sample openMP

#include <iostream>
#include <boost/timer.hpp>
#include <math.h>
#include <omp.h>

using namespace std;

double single_thread(double a[], const int &niter)
{
    double sum = 0.0;
    double x;
    for (int i = 0; i < niter; i++)
    {
        for (int j = 0; j < niter; j++)
        {
            for (int k = 0; k < niter; k++)
            {
                x = sin(a[i]) * cos(a[j]) * sin(a[k]);
                sum += x;
            }
        }
    }
    return sum;
}

double omp_reduction(double a[], const int &niter, const int &num_treads)
{
    double sum = 0;
    double x;
    int i, j, k;
    omp_set_num_threads(num_treads);

#pragma omp parallel for reduction(+:sum) shared(a) private(x, i, j, k)
    for (i = 0; i < niter; i++)
    {
        for (j = 0; j < niter; j++)
        {
            for (k = 0; k < niter; k++)
            {
                x = sin(a[i]) * cos(a[j]) * sin(a[k]);
                sum = sum + x;
            }
        }
    }
    return sum;
}

double omp_sections(double a[], const int &niter)
{
    double sum1 = 0;
    double sum2 = 0;
    //    double x;

#pragma omp parallel sections num_threads(2)
    {
#pragma omp section
        {
            for (int i = 0; i < niter / 2; i++)
            {
                for (int j = 0; j < niter; j++)
                {
                    for (int k = 0; k < niter; k++)
                    {
                        double x = sin(a[i]) * cos(a[j]) * sin(a[k]);
                        sum1 += x;
                    }
                }
            }
        }
#pragma omp section
        {
            for (int i = niter / 2; i < niter ; i++)
            {
                for (int j = 0; j < niter; j++)
                {
                    for (int k = 0; k < niter; k++)
                    {
                        double x = sin(a[i]) * cos(a[j]) * sin(a[k]);
                        sum2 += x;
                    }
                }
            }
        }
    }
    return sum1 + sum2;
}

int main()
{
    boost::timer t;
    const int niter = 1000;
    double a [niter];
    double sum = 0;
    for (long i = 0; i < niter; i++)
    {
        a[i] = (double)i / (double)niter;
    }

    t.restart();
    sum = single_thread(a, niter);
    cout << "Serial. Sum: " << sum << " time: " << t.elapsed() << endl;

    t.restart();
    sum = omp_reduction(a, niter, 2);
    cout << "Reduction, 2 threads. Sum: " << sum << " time: " << t.elapsed() << endl;

    t.restart();
    sum = omp_sections(a, niter);
    cout << "Two sections. Sum: " << sum << " time: " << t.elapsed() << endl;

    return 0;
}