Question

为计算pi的课程编写程序。完成所有编码并运行程序 - 但对于某些值，它不起作用。程序应该包含2个参数，第一个是计算pi的元素数，第二个是要使用的线程数。它对于许多数字都可以完美地运行，但是对于1,2,5,6,7,8和其他一些数据来说它的计算效果不佳。它可能是循环中的限制，但我无法弄明白。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 1
pi computed with 2 terms in 1 threads is 3.00000000000000000000
./pie1 3 1
pi computed with 3 terms in 1 threads is 3.16666666666666651864
 ./pie1 3 2
pi computed with 3 terms in 2 threads is 3.13333333333333330373
 ./pie1 4 2
pi computed with 4 terms in 2 threads is 3.00000000000000000000
 ./pie1 4 1
pi computed with 4 terms in 1 threads is 3.00000000000000000000
 ./pie1 4 3
pi computed with 4 terms in 3 threads is 3.14523809523809516620
 ./pie1 10 1
pi computed with 10 terms in 1 threads is 3.00000000000000000000
 ./pie1 10 2
pi computed with 10 terms in 2 threads is 3.13333333333333330373
 ./pie1 10 3
pi computed with 10 terms in 3 threads is 3.14523809523809516620
 ./pie1 10 4
pi computed with 10 terms in 4 threads is 3.00000000000000000000
./pie1 10 5
pi computed with 10 terms in 5 threads is 3.00000000000000000000
./pie1 10 6
pi computed with 10 terms in 6 threads is 3.14088134088134074418
 ./pie1 10 7
pi computed with 10 terms in 7 threads is 3.14207181707181693042
./pie1 10 8
pi computed with 10 terms in 8 threads is 3.14125482360776464574
 ./pie1 10 9
pi computed with 10 terms in 9 threads is 3.14183961892940200045
./pie1 11 2
pi computed with 11 terms in 2 threads is 3.13333333333333330373
 ./pie1 11 4
pi 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）对待整数dividion的方式与我们手工处理方式不同。

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双精度。

计算pi的线程程序

1 个答案: