Removing duplicates

[nojero]

Hi,

I've the following definitions.(* player(name,age) *)

player(peter,9).
player(paul,10).
player(chris,9).
player(susan,9).

games(X,Y) :- player(X,9),player(Y,9),X \== Y.

I want Prolog to return all possible games to play in a tournament, but only with people aged 9.

The problem is that, when asking

?- games(X,Y).

it gives both

X = chris
Y = susan

and

X = susan
Y = chris

I know I have to use the cut operator (!), and 'fail maybe, but I'm not so sure how to do it.

Can anyone help?

Thx in advance!

 

[kahleen]

You can make use of the standard ordering of Prolog terms to invalidate the undesired pair:

games(X, Y) :-
    player(X, 9),
    player(Y, 9),
    X @< Y.

You need the @< operator because you don't compare integers or strings, but atoms. The @< operator may seem strange, but it does its job. Here is the help page for @<:


+Term1 @< +Term2 [ISO]
True if Term1 is before Term2 in the standard order of terms.


Now chris @< susan, but not susan @< chris.
So only X = chris, Y = susan pair will be validated