filter.c

#include <stdlib.h>
#include <stdio.h>
#include "filter.h"

IntArray *create_int_array_from(int *values, int length)
{
  IntArray *array = (IntArray *)malloc(sizeof(IntArray));
  array->values = (int *)malloc(sizeof(int) * length);
  array->length = length;
  for (size_t i = 0; i < array->length; i++)
  {
    array->values[i] = values[i];
  }
  return array;
}

void display_int_array(IntArray array)
{
  for (size_t i = 0; i < array.length; i++)
  {
    printf("%d ", array.values[i]);
  }
  printf("\n");
}

IntArray *filter_even(int *numbers, int length)
{
  int temp_evens[length];
  int number_of_evens = 0;

  for (size_t i = 0; i < length; i++)
  {
    if (numbers[i] % 2 == 0)
    {
      temp_evens[number_of_evens] = numbers[i];
      number_of_evens++;
    }
  }

  return create_int_array_from(temp_evens, number_of_evens);
}

filter.h

#ifndef __FILTER_H_
#define __FILTER_H_

typedef struct
{
  int *values;
  int length;
} IntArray;

IntArray *create_int_array_from(int *values, int length);
void display_int_array(IntArray array);
IntArray *filter_even(int *numbers, int length);

#endif

main.c

#include <stdio.h>
#include <stdlib.h>
#include "filter.h"

int main(void)
{
  int a[5] = {1, 2, 3, 4, 5};

  IntArray *evens = filter_even(a, 5);
  display_int_array(*evens);
  return 0;
}