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;
      retur         


        

Answer 1

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 2

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.

Answer 3

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 4

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 5

You have to initialize a random seed with

void srand ( unsigned int seed );

You can take for example the system time.