How to check if string2 has characters not in string1?

Question

char String1 = "1234567890+-";
char String2 = "1+a";

String2 is an input and I want to check if it contains any character that is not in String1.

I have tried using strpbrk(String2,String1) but this returns true as long as any character in String1 exists in String2 .

Is there a better way to do it?

Answer 1

strspn will return the index of the first character in String2 that is not in String1. If all the characters match, the index will be to the terminating zero.

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

int main ( void) {
    char *String1 = "1234567890+-";
    char *String2 = "1+a";
    int index = strspn ( String2, String1);
    if ( String2[index]) {
        printf ( "character \'%c\' not found in %s\n", String2[index], String1);
    }
    return 0;
}

Answer 2

You can use a simplified version of union-find .

First, you loop over string1 and mark all the characters with 1 . I write simplified code, you can complete it.

char mark[255];
for(s=string1; s; s++) mark[*s]=1;

Next, you loop over string2 and check if the current char is marked.

for(s=string2; s; s++) if (!mark[*s]) printf("%c", *s);

This will print all chars from string2 that are not in string1 .