並行求pi (C++實現)

用OpenMP並行化求pi的代碼，這裏用的是公式法求pi。具體如下：

//公式法
#include<omp.h>
#include<stdio.h>
#include<stdlib.h>

#define n_threads  2   //這裏線程數設爲2
static int num_steps = 10000000;  //步數

int main() {
	int i;
	double sum = 0.0;
	double x;
	double pi;
	double factor;
	omp_set_num_threads(n_threads);  //並行化
#pragma omp parallel for reduction(+:sum) private(x, factor)
	for (i = 0;i<num_steps;i++)
	{
		if (i % 2 == 0)
			factor = 1.0;
		else
			factor = -1.0;
		x = factor / (2 * (double)(i)+1);
		sum = sum + x;
	}
	pi = 4 * sum;
	printf("when num_steps is %d pi is: %2.8f\n", num_steps, pi);
	return 0;
}

得到的結果爲：3.14159255