Thread: How works the rand function?

Hi all,
I'm using the rand function.. I used it correctly but I can't understand why it returns sometimes the same values of a previous computation.
I use to write:


When I use this code in a call to function like this:

Code:

buildListsOfRandom(){

for  (i=1 TO K){
   createRandomList()
}
}


createRandomList(){
    srand ( time(NULL) );
    randNum= rand() % 100;
}

The values returned for the lists are everytime the same... why??
Maybe I have to use the srand function in some other place?

Originally Posted by MK27

Moral: do not call srand() more than once in a single program, unless you are using a completely different seed each time.

You're right ...I've used the srand() function other times in my code. I was believing that the function srand was to use each time I use it for different reasons...
So the rule is: Use it once in every execution of a program..?

I do it ... but it stil doesn't work!

I erased completely the srand() function in this procedures (having used it another time in the program)... but the function createRandomList creates the first time some values and the second, third, and so on... the same values

Make sure you're still only calling srand once. If you call it before making each list, then just as before you're going to get the same list each time.

Yes.. I'm used it only when I launched th function buildListOfRandom. But in createRandomList i'm creating two different lists each time.. maybe I've to change the seed for the two different lists?

Try using the srand(time(NULL)); call once... right at the beginning of your program.

After that, only use rand() % whatever; to create your lists.

srand() creates a sequence of random numbers based on a seed value... if you are using time() as your seed, calling it twice in the same second will generate the same list twice. So you really only want to use it once in each run of your program. You want rand() to continue down the same list, not start at the top of a new (possibly duplicated) list.

uhmmm.. maybe I've to re-control my code because I use the rand function in much points of it. In other cases, in example, I use the srand with a different seed every time because I've different variables changing every time in these functions. I will try tomorrow to do some changes...
Thanks for your help!

srand is not tied to anything like variables or anything like that. You can use rand all you want, in all the functions you care to write, for all the variables you need, provided you've used srand once, and exactly once, in the program.

Originally Posted by angeloulivieri

uhmmm.. maybe I've to re-control my code because I use the rand function in much points of it. In other cases, in example, I use the srand with a different seed every time because I've different variables changing every time in these functions. I will try tomorrow to do some changes...
Thanks for your help!

What tabstop said... use srand() once and only once, first line of main in your program. From there use rand() just as often as your heart pleases.