Simple puzzle

puzzle.c

#include <stdio.h>
#include <assert.h>
#include <math.h>
#include <stdbool.h>

#define N 4

static bool
double_equal(double a, double b, double epsilon)
{
  return fabs(a - b) < epsilon;
}

static void
init_arrays(double *arr1, double *arr2)
{
  for (size_t i = 0; i < N; i++) {
    arr1[i] = 0.5;
    arr2[i] = 1.1;
  }
}

static double
do_seq(double start, double *in1, double *in2)
{
  double ans = start;
  for (size_t i = 0; i < N; i++)
    ans += in1[i] * in2[i];
  return ans;
}

static double
do_par(double start, double *in1, double *in2)
{
  double ans = start;
  #pragma omp parallel for reduction(+:ans) schedule(static, 2) num_threads(2)
  for (size_t i = 0; i < N; i++)
    ans += in1[i] * in2[i];
  return ans;
}

static void
test_parallel(void)
{
  double arr1[N], arr2[N];
  init_arrays(arr1, arr2);

  assert(double_equal(
    do_seq(10, arr1, arr2),
    do_par(10, arr1, arr2),
    1e-15
  ));
}

int
main()
{
  test_parallel();
  return 0;
}