Swapping and copying two strings into one another without using strcpy() in C

Question

this program is meant to take two strings and call a function that copies string 1 into string2, and vice versa, while fulfilling the following criteria:

• Uses pointers
• Uses only while loops
• does not use strcpy();

This is what I have, but nothing shows up. How can I fix this?

#include<stdio.h>

void string_copier(char *a, char *b);

int main(){
    char string1[100], string2[100];
    
    /*read strings*/
    printf("Enter a string less than 100 characters.\n");
    gets(string1);
    //scanf("%s", &string1);
    printf("Enter another string less than 100 characters.\n");
    gets(string2);
    //scanf("%s", &string2);
    
    /*call function*/
    string_copier(string1, string2);
    
    /*print new strings*/
    printf("%s\n", string1);
    printf("%s\n", string2);
}

void string_copier(char *a, char *b){
    int i = 0;
    char *swap;

    /*assign string a to swap variable*/
    while(*(a + i) != '\0'){
        *(swap + i) = *(a + i);
        i++;
    } 
    *(swap + i) = '\0';
    
    /*redefine i to use in next while loop*/
    i = 0;
    
    /*assign b to a*/
    while(*(b + i) != '\0'){
        *(a + i) = *(b + i);
        i++;
    }
    *(a + i) = '\0';
    
    i = 0;
    
    /*assign swap to b*/
    while(*(swap + i) != '\0'){
        *(b + i) = *(swap + i);
        i++;
    }
    *(b + i) = '\0';
}

Thank you.

Answer 1

For starters the function gets is unsafe and is not supported by the C Standard. Instead use either fgets or scanf.

The pointer swap is uninitialized and has an indeterminate value

char *swap;

So the following code within the function where the pointer is dereferenced invokes undefined behavior.

Also using expressions like this *(a + i) with the auxiliary variable i is the same as using the subscript operator a[i] that is the pointers themselves in the function are not increased. But according to the assignment you need to use only pointers to access elements of the passed strings.

The function can be defined for example the following way as shown in the demonstration program below.

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

void string_swap( char *s1, char *s2 )
{
    while ( *s1 && *s2 )
    {
        char c = *s1;
        *s1++ = *s2;
        *s2++ = c;
    }

    char c = *s1;
    *s1 = *s2;
    *s2 = c;

    if (*s1 != '\0')
    {
        while (*s1 != '\0')
        {
            *++s1 = *++s2;
        }
    }
    else
    {
        while (*s2 != '\0')
        {
            *++s2 = *++s1;
        }
    }
}

int main( void )
{
    char s1[10] = "Hello";
    char s2[10] = "everybody";

    printf( "%zu: %s\n", strlen( s1 ), s1 );
    printf( "%zu: %s\n", strlen( s2 ), s2 );

    string_swap( s1, s2 );

    printf( "%zu: %s\n", strlen( s1 ), s1);
    printf( "%zu: %s\n", strlen( s2 ), s2 );

    string_swap( s1, s2 );

    printf( "%zu: %s\n", strlen( s1 ), s1 );
    printf( "%zu: %s\n", strlen( s2 ), s2 );
}

The program output is

5: Hello
9: everybody
9: everybody
5: Hello
5: Hello
9: everybody

As you can see the function itself does not use any C standard string function.