OpenMP求解Pi的例子

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#include <iostream>
#include "omp.h"
using namespace std;
/*
    Exercise 2, Serial PI program to parallel PI program
*/
static long num_steps = 1000000;
double step;
void main()
{
    int i;
    double x, pi, sum = 0;
    double time_start, time_end;
    step = 1.0/(double)num_steps;
    //Serial version
    time_start = omp_get_wtime();
    for (i = 0; i < num_steps; i++)
    {
        x = (i+0.5)*step;
        sum = sum + 4.0/(1.0+x*x);
    }
    pi = step*sum;
    time_end = omp_get_wtime();
    cout << scientific << "Calculated results = " << pi << endl;
    cout << "Calculated times = " << (time_end - time_start)
        << endl;
    // Parallel version
    sum = 0.0;
    omp_set_num_threads(4);
    time_start = omp_get_wtime();
#pragma omp parallel for reduction(+:sum)
    for (i = 0; i < num_steps; i++)
    {
        x = (i+0.5)*step;
        sum = sum + 4.0/(1.0+x*x);
    }
    pi = step*sum;
    time_end = omp_get_wtime();
    cout << "Calculated results = " << pi << endl;
    cout << "Calculated times = " << (time_end - time_start)
        << endl;
    cin.get();
    return;
}

VS2012结果：
Calculated results = 3.141593e+000
Calculated times = 4.392572e-002
Calculated results = 3.141593e+000
Calculated times = 3.620780e-002