c - calling rand() returning non-random results

Question

I am writing a simple C program that tosses a coin 100000 times and calculates how many heads and how many tails resulted from the toss using srand and rand. Here is what I have:

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

int main(void){

    int i;
    int h=0;
    int t=0;
    int n;

    for(i=0;i<100000;i++){
        srand(time(NULL));
        n=rand()%2;
        if(n==0){
            h++;
        }
        else {   
            t++;
        }
    }

    printf("%d heads
", h);
    printf("%d tails
", t);

    return EXIT_SUCCESS;
}

However when running the program I sometimes get 100000 heads and 0 tails and sometimes 0 heads and 100000 tails and sometimes I get random numbers in heads and tails but not in any way a 50% heads 50% tails.

Am I missing something? Thanks!

Answer 1

You are seeding the pseudo-random number generator with the same seed each time around the look. Simply move the call to srand outside the loop.

srand(time(NULL));
for(i=0;i<100000;i++){
    n=rand()%2;
    .....

---

A pseudo-random number generator (PRNG) is deterministic. That is, given the same initial seed, it will return the same sequence of numbers each time is is called. In your code, since you seed every time you call rand, you get the same sequence of 1 number for every iteration of your loop. Now, different executions of the program will have different results, because time will have changed.

And in the cases when you were not seeing 100% heads or 100% tails, that would be when the loop took sufficiently long for time to have advanced.