Confused about this pointer arithmatic

[csstudent123]

/* strchr example */
#include <stdio.h>
#include <string.h>

int main ()
{
  char str[] = "This is a sample string";
  char * pch;
  printf ("Looking for the 's' character in \"%s\"...\n",str);
  pch=strchr(str,'s');
  while (pch!=NULL)
  {
    printf ("found at %d\n",pch-str+1);///what is this pch-str+1 doing?
    pch=strchr(pch+1,'s');
  }
  return 0;
}

[jlb]

///what is this pch-str+1 doing?

It is subtracting the address of str from pch then adding 1 to that result. Remember pch is a pointer to somewhere in the string whose starting address is str, this number should be the "index" value of the array.

[csstudent123]

thanks so he should use str type int, not string...