OpenMP 之reduction 子句求数值积分圆周率（pi）（学习笔记）

#include "stdafx.h"
#include <stdio.h>
#include <time.h>
#include <stdlib.h>
#include <omp.h>

static long num_steps = 100000000;
double step;
#define NUM_THREADS 2

int _tmain(int argc, _TCHAR* argv[])
{
	int i;
	clock_t t1,t2;
	double x,pi,sum = 0.0;

	step = 1.0/(double) num_steps;
	omp_set_num_threads(NUM_THREADS);

	t1 = clock();
#pragma omp parallel for reduction(+:sum) private(x)
	for(i= 0;i< num_steps ;i++)
	{
		x=(i+0.5)*step;
		sum = sum + 4.0/(1.0 + x*x);
	}
	pi = step * sum;
	t2 = clock();
	printf("pi = %.15f\n",pi);
	printf("parallel time=%d\n",(t2-t1));

	t1 = clock();
	sum =0;
	for (int i=0;i <num_steps;i++)
	{
		x=(i+0.5)*step;
		sum = sum + 4.0/(1.0 + x*x);
	}
	pi = step * sum;
	t2 = clock();
	printf("pi = %.15f\n",pi);
	printf("serial time=%d\n",(t2-t1));
	printf("wo shi xingxing\n");
	system("pause");
	return 0;
}

#define NUM_THREADS 4