Get index of substring

Question

I have char * source, and I want extract from it subsrting, that I know is beginning from symbols "abc", and ends where source ends. With strstr I can get the poiner, but not the position, and without position I don't know the length of the substring. How can I get the index of the substring in pure C?

Solution 1

Use pointer subtraction.

char *str = "sdfadabcGGGGGGGGG";
char *result = strstr(str, "abc");
int position = result - str;
int substringLength = strlen(str) - position;

Solution 2

newptr - source will give you the offset.

Solution 3

char *source = "XXXXabcYYYY";
char *dest = strstr(source, "abc");
int pos;

pos = dest - source;

Solution 4

If you have the pointer to the first char of the substring, and the substring ends at the end of the source string, then:

• strlen(substring) will give you its length.
• substring - source will give you the start index.

Solution 5

Here is a C version of the strpos function with an offset feature...

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int strpos(char *haystack, char *needle, int offset);
int main()
{
    char *p = "Hello there all y'al, hope that you are all well";
    int pos = strpos(p, "all", 0);
    printf("First all at : %d\n", pos);
    pos = strpos(p, "all", 10);
    printf("Second all at : %d\n", pos);
}


int strpos(char *hay, char *needle, int offset)
{
   char haystack[strlen(hay)];
   strncpy(haystack, hay+offset, strlen(hay)-offset);
   char *p = strstr(haystack, needle);
   if (p)
      return p - haystack+offset;
   return -1;
}

View more solutions