Copying odd-index elements from one array to another in C

Question

So I have a program where I have user-input string (a char array), and I need to copy all of the odd indexed elements to another array using a memory-allocated pointer-to-char-array.

(the charPointer needs to be allocated to the exact amount of space required using malloc)

char charArray[16];

printf("Enter string: ");
scanf("%16s", charArray);

char *charPointer = malloc((sizeof(charArray) / 2 ));
char *d = charPointer;

for (char *p = charArray; *p != '\0'; p++) {

    // Calculates odd-index element using bit operator
    if ((p - charArray) % 2 != 0){
        // Shows which elements should be copied
        printf("%c", *p);

        // Copy *p value to charPointer
        charPointer[p-charArray] = *p;
        charPointer++;
    }
}

while(*d != '\0')
   printf("%c\n",*d++);

But I'm getting strange results, like charPointer it's only copying the first odd index and not the rest. I thought I understood pointers but this really has me puzzled. I appreciate any help!

Answer 1

For starters this call

scanf("%16s", charArray);
       ^^^^ 

is incorrect. You need to write

scanf("%15s", charArray);
       ^^^^

This memory allocation

char *charPointer = malloc((sizeof(charArray) / 2 ));

allocates redundant memory. You should write

char *charPointer = malloc( strlen(charArray) / 2 + 1 );

These statements

    charPointer[p-charArray] = *p;
    charPointer++;

do not make a sense because they do not write sequentially characters in the dynamically pointed character array. You should write at least like

    *charPointer++ = *p;

As the terminating zero character '\0' was not appended to the dynamically allocated array then this while loop

while(*d != '\0')
   printf("%c\n",*d++);

invokes undefined behavior.

Also you should to free the allocated memory.

Here is a demonstration program.

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

int main(void) 
{
    char charArray[16]; 
    
    printf( "Enter a string: " );
    scanf( "%15s", charArray );
    
    char *charPointer = malloc( strlen( charArray ) / 2 + 1 );
    
    char *q = charPointer;
    
    for ( const char *p = charArray; *p != '\0'; ++p  )
    {
        if ( ( p - charArray ) % 2 != 0 )
        {
            *q++ = *p;
        }
    }
    
    *q = '\0';
    
    puts( charPointer );
    
    free( charPointer );
    
    return 0;
}

The program output might look like

Enter a string: 012345678901234
1357913