On Sun, 15 Jun 2014, J-P. Rosen wrote:

> Le 15/06/2014 20:54, Stefan.Lucks@uni-weimar.de a écrit :
>> The alternative is to (2n) instantiate a single random generator (I've
>> forgotten the package name, but it is not A.N.Discrete_Random), which
>> just generates a Float F_Rand between 0 and 1. Then set R_I :=
>> Truncate(1 + F_Rand * (50-I)).
> This is not equivalent. I don't remember the details, but in the making
> of Ada 95 there was a furious discussion between numericists about
> whether it was possible to make a discrete random generator given a
> floating point random generator.
>
> After megabytes of E-mail discussion, they concluded that NO, it was not
> possible, and therefore that Ada should have both.

I am not aware of that discussion, but mathematically, given a uniformly 
distributed real number F between 0.0 and 1.0, one can get a uniformly 
distributed discrete D between Low and high: D := Truncate(Low + F * 
(High-Low + 1)).

I guess, the issue that has been raised in that discussion that if F is of 
type float or even double-float, it has a mantissa of limited size, and 
then the distribution of D would significantly deviate from the uniform 
distribution if High-Low happens to be large. In the current case, 
however, High-Low < 50, so the solution using the uniformly distributed 
float should work properly. If D would significantly deviate from the 
uniform distribution, then so would F.



------  I  love  the  taste  of  Cryptanalysis  in  the morning!  ------
     <http://www.uni-weimar.de/cms/medien/mediensicherheit/home.html>
--Stefan.Lucks (at) uni-weimar.de, Bauhaus-Universität Weimar, Germany--