#include <stdio.h>
#include <string.h>
int main(void)
{
char s1[] = "f35";
char s2[] = "121258y";
// found the number 5 from s2[] in s1[2]
/* return the number of characters of s1[] that read before first occurrence
* of the number 5
*/
printf("The first occurrence in s1 is at index %u\n", (int)strcspn(s1, s2));
return 0;
}
/*
run:
The first occurrence in s1 is at index 2
*/