c - 计算 pi 的线程程序

Question

为类编写一个计算 pi 的程序。完成所有编码并运行程序 - 但对于某些值它不起作用。该程序应该接受 2 个参数，第一个是计算 pi 的元素数量，第二个是要使用的线程数量。它对于许多数字都可以完美地工作，但对于 1、2、5、6、7、8 和其他一些数字，它的计算效果不佳。这可能是循环中的限制，但我无法弄清楚。任何帮助表示赞赏。代码:

#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>

//global variables
int N, T;
double *vsum;

//pie function
void* pie_runner(void* arg)
{
    long j = (long)arg;    
    //double *limit_ptr = (double*) arg;
    //double j = *limit_ptr;

    //for(int i = (j-1)*N/T; i < N*(j) /T; i++)
    for(int i = (N/T)*(j-1); i < ((N/T)*(j)); i++)
    {

        if(i % 2 == 0){
            vsum[j] += 4.0/((2*j)*(2*j+1)*(2*j+2)); 
            //printf("vsum %lu = %f\n", j, vsum[j]);
                  }
        else{
            vsum[j] -= 4.0/((2*j)*(2*j+1)*(2*j+2));
            //printf("vsum %lu = %f\n", j, vsum[j]);
                  }


    }

    pthread_exit(0);
}

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

    if(argc != 3) {
        printf("Error: Must send it 2 parameters, you sent %d\n", argc-1);
        exit(1);
    }
    N = atoi(argv[1]);
    T = atoi(argv[2]); 

    vsum = malloc((T+1) * sizeof(*vsum));
    if(vsum == NULL) {
        fprintf(stderr, "Memory allocation problem\n");
        exit(1);
    }

    if(N <= T) {
        printf("Error: Number of terms must be greater then number of threads.\n");
        exit(1);    
    }

    for(int p=1; p<=T; p++)        //initialize array to 0
    {
        vsum[p] = 0;
    }

    double pie = 3.0;
    //launch threads
    pthread_t tids[T+1];

    for(long i = 1; i<=T; i++)
    {
        pthread_attr_t attr;
        pthread_attr_init(&attr);
        pthread_create(&tids[i], &attr, pie_runner, (void*)i);
    }

    //wait for threads...
    for(int k = 1; k<=T; k++)
    {
        pthread_join(tids[k], NULL);
    }

    for(int x=1; x<=T; x++)
    {
        pie += vsum[x];
    }

    printf("pi computed with %d terms in %d threads is %.20f\n", N, T, pie);

    //printf("pi computed with %d terms in %d threads is %20f\n", N, T, pie);

    free(vsum);
}

值不起作用:

./pie1 2 1pi computed with 2 terms in 1 threads is 3.00000000000000000000./pie1 3 1pi computed with 3 terms in 1 threads is 3.16666666666666651864 ./pie1 3 2pi computed with 3 terms in 2 threads is 3.13333333333333330373 ./pie1 4 2pi computed with 4 terms in 2 threads is 3.00000000000000000000 ./pie1 4 1pi computed with 4 terms in 1 threads is 3.00000000000000000000 ./pie1 4 3pi computed with 4 terms in 3 threads is 3.14523809523809516620 ./pie1 10 1pi computed with 10 terms in 1 threads is 3.00000000000000000000 ./pie1 10 2pi computed with 10 terms in 2 threads is 3.13333333333333330373 ./pie1 10 3pi computed with 10 terms in 3 threads is 3.14523809523809516620 ./pie1 10 4pi computed with 10 terms in 4 threads is 3.00000000000000000000./pie1 10 5pi computed with 10 terms in 5 threads is 3.00000000000000000000./pie1 10 6pi computed with 10 terms in 6 threads is 3.14088134088134074418 ./pie1 10 7pi computed with 10 terms in 7 threads is 3.14207181707181693042./pie1 10 8pi computed with 10 terms in 8 threads is 3.14125482360776464574 ./pie1 10 9pi computed with 10 terms in 9 threads is 3.14183961892940200045./pie1 11 2pi computed with 11 terms in 2 threads is 3.13333333333333330373 ./pie1 11 4pi computed with 11 terms in 4 threads is 3.00000000000000000000

Answer 1

我认为问题在于您在代码中使用了类似 N/T 的表达式。这里N和T是整数。 C 语言(以及许多其他语言)处理整数除法的方式与我们手工处理整数除法的方式不同。

例如，5/3 = 1，而不是 1.66667。要获得 1.66667，您可以将其中一个整数转换为 double。