Re: Discrete random with given distribution ?

[tmoran@acm.org]

> >   Unfortunately, roundoff complicates it a bit.   Consider:
> > Size = 10
>
> If size is ten, you're only asking for one digit of accuracy.
  Size = 10 was just a small example.  Consider:
Size : constant := 1000;
type Discrete_Things is range 1 .. 50;
Probability : constant array(Discrete_Things) of Float
  := (1 .. 49 => 1.0/1024.0, 50 => 1.0-49.0/1024.0);
the first 49, 1000*(1.0/1024.0), will each round to 1
the 50th, 1000*(1.0-49.0/1024.0), rounds to 952.
So, ignoring the roundoff problem, the simple algorithm will try to put
49+952= 1001 entries in Lookup, which only has room for 1000 entries.
While a programmer would, of course, recognize the problem when he wrote
"1.0/1024.0" and knew he was dealing with 3 digit accuracy, the problem
could easily creep in if the Probability array was calculated at run time.