Why use srand when rand() generates all random numbers in C

Question

I am generating random numbers , and someone told me to use srand. I searched about srand and found it is used on top of rand(). But I do not understand why not use just rand() since I get random numbers in my below code:

#include<stdio.h>
#include<stdlib.h>

int main(){

    int i=0;

    for(i=0;i<5;i++){
        printf("random number is %d\n",rand());
    }
}

And I get a set of 5 random numbers. So rand() does gives random numbers. Why do we need srand on top of that then?

The [documentation](http://linux.die.net/man/3/rand) should explain what happens when srand is *not* explicitly called first. — user2864740, May 06 '15 at 01:57
See also [`srand()` — why call it only once?](http://stackoverflow.com/questions/7343833) — Jonathan Leffler, May 06 '15 at 02:40

score 2 · Answer 1 · answered May 06 '15 at 01:57

2

If you simply use rand() and run your code multiple times you will notice that you tend to get the same sequence of random numbers every time.

srand is used to seed the random number generator. This allows you to generate different sequences. I'd suggest that you read the manual page for srand.

answered May 06 '15 at 01:57

David Hoelzer

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You use `srand` to generate `semi-random` numbers with a `seed` to produce a **reproducible** random set to allow debugging, etc. – David C. Rankin May 06 '15 at 02:38

Why use srand when rand() generates all random numbers in C

1 Answers1