Uniform random number between(0.1]

[ozan]

Hi,
I try to generate uniform random number between 0 and 1.
I wrote this code but it creates only numbers at the 0.1***.
Generated number always have 0.1 and remaning numbers change,
like 0.1234, 0.13444,0.143334....
Can you show me show how can I generate these numbers (uniformly) at (0,1].

#include <cmath>
#include <iostream>
#include <limits>
#include <cstdlib>
#include <ctime>
using namespace std;
main ()
{


double AOD;
srand((unsigned)time(NULL));
AOD=((double) rand() / (RAND_MAX+1)) ;
cout<<endl<<AOD;

  std::cout <<"   Press ENTER to continue...";
  std::cin.ignore( std::numeric_limits<std::streamsize>::max(), '\n' );

return(0);
}

[anders43]

try this

  double AOD;
  srand((unsigned)time(NULL));
  for (int i =0; i<10;++i)
  {
    AOD=((double) rand() / (RAND_MAX+1)) ;
    cout<<endl<<AOD;
  }

  std::cout <<"   Press ENTER to continue...";
  std::cin.ignore( std::numeric_limits<std::streamsize>::max(), '\n' );

check up the doc on srand() ... normally using time(NULL) isn't so good as initial value, especially if you run the program quickly several times after another...

[z05DSL3A]

#include <iostream>
#include <ctime>
using namespace std;

//
// Generate a random number between 0 and 1
// return a uniform number in [0,1].
double unifRand()
{
    return rand() / double(RAND_MAX);
}
//
// Generate a random number in a real interval.
// param a one end point of the interval
// param b the other end of the interval
// return a inform rand numberin [a,b].
double unifRand(double a, double b)
{
    return (b-a)*unifRand() + a;
}
//
// Generate a random integer between 1 and a given value.
// param n the largest value 
// return a uniform random value in [1,...,n]
long unifRand(long n)
{
    
    if (n < 0) n = -n;
    if (n==0) return 0;
    /* There is a slight error in that this code can produce a return value of n+1
    **
    **  return long(unifRand()*n) + 1;
    */
    //Fixed code
    long guard = (long) (unifRand() * n) +1;
    return (guard > n)? n : guard;
}
//
// Reset the random number generator with the system clock.
void seed()
{
    srand(time(0));
}


int main()
{
    seed();
    for (int i = 0; i < 20; ++i)
    {
        cout << unifRand() << endl;
    }
    return 0;
}

[ozan]

anders43, Grey Wolf;
thanks for replies.

[z05DSL3A]

Fixed an error in previouse code posting!

[jsmith]

But your fix means that it is no longer uniformly distributed. There is now a greater chance of returning n than any other number.

[z05DSL3A]

Yes and no...The chance of getting an n+1 result is small and I tested the mod on variouse ranges taking 10⁸ samples and found the distrubution to be acceptable.

[helios]

How about return unifRand()*(n-1)+1;?

[Zaita]

http://www.boost.org/doc/libs/1_35_0/libs/random/index.html