获取输入数组返回错误结果

Question

I try to evaluate the pi value using Monte Carlo method. More precisely, using the circumference method.

Initially I wrote a code that the number of iteration is fixed.Now, I'm trying to simulate the behavior of the code at the growing of the input.

To do this, I write this code, but it doesn't work:

#define _XOPEN_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <mpi.h>

#define SEED time(NULL)

int main(int argc, char **argv) {

  int rank, size;
  unsigned int seed;
  double x, y, start, end;
  long long int i, j, all_point, points = 0, all_intern;

  long long int dim[] = {5, 50, 500, 5000, 50000, 500000, 5000000, 50000000};

  MPI_Init(&argc, &argv);
  start = MPI_Wtime();

  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
  MPI_Comm_size(MPI_COMM_WORLD, &size);

  seed = SEED + rank;

  for ( j = 0; j < 8; j++ ) {
    for ( i = 0; i < dim[j]; i++ ) {
      x = (double) rand_r(&seed) / RAND_MAX;
      y = (double) rand_r(&seed) / RAND_MAX;

      if ( x * x + y * y <= 1.0 ) points++;
    }

    MPI_Reduce(&points, &all_intern, 1, MPI_LONG_LONG_INT, MPI_SUM, 0, MPI_COMM_WORLD);
    all_point = dim[j] * size;

    MPI_Barrier(MPI_COMM_WORLD);
    end = MPI_Wtime();

    if ( rank == 0 ) {
      printf("\u03C0 \u2248 %Lf\n", (long double) all_intern / all_point * 4.0);
      printf("Time elapsed: %.4f\n", end - start);
    }
  }

  MPI_Finalize();
  return 0;
}

Answer 1

The issue is that the the value of points is not reset to zero between each iteration, while all_point counts only the total number of point of the current iteration. So either you reset the value of points , either you correctly compute all_point . Since it was no clear in the comments, below you find the two variants with comments in the code where I made modifications. I removed all the MPI content since it's not an MPI issue.

Variant 1: Reset the counter of points in the circle

#define _XOPEN_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define SEED time(NULL)

int main(int argc, char **argv) {

  unsigned int seed;
  double x, y, start, end;
  long long int i, j, all_point, points = 0, all_intern;

  long long int dim[] = {5, 50, 500, 5000,50000,500000,5000000,50000000};

  seed = SEED + 1;

  for ( j = 0; j < 8; j++ ) {
    // reset points to 0 here
    points=0;
    for ( i = 0; i < dim[j]; i++ ) {
      x = (double) rand_r(&seed) / RAND_MAX;
      y = (double) rand_r(&seed) / RAND_MAX;

      if ( x * x + y * y <= 1.0 ) points++;
    }

    all_point = dim[j];

    printf("\u03C0 \u2248 %Lf\n", (long double) points / all_point * 4.0);

  }

  return 0;
}

Variant 2: Keep results from previous iterations but adjust the total count of points

#define _XOPEN_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define SEED time(NULL)

int main(int argc, char **argv) {

  unsigned int seed;
  double x, y, start, end;
  long long int i, j, all_point, points = 0, all_intern;

  long long int dim[] = {5, 50, 500, 5000,50000,500000,5000000,50000000};

  seed = SEED + 1;

  for ( j = 0; j < 8; j++ ) {
    for ( i = 0; i < dim[j]; i++ ) {
      x = (double) rand_r(&seed) / RAND_MAX;
      y = (double) rand_r(&seed) / RAND_MAX;

      if ( x * x + y * y <= 1.0 ) points++;
    }
    // Correctly count the total of points used
    // since the beginning
    all_point=0;
    for ( int k = 0; k <= j; k++ ) {
        all_point += dim[k];
    }

    printf("\u03C0 \u2248 %Lf\n", (long double) points / all_point * 4.0);

  }

  return 0;
}

In both cases, the output is similar and correct.