :)