[boost] [Random] uniform_int vs. uniform_real vs. uniform_01

From: Doug Gregor (dgregor_at_[hidden])
Date: 2004-12-30 17:24:34

I'm having a few problems dealing with uniform_real and uniform_01.

The basic issue is that uniform_real does not always produce values 
between its min and its max. The problem consistently occurs when I'm 
using minstd_rand as my generator (one of the linear_congruential 
variants). For instance, if I write:

   template<typename RandomNumberGenerator>
   void foo(RandomNumberGenerator& gen)
   {
     uniform_int<int> rand_int(some_min_int, some_max_int);
     // Okay, some_min_int <= x <= some_max_int
     int x = rand_int(gen);

     uniform_real<double> rand_double(some_min_double, some_max_double);
     // Uh-oh, some_min_double <= y <= some_max_double doesn't always 
hold
     double y = rand_double(gen);
   }

Looking at the source to uniform_real, the problem is obvious:

   result_type operator()(Engine& eng) { return eng() * (_max - _min) + 
_min; }

uniform_real assumes that the engine returns a value in [0, 1], but 
doesn't check that. If this is correct (I hope it isn't!), the 
"obvious" answer is to use uniform_01<RandomNumberGenerator...> as 
input to uniform_real. However, uniform_01 is also broken in several 
ways:

        1) It isn't a random distribution because its operator() does not 
accept an engine. Instead, the engine is embedded in the uniform_01 
object.
        2) uniform_01 stores the incoming random number generator by _value_, 
so the original generator "gen" does not advance. You can't even fake 
it with uniform_01<RandomNumberGenerator&>, because that creates 
reference-to-reference problems.

Below is a small patch to make uniform_real<...> behave nicely. As for 
uniform_01: we should deprecate it in 1.33.0 and remove it afterwords, 
because it is hopelessly broken and not even included in the TR.

        Doug

Index: uniform_real.hpp
===================================================================
RCS file: /cvsroot/boost/boost/boost/random/uniform_real.hpp,v
retrieving revision 1.14
diff -u -r1.14 uniform_real.hpp
--- uniform_real.hpp    27 Jul 2004 03:43:32 -0000      1.14
+++ uniform_real.hpp    30 Dec 2004 22:11:33 -0000
@@ -50,7 +50,11 @@
    void reset() { }

    template<class Engine>
-  result_type operator()(Engine& eng) { return eng() * (_max - _min) + 
_min; }
+  result_type operator()(Engine& eng) {
+    return static_cast<result_type>(eng() - eng.min())
+           / (eng.max() - eng.min())
+           * (_max - _min) + _min;
+  }

  #if !defined(BOOST_NO_OPERATORS_IN_NAMESPACE) && 
!defined(BOOST_NO_MEMBER_TEMPLATE_FRIENDS)
    template<class CharT, class Traits>