#include <stdio.h>

int is_even(int);

void sum (double a[],
          int n,
          double *even_index_sum_ptr,
          double *odd_index_sum_ptr){
  int i;

  for(i = 0; i < n; i++){
    if (is_even(i))  
      *even_index_sum_ptr += a[i];
    else *odd_index_sum_ptr += a[i];
  }
}

int main(void) {
  double even_sum = 0.0, odd_sum = 0.0;

  double a[5] = {0.5, 7.7, 8.3, 3.2, 4.0}; 

  sum(a, 5, &even_sum, &odd_sum);

  printf("Even sum: %f. Odd sum: %f.\n\n", even_sum, odd_sum);
  
  return 0;
}

int is_even(int i){
  return i%2 == 0;
}