How to generate random float number in C

Question

I can't find any solution to generate a random float number in the range of [0,a], where a is some float defined by a user.

I have tried the following, but it doesn't seem to work correctly.

float x=(float)rand()/((float)RAND_MAX/a) 

Answer 1

Try:

float x = (float)rand()/(float)(RAND_MAX/a); 

To understand how this works consider the following.

N = a random value in [0..RAND_MAX] inclusively. 

The above equation (removing the casts for clarity) becomes:

N/(RAND_MAX/a) 

But division by a fraction is the equivalent to multiplying by said fraction's reciprocal, so this is equivalent to:

N * (a/RAND_MAX) 

which can be rewritten as:

a * (N/RAND_MAX) 

Considering N/RAND_MAX is always a floating point value between 0.0 and 1.0, this will generate a value between 0.0 and a.

Alternatively, you can use the following, which effectively does the breakdown I showed above. I actually prefer this simply because it is clearer what is actually going on (to me, anyway):

float x = ((float)rand()/(float)(RAND_MAX)) * a; 

Note: the floating point representation of a must be exact or this will never hit your absolute edge case of a (it will get close). See this article for the gritty details about why.

Sample

#include <stdio.h> #include <stdlib.h> #include <time.h>  int main(int argc, char *argv[]) {     srand((unsigned int)time(NULL));      float a = 5.0;     for (int i=0;i<20;i++)         printf("%f\n", ((float)rand()/(float)(RAND_MAX)) * a);     return 0; } 

Output

1.625741 3.832026 4.853078 0.687247 0.568085 2.810053 3.561830 3.674827 2.814782 3.047727 3.154944 0.141873 4.464814 0.124696 0.766487 2.349450 2.201889 2.148071 2.624953 2.578719