How do I delete each character from a string that matches any character in another string

Question

This is the code that I came up with, but sometimes it does not work.

For example, if string1 = highway to hell, and string2 = stairway to heaven, the output is 'ghll' despite 'h' being in the second string as well as the first string. I cant figure out why h is not being removed like other characters that are in both strings.

#include <stdio.h>
#include <string.h>
main()
{
    char string1[30], string2[30];
    int length1, length2;

    printf("Enter string 1: ");
    gets(string1);

    printf("Enter string 2: ");
    gets(string2);

    length1 = strlen(string1);
    length2 = strlen(string2);

    for (int i = 0; i <= length1; i++)
    {
        for (int j = 0; j <= length2; j++)
        {
            if (string1[j] == string2[i])
            {
                for (int k = j; k <= length1; k++)
                {
                    string1[k] = string1[k + 1];
                }
            }
        }
    }

    printf("Deleted characters of string 2 from string 1: %s", string1);
}

`gets()` is bad. Use [`fgets()`](https://en.cppreference.com/w/c/io/fgets) instead. — Zakk, Apr 09 '22 at 19:48
You may want to read this: [Why is the gets function so dangerous that it should not be used?](https://stackoverflow.com/q/1694036/12149471) — Andreas Wenzel, Apr 09 '22 at 21:04
If one string contains the upper-case character `H`, should the corresponding lower-case character `h` be filtered from the other string? — Andreas Wenzel, Apr 09 '22 at 21:06

जलजनक · Accepted Answer · 2022-04-09T21:30:21.243

Use a lookup table to efficiently apply character filter.

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

int main() {
    char s1[] = "highway to hell";
    char s2[] = "stairway to heaven";

    printf ("S1: %s\nS2: %s\n", s1, s2);

    char alt[256] = {0};
    //build a lookup table for efficient filtering
    for (int ci = 0; s2[ci]; )
        alt[(unsigned char)s2[ci++]] = 1;

    int slen = 0;
    for (int ci = 0; s1[ci]; ++ci) {
        if (alt[(unsigned char)s1[ci]]) continue;
        s1[slen++] = s1[ci];
    }
    s1[slen] = '\0';
    printf ("S1 after S2 filter : %s\n", s1);

    return 0;
}

gets() has been deprecated, use fgets() instead.
fgets() : If a newline(\n) is read, it is stored into the buffer. Trim the buffer before use : Removing trailing newline character from fgets() input

`int s1len = strlen (s1); int s2len = strlen (s2);` not needed. Perhaps `for (int ci = 0; ci < s2len; )` --> `for (int ci = 0; s2[ci]; )` instead. — chux - Reinstate Monica, Apr 09 '22 at 20:06

How do I delete each character from a string that matches any character in another string

1 Answers1