Assigning value to pointer correctly in C using strcpy() - arrays

Question

I need to take only the odd values from a char array and copy them into correctly sized dynamic memory using a pointer.
However when running my program it works correctly with certain input strings and not with others. Is there something that I'm doing wrong? I can't seem to figure out what's going on.
/* A.) Include the necessary headers in our program */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_STRING_LENGTH 32
int main() {
/* B.) Declare char array with inital size of 32 */
char input_string[MAX_STRING_LENGTH];
/* C.) Recieve user input.
Can save the first 31 characters in the array with 32nd reserved for '\0' */
printf("Enter a string of characters: ");
/* D.) Using the technique we discussed to limit the string to 31 charaters */
scanf("%31s", input_string);
printf("\n");
/* Will be used to determine the exact amount of dynamic memory that will be allocated later */
int odd_value_count = 0;
printf("Odd Characters: ");
for(int i = 0; i < strlen(input_string); i++) {
if(i % 2 != 0) {
printf("%c ", input_string[i]);
odd_value_count++;
}
}
printf("\n");
printf("Odd value count: %d\n", odd_value_count);
/* E.) Delecaring the pointer that will hold some part of the input_string
Pointer will be a char type */
char *string_pointer;
/* G.) Allocating the space before the copy using our odd value count */
/* H.) The exact amount of space needed is the sizeof(char) * the odd value count + 1 */
string_pointer = (char *)malloc(sizeof(char) * (odd_value_count + 1));
if (string_pointer == NULL) {
printf("Error! Did not allocte memory on heap.");
exit(0);
}
/* F.) Copying all charcters that are on the odd index of the input_string[] array
to the memory space pointed by the pointer we delcared */
printf("COPIED: ");
for (int i = 0; i < strlen(input_string); ++i) {
if(i % 2 != 0) {
strcpy(string_pointer++, &input_string[i]);
printf("%c ", input_string[i]);
}
}
/* Printing out the string uses the pointer, however we must subtract odd_value_count to
position the pointer back at the original start address */
printf("\n%s\n", string_pointer - odd_value_count);
return 0;
}
This input string: 01030507
works fine and copies & prints: 1357
The input string: testing
Copies etn but prints etng.
I cant figure out why for some strings it prints out the extra character at the end when I never even copy the value over.

Answer 1

You need to Null Terminate your string, like this *string_pointer = '\0';, just after you are done copying the odd characters in your string pointer - after that loop, null terminate your string.
Read more in How to add null terminator to char pointer, when using strcpy?

Answer 2

In the end of your routine you will need to null terminate the string, otherwise you don't have a string you just have a char array, you can use string_pointer which is already pointing to one past the end of the string you want to save:
//...
for (int i = 0; i < strlen(input_string); ++i) {
if(i % 2 != 0) {
strcpy(string_pointer++, &input_string[i]);
//as you are copying characters, you can do this:
//*string_pointer++ = input_string[i];
//instead of strcpy
printf("%c ", input_string[i]);
}
}
*string_pointer = '\0'; // <-- here
//...