Generation of random integers between two set values in C

Question

I'm looking to implement something similar to python's random.randint in C.

I would do something like:

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
srand(time(NULL));

int randInt(int lBound, int uBound){
    return (rand()%(uBound-lBound+1))+lbound;
}

but if RAND_MAX is not a multiple of uBound, then distribution will be slight skewed.

Is there a quick and dirty way that's better?

That function is wrong, it should be `(rand()%(uBound-lBound+1))+lbound` — interjay, Oct 31 '13 at 12:36
@Alexandre&interjay. Thanks, I missed those, and have edited the question — John Gann, Oct 31 '13 at 12:43
@TomSwifty I was unaware of it's existence and will take a look at it, but I'm still curious as to other ways of going about it. — John Gann, Oct 31 '13 at 12:46
gsl will help you avoid reinventing the wheel, but it is always a good idea to aim for a deeper understanding by writing your own — Tom Swifty, Oct 31 '13 at 12:48
@JohnGann [`rand()` should be considered harmful](http://channel9.msdn.com/Events/GoingNative/2013/rand-Considered-Harmful) I would highly suggest looking at library functions if you want a uniform distribution — Mgetz, Oct 31 '13 at 12:48
The issue is when `RAND_MAX` isn't a multiple of `(uBound - lBound + 1)`, not just `uBound`. — godel9, Oct 31 '13 at 12:49

NovaDenizen · Answer 1 · 2013-10-31T15:15:36.390

0

If avoiding skew is important then you need to do something like this.

int randInt(int lBound, int uBound){
    int rangesize = uBound-lBound+1; 
    if ((rangesize - 1 > RAND_MAX) || (rangesize < 0) || (lBound > uBound)) {
        // run in circles, scream and shout
    }
    int limit;
    if (rangesize < RAND_MAX) {
        limit = RAND_MAX - (((RAND_MAX % rangesize) + 1) % rangesize);
    } else {
        limit = RAND_MAX;
    ]
    int rv;
    do {
        rv = rand();
    } while (rv > limit);
    return rv % rangesize + lbound;
}

edited Oct 31 '13 at 15:15

answered Oct 31 '13 at 15:02

NovaDenizen

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1

That's a good way to approach the solution, but it has some issues: It doesn't work on platforms where `RAND_MAX == INT_MAX`, because `RAND_MAX+1` will overflow. It also doesn't work when `uBound-lBound > RAND_MAX`, or when `uBound-lBound+1` doesn't fit in an `int`. – interjay Oct 31 '13 at 15:09
Issues fixed, I think. – NovaDenizen Oct 31 '13 at 15:15

Generation of random integers between two set values in C

1 Answers1