Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!stretch.cs.mun.ca!leif!jgarland
From: jgarland@kean.ucs.mun.ca
Newsgroups: comp.lang.prolog
Subject: Re: Having trouble with disjunction
Message-ID: <125275@kean.ucs.mun.ca>
Date: 15 Aug 90 00:49:29 GMT
References: <90226.091749F0O@psuvm.psu.edu>
Organization: Memorial University. St.John's Nfld, Canada
Lines: 64

In article <90226.091749F0O@psuvm.psu.edu>, F0O@psuvm.psu.edu writes:
The structure:

>          gofirst(Player) :-
>              random(RandNum),
>              RandNum < 0.5, Player = computer ;
>              RandNum >= 0.5, Player = opponent.
> 
>                        [Tim]


Nice idea, the problem is you're thinking in Pascal.  Disjunction 
works more like a case statement in PDC.  I.e. your structure is 
equivalent to:

   gofirst(Player) :- random(Ran),Ran < 0.5,Player=computer.
   gofirst(Player) :- Ran >= 0.5, Player=opponent.

Ran is indeed free.

Listing 1 clumsily implements your ideas using the cut

**************** Listing 1 ********************
predicates

 gofirst(symbol)
 
clauses

 gofirst(Player) :-
   random(Ran),Ran <= 0.5,
   !,
   Player = computer;
   
   Player = opponent.
   
goal

   gofirst(Player).
*************** end ****************

Listing 2 is more elegant if I may say and doesn't mimic Pascal
**************** Listing 2 ********************

predicates

   gofirst(symbol,integer)
   
clauses

   gofirst(computer,0).
   gofirst(opponent,1).
   
   
goal

   random(2,Ran),            %integer random number generator--fast
   gofirst(Player,Ran).


John Garland

Bitnet:  jgarland@mun
Internet: jgarland@kean.ucs.mun.ca