function which returns a random value of 1 or 0

Question

int random_var(void);

int main(void)
{
    int i;
    for (i = 1; i <= 50; i++)
        printf("%d\n", random_var());

return 0;
}


int random_var(void)
{
    srand((unsigned) time(NULL));
    return rand() % 2;
}

I am tasked to write a function that returns a random value of 1 or 0 everytime it is called. My code keeps returning a fixed value even though I used a random generator for the seed, why??

You could just `return !rand();`. Nobody said anything about the relative likelihood of 1 and 0. — EOF, Oct 25 '16 at 21:02
@EOF: You could just `return 0;`. Nobody said anything about the relative likelihood of 1 and 0. — Keith Thompson, Oct 25 '16 at 21:04
@KeithThompson I presume you'll be relying on the occasional cosmic ray flipping a bit to supply the required randomness? — EOF, Oct 25 '16 at 21:09
@KeithThompson: Randomness is not a property of a single number but of a sequence. The sequence containing only zeros is considerably non-random. — EOF, Oct 25 '16 at 21:22
@EOF If you didn't like Keith's joke, you won't like http://xkcd.com/221/ either. — Steve Summit, Oct 25 '16 at 21:25
@SteveSummit: I like it just fine. I am just currently in a pedantic mood. — EOF, Oct 25 '16 at 21:27

StoryTeller - Unslander Monica · Answer 1 · 2016-10-25T21:04:29.427

1

You need to call srand() only once!

int random_var(void);

int main(void)
{
    int i;
    srand((unsigned) time(NULL));
    for (i = 1; i <= 50; i++)
        printf("%d\n", random_var());

    return 0;
}


int random_var(void)
{
    return rand() % 2;
}

Alternatively (or additionally), as Keith Thompson pointed out, you can try taking higher order bits instead, which may distribute better.

edited Oct 25 '16 at 21:04

answered Oct 25 '16 at 20:57

StoryTeller - Unslander Monica

165,132
21
377
458

1

*Some* implementations of `rand()` will return alternating even and odd numbers. [comp.lang.c FAQ](http://www.c-faq.com} question 13.18. – Keith Thompson Oct 25 '16 at 21:00
@KeithThompson, I'd upvote your comment if I hadn't have reached the daily limit today. However, sadly, [some popular implementations won't...](https://ideone.com/ajAD1Y) – StoryTeller - Unslander Monica Oct 25 '16 at 21:01
Of course if you re-seed with the same value you'll get the same results. – Keith Thompson Oct 25 '16 at 21:03
2

To expand on the problem, the OP is calling `srand` which seeded with the time in seconds. The tight `for` loop from 1 to 50 will execute in much less than a second, so `srand` is getting seeded with the same value 50 times. Therefore `rand()` will return the same value 50 times. If you happen to execute this loop during the transition to then next second, `rand` would return a different number, then the new one would be repeated until the loop terminated. – yano Oct 25 '16 at 21:04
@yano, you should post that as an answer. It's spot on. – StoryTeller - Unslander Monica Oct 25 '16 at 21:06
heh, thought about it, but it would've been the same as your answer .. think it's all covered now – yano Oct 25 '16 at 21:19

function which returns a random value of 1 or 0

1 Answers1