pseudo-random number generator rand() in .mex

In standard C, the following code generate 5 random numbers.

#include <stdio.h>
#include <stdlib.h>
int main (int argc, char *argv[])
{
/* Now generate 5 pseudo-random numbers */
int i;
for (i=0; i<5; i++)
{
  printf ("rand[%d]= %u\n",
    i, rand ());
}
return 0;
}

Each time you run the program, same serial of 5 numbers will be generated since the seed is retested. However, the same code as a mex function does not behave like this. Here is the mex code:

#include <stdio.h>
#include <stdlib.h>
#include "mex.h" 

void mexFunction(int nlhs, mxArray *plhs[],
    int nrhs, const mxArray *prhs[]) {

    /* Now generate 5 pseudo-random numbers */
    int i;
    for (i=0; i<5; i++)
    {
      mexPrintf ("rand[%d]= %u\n",
        i, rand ());
    }
    return;
  }

Every time this mex function produces different serial of 5 random numbers. Does anybody know why? How the mex function resets the seed number each time? Thanks in advance.

[Elliott Rachlin]

I notice that in a mex file, the code "rand()" seems to return an integer between 0 and 32767, not the value between 0.0 and 1.0 that I was expecting. Does anybody know why this is (and why the documentation does not seem to mention this)?

[Walter Roberson]

https://www.tutorialspoint.com/c_standard_library/c_function_rand.htm

That is, you are invoking C's rand(), not invoking MATLAB's rand()

Answer 1

[James Tursa]

When you first run a mex function it loads into memory and stays in memory until it gets cleared. So any variables that are global or persistent in the mex function, like a random number generator seed that the rng internally maintains, will retain the last value between calls. So you get a new set of numbers with each call. If you want to start fresh each time you can either clear the mex function from memory or in the mex code itself reinitialized the rng each time you enter the mex function.

Answer 2

[James]

Thanks.

If I use the following mex function, seedit_mex, to set the seed each time before calling rand() in another mex functions.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include "mex.h" 

void mexFunction(int nlhs, mxArray *plhs[],
    int nrhs, const mxArray *prhs[]) {

    unsigned int iseed;
    /* Simple "srand()" seed: just use "time()" */
  iseed = (unsigned int)time(NULL);
  srand (iseed);
  mexPrintf ("%d\n", iseed);
  return;
}

I guess it will work--seedit_mex will reset the global seed and will affect all other mex function calling rand(), until I do:

clear seedit_mex
clear other_mex_function

Am I right?

[James Tursa]

Yes and no. Using srand (or rand) in one mex routine will affect other mex routines that use rand. However, there is apparently another piece of the library hanging around in memory even after you clear the mex routines, so at least in my testing you will not get a fresh start this way by just doing clear. You must use srand.