			#include <chrono>
			#include <cmath>
			#include <cstdio>

			#if defined(_OPENMP)
			#include <omp.h>
			#endif

			using clk = std::chrono::high_resolution_clock;
			using second = std::chrono::duration<double>;
			using time_point = std::chrono::time_point<clk>;

			inline int digit(double x, int n) {
			return std::trunc(x * std::pow(10., n)) -
			std::trunc(x * std::pow(10., n - 1)) * 10.;
			}

			inline double f(double a) { return (4. / (1. + a * a)); }

			const int n = 10000000;

			int main(int /* argc /, char * /* argv */) {
			int i;
			double dx, x, sum, lsum, pi;

			#if defined(_OPENMP)
			int num_threads = omp_get_max_threads();
			#endif

			#if defined(_OPENMP)
			auto omp_t1 = omp_get_wtime();
			#endif
			auto t1 = clk::now();

			sum = 0.;
			#pragma omp parallel shared(sum) private(x, lsum)
			{
			/* calculate pi = integral [0..1] 4 / (1 + x*2) dx /
			dx = 1. / n;
			lsum = 0.0;
			#pragma omp for
			for (i = 1; i <= n; i++) {
			x = (1. * i - 0.5) * dx;
			lsum = lsum + f(x);
			}

			#pragma omp critical
			sum += lsum;
			}

			pi = dx * sum;

			#if defined(_OPENMP)
			auto omp_elapsed = omp_get_wtime() - omp_t1;
			#endif
			second elapsed = clk::now() - t1;

			std::printf("computed pi = %.16g\n", pi);
			#if defined(_OPENMP)
			std::printf("wall clock time (omp_get_wtime) = %.4gs on %d threads\n",
			omp_elapsed, num_threads);
			#endif
			std::printf("wall clock time (chrono) = %.4gs\n", elapsed.count());

			for (int d = 1; d <= 15; ++d) {
			std::printf("%d", digit(pi, d));
			}

			return 0;
			}

scitas-intro-to-parallelism/openmp/C/.solutions/pi.cc
327098198a93complete

pi.cc
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