c - 生成允许在给定范围内重复的随机数

Question

我正在编写一个 MPI 程序，其中我允许每个进程在我想要一些进程的耗时内生成它的自己的随机数由一些其他进程生成相同的随机数。例如，一个简单的 printf 语句:

Process 1 has generated the number 19
Process 2 has generated the number 8
Process 3 has generated the number 19
.
.

因此，进程1和3产生了相同的随机数。此外，为了确保我可以获得相同的随机数生成，我将限制为仅整数值，而不是 double 值或 float 。

我写过:

#include <stdio.h>
#include "mpi.h"
#include <stdlib.h>
#include <time.h>
#define numprocess 20

int main(int argc, char *argv[])
{
    int rank,size;
    int rand_value;
    int lower,upper;

    MPI_Status status;

    MPI_Init(&argc,&argv);
    MPI_Comm_rank(MPI_COMM_WORLD,&rank);
    MPI_Comm_size(MPI_COMM_WORLD,&size);

    upper = 20;
    lower = 0;
    rand_value = rand()%(upper-lower);
    printf("Process %d generated a random number of %d\n",rank,rand_value);

    MPI_Finalize();
    return 0;
}

但我得到的输出是:

Process 0 generated a random number of 7
Process 2 generated a random number of 7
Process 8 generated a random number of 7
Process 1 generated a random number of 7
Process 3 generated a random number of 7
Process 4 generated a random number of 7
Process 5 generated a random number of 7
Process 6 generated a random number of 7
Process 7 generated a random number of 7
Process 9 generated a random number of 7
Process 11 generated a random number of 7
Process 10 generated a random number of 7
Process 16 generated a random number of 7
Process 17 generated a random number of 7
Process 19 generated a random number of 7
Process 13 generated a random number of 7
Process 12 generated a random number of 7
Process 18 generated a random number of 7
Process 15 generated a random number of 7
Process 14 generated a random number of 7

Answer 1

你可以这样做:

srand(time(NULL)+rank * 10000);

使用 time 将确保您在每次运行时获得不同的输出，而使用 rank 可以确保(很有可能但不是 100%)每个线程都有一个不同的种子。常数 10000 是一个任意常数，只是为了降低两个线程意外获得相同种子的可能性。

但这是一个更好的方法:

const int current_time = time(NULL);

MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD,&rank);
MPI_Comm_size(MPI_COMM_WORLD,&size);

srand(current_time+rank);

或者使用MPI_Bcast

int current_time;

MPI_Init(&argc,&argv);
MPI_Comm_rank(MPI_COMM_WORLD,&rank);
MPI_Comm_size(MPI_COMM_WORLD,&size);

if(rank == 0)
    current_time = time(NULL);

MPI_Bcast(&current_time, 1, MPI_INT, 0, MPI_COMM_WORLD);
srand(current_time+rank);