Function for generate random number, not so random

Question

I write a function to generate random number, I call this function every 10 mS.

What I get is surprising, my number generate are a sequence of 3 ( 0,3,6,9,12,...)

Here's my function :

int utilitaireUDP::genereNombreAleatoire(int a, int b){
    int nombre_aleatoire = 0; //Sert a remplir la structure
    srand(time(NULL)); // initialisation de rand
    nombre_aleatoire = rand()%(b-a) +a;;
    return nombre_aleatoire;
}

Do I forget something ? Is the problem linked to my CPU ( because random number are generate from my internal clock ) ?

EDIT : Thanks everyone !

I use this post to add something, I've got this function :

bool utilitaireUDP::genererBooleen(){
    if (genereNombreAleatoire(0,1) == 0){
        return true;
    }
    else {
        return false;
    }
}

And this generate me only true, but I put the srand in my main now ... And it works perfectly for int ...

Have you tried using [the random header](http://stackoverflow.com/a/19553318/1708801)? — Shafik Yaghmour, Jul 01 '14 at 13:46
Seed only 1 time in your application. Never ever seed in a loop. — drescherjm, Jul 01 '14 at 13:46
This is silly-mistake #1 related to `rand` with approx. 1 million questions per week. Only `srand` once, not every time. — Damon, Jul 01 '14 at 13:48
Be aware `rand` is usually not a high quality generator. If you have C++11 there are many better ones included. — Neil Kirk, Jul 01 '14 at 13:50
BTW, you may use `std::uniform_int_distribution<>` in C++11. — Jarod42, Jul 01 '14 at 13:51
I think all the explanations you need can be found here: [how to generate different random number in a loop in C++?](http://stackoverflow.com/questions/4926622/how-to-generate-different-random-number-in-a-loop-in-c). It was right of findable from the **Related** section at the right. Pleas do a bit more research efforts before asking. — πάντα ῥεῖ, Jul 01 '14 at 14:16

haccks · Accepted Answer · 2014-07-01T15:00:22.677

6

It's rarely useful to call srand more than once during a run of a program; in particular, don't try calling srand before each call to rand, in an attempt to get "really random" numbers.

Remove srand(time(NULL)); from your function and place it at program start up so that it called only once.

edited Jul 01 '14 at 15:00

answered Jul 01 '14 at 13:47

haccks

104,019
25
176
264

Function for generate random number, not so random

1 Answers1