CODEWITHSUNDEEP
c
DewinDell
DewinDell

Reputation: 1028

calling rand() returning non-random results

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\n", h);
    printf("%d tails\n", 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!

Upvotes: 2

Views: 377

Answers (8)

dzamudio1
dzamudio1

Reputation: 21

Yes, if you are using srand, it's getting a number randomly using time, so if you or the computer chooses a value randomly in less than a second, the number will come out to be the same number every time.

Upvotes: 2

David Heffernan
David Heffernan

Reputation: 613501

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.

Upvotes: 7

Bacardi
Bacardi

Reputation: 13

Put srand outside for loop. Call srand(time(null) all only one time.

Upvotes: 0

instanceOfObject
instanceOfObject

Reputation: 2994

Duplicate to srand(time(NULL)) doesn't change seed value quick enough. And yes, as said above, move

srand(time(NULL));

outside(before ;)) the loop :)

Upvotes: 0

trojanfoe
trojanfoe

Reputation: 122458

I think the problem is calling srand() for every iteration; you only need to call it once.

Try this version:

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

int main(void){


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

srand(time(NULL));
for(i=0;i<100000;i++){
    if((rand() & 1) == 1){
        h++;
    }
    else{
        t++;
    }
}

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

return EXIT_SUCCESS;
}

This gives fairly consistent results:

$ ./rand
49980 heads
50020 tails
$ ./rand
50131 heads
49869 tails
$ ./rand
50000 heads
50000 tails
$ ./rand
50000 heads
50000 tails
$ ./rand
50093 heads
49907 tails
$ ./rand
50093 heads
49907 tails

EDIT: Stratch my previous comment about rand() - it doesn't return negative values.

Upvotes: 1

Pawel Zubrycki
Pawel Zubrycki

Reputation: 2713

Yeah. It's all about this line:

srand(time(NULL));

Move it before for loop and it will be working. srand sets seed so if you call it like that and time(NULL) won't change (and it's quite possible) rand() will always return the same value.

Upvotes: 0

jglouie
jglouie

Reputation: 12880

you're using the identical seed value at each iteration of the loop.

move srand(time(NULL)); to before the for loop.

Upvotes: 0

Charles Duffy
Charles Duffy

Reputation: 295815

Don't call srand within your loop. It should be called only once, at the very beginning of execution, if you're going to invoke it at all.

As it is, you're resetting the random number generator to the same seed (unless a second boundary passes) before each time you use it... so it should be no surprise when it comes up with consistent results!

Upvotes: 0

Related Questions