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 ...

Seed only 1 time in your application. Never ever seed in a loop. — drescherjm
– drescherjm, Commented Jul 1, 2014 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
– Damon, Commented Jul 1, 2014 at 13:48
BTW, you may use std::uniform_int_distribution<> in C++11. — Jarod42
– Jarod42, Commented Jul 1, 2014 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++?. It was right of findable from the Related section at the right. Pleas do a bit more research efforts before asking. — πάντα ῥεῖ
– πάντα ῥεῖ, Commented Jul 1, 2014 at 14:16

haccks · Accepted Answer · 2014-07-01 15:00:22Z

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.

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