Random float number

Question

I wrote this function to get a pseudo random float between 0 .. 1 inclusive:

float randomFloat()
{
      float r = (float)rand()/(float)RAND_MAX;
      return r;
}

However, it is always returning 0.563585. The same number no matter how many times I run my console application.

EDIT:

Here is my entire application if needed:

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

float randomFloat()
{
      float r = (float)rand() / (float)RAND_MAX;
      return r;
}

int main(int argc, char *argv[])
{
  float x[] = {
        0.72, 0.91, 0.46, 0.03, 0.12, 0.96, 0.79, 0.46, 0.66, 0.72, 0.35, -0.16,
        -0.04, -0.11, 0.31, 0.00, -0.43, 0.57, -0.47, -0.72, -0.57, -0.25,
        0.47, -0.12, -0.58, -0.48, -0.79, -0.42, -0.76, -0.77
  };

  float y[] = {
        0.82, -0.69, 0.80, 0.93, 0.25, 0.47, -0.75, 0.98, 0.24, -0.15, 0.01,
        0.84, 0.68, 0.10, -0.96, -0.26, -0.65, -0.97, -0.03, -0.64, 0.15, -0.43,
        -0.88, -0.90, 0.62, 0.05, -0.92, -0.09, 0.65, -0.76      
  };

  int outputs[] = {
      -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
      1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1
  };

  int patternCount = sizeof(x) / sizeof(int);

  float weights[2];
  weights[0] = randomFloat();
  weights[1] = randomFloat();

  printf("%f\n", weights[1]);

  float learningRate = 0.1;

  system("PAUSE");
  return 0;
}

Answer 1

drand48 is perfect for your usage, better than (float)rand()/RAND_MAX, because

The drand48() and erand48() functions return non-negative, double-precision, floating-point values, uniformly distributed over the interval [0.0 , 1.0].

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
int main() {
    int i;

    srand48(time(NULL));

    for (i = 0; i < 10; i++)
        printf("%g\n", drand48());

    return 0;
}

Answer 2

You have to initialize a random seed with

void srand ( unsigned int seed );

You can take for example the system time.

Answer 3

That is because the C random-generator is a pseudo-random generator (and that is, BTW, a very good thing, with many applications). (The ANSI C standard requires such a pseudo-random generator)

Essentially each time your console application is re-started it uses the same [default] seed for the random generator.

by adding a line like

  srand(time(NULL));

you will get a distinct seed each time, and the sequence of number produced with vary accordingly.
Note: you only need to call srand() once, not before each time you call rand().

Edit: (Test/Debug-time hint)
[from a remark by Artelius] For testing purposes, it's best to srand(SOME_CONST), and change the value of the constant between runs. This way if a bug manifests itself due to some combination of random numbers, you'll be able to reproduce it.

Answer 4

You need to call

srand(time(NULL));

before using rand for the first time. time is in <time.h>.

EDIT: As Jonathan Leffler points out, this is easily predicted, so don't try using it for cryprography.

Answer 5

Do you call it more than once? rand() will always return the same pseudo random. You might want to call srand() with some nice seed, like the current time.