Question

我尝试使用多个进程编写Pi计算模拟。我有一个函数，生成从1到-1的随机双x，y数字，当我试图计算x ^ 2 + y ^ 2 <= 1时，但结果是一个巨大的数字，它总是大于1。

icpi.cpp（主文件）：

#include <mpi.h>
#include <iostream>
#include "main_header.h"
#include "dynamic.h"
using namespace std;

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

int numprocs, myid, i, root = 0, pointsInCircle = 0, pointsOutOfCircle = 0;
boolean pointInside = TRUE;
double point[DIMENSION], scatterTable[NUMBER_OF_SLAVES][DIMENSION], pi, t1, t2;
MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD,&myid);
MPI_Comm_size(MPI_COMM_WORLD,&numprocs);    
MPI_Status status;
srand(time(NULL));
t1 = MPI_Wtime();
printf("Trying to scatter..\n");
fflush(stdout);
MPI_Scatter(&scatterTable[0][0], DIMENSION, MPI_DOUBLE,     // Master process sends the first tasks to the slaves.
    &point[0], DIMENSION, MPI_DOUBLE, root, MPI_COMM_WORLD);
printf("Scatter successful\n");
fflush(stdout);
if (myid == 0) {
    for (i = 0; i < TOTAL_NUM_OF_POINTS; i++) {
        MPI_Recv(&pointInside, 1, MPI_INT, MPI_ANY_SOURCE, 0, MPI_COMM_WORLD, &status);
        if (pointInside) {
            pointsInCircle++;
            printf("point inside\n");
            fflush(stdout);
        }
        else {
            pointsOutOfCircle++;
            printf("point outside\n");
            fflush(stdout);
        }
        point[0] = randomPoint();       //  X component
        point[1] = randomPoint();       //  Y component
        printf("x = %f, y = %f\n", point[0], point[1]);
        fflush(stdout);
        MPI_Send(&point[DIMENSION], 2, MPI_DOUBLE, status.MPI_SOURCE, 0, MPI_COMM_WORLD);
    }
    point[0] = point[1] = END_OF_PI_CALC;
    for (i = 0; i < NUMBER_OF_SLAVES; i++)
        MPI_Send(&point[DIMENSION], 2, MPI_DOUBLE, status.MPI_SOURCE, 0, MPI_COMM_WORLD);
    t2 = MPI_Wtime();
    pi = calculatePi(pointsInCircle, pointsOutOfCircle);
    printf("Pi = %f.\n Time to calculate: %f", pi, (t2-t1));
}
else {
    while (point[0] != END_OF_PI_CALC || point[1] != END_OF_PI_CALC) {
        pointInside = withinCircle(point[0], point[1]);
        MPI_Send(&pointInside, 1, MPI_INT, 0, 0, MPI_COMM_WORLD);
        MPI_Recv(&point[DIMENSION], 2, MPI_DOUBLE, 0, 0, MPI_COMM_WORLD,     &status);
    }
}

if (myid == 0)
    MPI_Finalize();
return 0;
}

dynamic.cpp：

#include "dynamic.h"
#include <stdio.h>;
#include <math.h>
boolean withinCircle(double x, double y) {
    double result = (pow(x,(double)2) + pow(y, (double)2));
    printf("result = %f", result);
    fflush(stdout);
    if (result <= 1)
        return TRUE;
    return FALSE;
}

double randomPoint() {
    double range = (RANDOM_MAX - RANDOM_MIN);
    double div = RAND_MAX / range;
    return RANDOM_MIN + (rand() / div);
}

double calculatePi(int pointsWithin, int pointsOutside) {
    double pi = (pointsWithin / pointsOutside) * 4;
    return pi;
}

dynamic.h：

#ifndef DYNAMIC_H
#define DYNAMIC_H

#include "main_header.h"
#define TOTAL_NUM_OF_POINTS 20000
#define NUM_OF_POINTS_TO_SEND 1 
#define RANDOM_MAX 1
#define RANDOM_MIN -1
#define NUMBER_OF_SLAVES 3
#define DIMENSION 2
#define END_OF_PI_CALC 2

boolean withinCircle(double x, double y);

double randomPoint();

double calculatePi(int pointsWithin, int pointsOutside);

#endif DYNAMIC_H

main_header.h：

#ifndef MAIN_H
#define MAIN_H

#include <time.h>
#include <stdlib.h>

#define TRUE 1
#define FALSE 0


typedef int boolean;        //  Defines a boolean datatype.

#endif MAIN_H

来自wmpiexec的示例结果：

x = -0.944151, y = 0.389386
result = 17134570711043240115048918967982918960341742262641887592820622432866394633762913354831186160392586118427086670840176838705152.000000
point outside

（我知道实际计算的结果可能不对，只是一个例子）

Answer 1

double calculatePi(int pointsWithin, int pointsOutside) {
    double pi = (pointsWithin / pointsOutside) * 4;
    return pi;
}

这是错误的。发生的事情是你正在进行整数除法然后乘以4，然后然后将它转换为double。所以，你得到了地板行为。

在进行除法之前，您需要将pointsWithin或pointsOutside投射到双倍。

（这也是错误的公式，但这是另一回事。）

Answer 2

我误用了MPI_Send和MPI_Recv，现在正在计算权力。

计算pow（double，2）给出了一个巨大的数字（C）

2 个答案: