Why is my code not giving me correct results

Question

I have this code that is supposed to swap between two consecutive strings in an array of strings. I am getting this output:

adf
nmmmmmmm
gh
11kk%$#
abcd

It's supposed to give me this output

adf
nmmmmmmmMMMMMM
gh
11kk%$#
abcd

This is my code:

#include <stdio.h>

void swap( char **t1,char **t2)
{
    char *t;
    t = *t1;

    *t1 = *t2;
    *t2 =t;
}

int main() {
    char array[][21]={"abcd","adf","nmmmmmmmMMMMMM","gh","11kk%$#"};
    
    for(int i=0;i<4;i++){
        swap(&array[i],&array[i+1]);
    }
    
    for(int i=0;i<5;i++){
        printf("%s\n",array[i]);
    }
    
    return 0;
}

Anyone have an idea why?

Answer 1

The function declaration and its call are incorrect.

In this call

swap(&array[i],&array[i+1]);

the arguments have the type char ( * )[21] while the function parameters have the type char **. And there is no implicit conversion from one type to another.

What you need is to swap strings by exchanging contents of character arrays using the standard C string function strcpy.

The function can be declared and defined the following way

#include <string.h>

//...

enum { N = 21 };

void swap( char *t1, char *t2 )
{
    char t[N];
    strcpy( t, t1 );
    strcpy( t1, t2 );
    strcpy( t2, t );
}

and the function should be called like

swap( array[i], array[i+1]);

Here is a demonstrative program.

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

enum { N = 21 };

void swap( char *t1, char *t2 )
{
    char t[N];
    strcpy( t, t1 );
    strcpy( t1, t2 );
    strcpy( t2, t );
}

int main(void) 
{
    char array[][N] = { "abcd", "adf", "nmmmmmmmMMMMMM", "gh", "11kk%$#" };
    
    for(int i=0;i<4;i++){
        swap( array[i], array[i+1] );
    }
    
    for(int i=0;i<5;i++){
        puts( array[i] );
    }
    
    return 0;
}

The program output is

adf
nmmmmmmmMMMMMM
gh
11kk%$#
abcd