String allocated with malloc is only returning the first character

Question

I'm writing a function to rearrange a string's characters. I allocated the new string with malloc and initialize it, and then return it. But when I return it, it always only returns the first character.

I've tried printing the characters of the new string one by one and they print out correctly, but when I call the function, only the first character is returned. I can't figure out why. Any help would be appreciated! Here's my code:

char * solution(char * s) {
    int len = strlen(s);
    int i;
    int index = 0;
    
    char *ans = (char *) malloc(sizeof(char) * (len + 1));
    if (ans == NULL) {
        fprintf(stderr, "Ran out of space in some function \n");
        exit(1);
    }
    //char* ans = (char *) malloc(len + 1);
    ans[len] = '\0';
    
    for(i = 0; i < len/2; i++){
        ans[index++] = s[i];
        
        ans[index++] = s[len - i];
    }
    if(len % 2 == 1){
        ans[index] = s[len/2];
    }
    return ans;
}

Answer 1

In the first iteration of this for loop

for(i = 0; i < len/2; i++){
    ans[index++] = s[i];
    
    ans[index++] = s[len - i];
}

the character ans[1] is set to s[len] (i is equal to 0 in the first iteration of the loop) that is to '\0'.

As a result you will get a string that contains only one character.

What you do is what you get.:)

It seems you mean

ans[index++] = s[len - i - 1];

Pay attention to that as the source string is not changed within the function then the function should be declared like

char * solution( const char * s );

The original declaration

char * solution(char * s);

means that the string will be changed in place.

If you want to change a string in place then the function can look the following way as shown in the demonstration program below.

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

char * solution( char *s )
{
    size_t n = strlen( s );

    for (size_t i = 1; i < n; i += 2)
    {
        char c = s[n - 1];
        memmove( s + i + 1, s + i, n - i - 1);
        s[i] = c;
    }

    return s;
}

int main( void )
{
    char s[] = "0123456789";
    puts( s );
    puts( solution( s ) );
}

The program output is

0123456789
0918273645