time dependent uniformly distributed random number

Question

I need a uniformly distributed random number generator... Here is what I've tried its output is a constant number no matter how many time i run the .exe

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


int randr( int min,  int max);

int main()
{
    srand(time(NULL));
    int rr=randr(0,10);
    printf("rr=%d\n",rr)

    return 0;
}

int randr( int min,  int max)
{
    double scaled = (double)rand()/RAND_MAX;        
    return (max - min +1)*scaled + min;
}

Thanks in advance.

What is uniformly distributed random numbers ? Can you please i give an example ? — mahendiran.b, Aug 07 '14 at 10:14
http://en.wikipedia.org/wiki/Uniform_distribution_(discrete) — user1740587, Aug 07 '14 at 10:15
If you can use C++11, try [std::uniform_int_distribution](http://en.cppreference.com/w/cpp/numeric/random/uniform_int_distribution) (but it seems that you're in C, unfortunately ..) — ikh, Aug 07 '14 at 10:17
example: a dice is a uniformly distributed random number generator with 1-6 range — user1740587, Aug 07 '14 at 10:17
my code is in C and translate it would be a pain in the ass( — user1740587, Aug 07 '14 at 10:19
http://stackoverflow.com/questions/11641629/generating-a-uniform-distribution-of-integers-in-c — dragosht, Aug 07 '14 at 10:32
dragosht it is giving a uniform random number but it doesn't change with time(ive run it 10 times its giving the same 9) — user1740587, Aug 07 '14 at 10:36
That's weird. What does time(NULL) return? Does it change it's value? — dragosht, Aug 07 '14 at 10:37
Its strange when i run it on a online compiler its working fine... — user1740587, Aug 07 '14 at 10:43
Assuming your program executes fast enough (and you run it like 10 times in a second, which is unlikely though, unless you're running with a script) you could get the behavior you describe. man time: `time(NULL)` returns the number of seconds since the Epoch. — dragosht, Aug 07 '14 at 10:50

takeo999 · Answer 1 · 2014-08-07T14:15:20.403

The problem with your code is that the time function doesn't use milliseconds, so each call to your function set the same seed and generate the same first number (assuming it's called at the same time, i.e. the same second).

One way to avoid this is to give a seed only once in your program (srand must be called only once), you can verify that by trying this code :

int main()
{
    int a = 0;
    srand(time(NULL));
    for (int i=0;i<10000;i++){
        int rr=randr(0,10);
        a+=rr;
        printf("rr=%d\n",rr);
    }

    printf("mean : %d\n", a/10000); // to quickly check the uniformity
    return 0;
}

Another way is to use a function that can give you a different seed at each call (time based on milliseconds for example). A possible implementation on a POSIX system :

struct timespec tmp;
clock_gettime(CLOCK_MONOTONIC,&tmp);
srand(tmp.tv_nsec);

This will be based on nanoseconds (as suggested by R..), to compile you'll probably need to link with librt (-lrt on gcc).

You're right, I edited with an example on how to access the nanoseconds. — takeo999, Aug 07 '14 at 14:16

time dependent uniformly distributed random number

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