F0O@psuvm.psu.edu (08/14/90)
Last night, in PDC prolog I was trying to write the following predicate:
gofirst(Player) :-
random(RandNum),
RandNum < 0.5, Player = computer ;
RandNum >= 0.5, Player = opponent.
When I tried to run the program, I got the message 'Free variables
not allowed here', at Player = opponent.
Is this a quirk of PDC prolog, or don't I fully understand disjunction?
[Tim]jgarland@kean.ucs.mun.ca (08/15/90)
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
ok@goanna.cs.rmit.oz.au (Richard A. O'Keefe) (08/15/90)
In article <90226.091749F0O@psuvm.psu.edu>, F0O@psuvm.psu.edu writes: > gofirst(Player) :- > random(RandNum), > RandNum < 0.5, Player = computer ; > RandNum >= 0.5, Player = opponent. > > When I tried to run the program, I got the message 'Free variables > not allowed here', at Player = opponent. > Is this a quirk of PDC prolog, or don't I fully understand disjunction? This is easily the best thing I have ever heard about PDC Prolog. Your problem is that you have a terrible layout convention. Let's put your code through a Prolog pretty-printer and see what comes out: gofirst(Player) :- ( random(RandNum), RandNum < 0.5, Player = computer ; RandNum >= 0.5, Player = opponent ). See what's going on? Use this indentation rule: always write disjunctions as ( <disj1> ; <disj2> ... ; <disjn> ) exactly like you'd write an Ada if statement, with "(" acting like "if", "->" [when using it] acting like "then", ";" acting like "else if" or "else", and ")" acting like "end if". Following that rule, and writing what you meant, we get gofirst(Player) :- random(RandNum), ( RandNum < 0.5, Player = computer ; RandNum >= 0.5, Player = opponent ). -- The taxonomy of Pleistocene equids is in a state of confusion.
stefan@hpbbi4.BBN.HP.COM (#Stefan Bachert) (08/15/90)
/ hpbbi4:comp.lang.prolog / F0O@psuvm.psu.edu / 3:17 pm Aug 14, 1990 /
Watch the precedence. I added brackets to show you what you
told your prolog.
gofirst(Player) :-
( random(RandNum),
RandNum < 0.5, Player = computer );
( RandNum >= 0.5, Player = opponent).
I think you intended the following
gofirst(Player) :-
random(RandNum),
(RandNum < 0.5, Player = computer ;
RandNum >= 0.5, Player = opponent).
Stefan