Re: Random number generation

[Brian Drummond <brian_drummond@btconnect.com>]

On Thu, 30 Dec 2010 11:43:40 +0100, Mart van de Wege <mvdwege@mail.com> wrote:

>Beginner's question: I'm trying to implement a function that rolls a
>number of dice and then adds a modifier. Somehow, it produces the same
>number every time. I can't see where I'm going wrong.
>
>Here's the proof of concept code:
>
>with Ada.Numerics.Discrete_Random,Ada.Integer_Text_IO;
>use Ada.Integer_Text_IO;
>
>procedure Rolltest is
>   function Roll ( Number : in Positive;
...
>      package Die is new Ada.Numerics.Discrete_Random( Die_Size );
...
>   begin
>      Die.Reset(G);
...
>   end Roll;
>begin
>   for I in 1..10 loop
>      Put(Roll( Number => 3, Size => 6 ));
>   end loop;
>end Rolltest;
>
>Anyone care to at least point me to some documentation that explains
>what I'm doing wrong? 

Check the documentation for  Ada.Numerics.Discrete_Random or at least its
specification ( .ads file) but I think you'll find you need to reset the Die
exactly once, rather than every call of the Roll function.

As I understand, you are resetting the seed each time, so you get the same
results!

The shortest way to resolve this is to make Die a global variable and reset it
at the start of the main program - in the context of a one-page beginner's
program it's a reasonable thing to do.

We're all taught "Global Variables are BAD" with good reason, so - once you have
resolved the immediate problem - it might be a good time to learn a bit about
packages, to hide the Die and expose only what you need to operate on it.

Essentially, within the package, Die can safely become a "global" variable, but
invisible and inaccessible outside the package, so that it appears more like a
static variable in a C function (holds the current random seed value between
calls).

- Brian