🚀 go-pugleaf

In article <1992Oct29.140104.7751@coli.uni-sb.de> ulma@coli.uni-sb.de
(Ulrich Mayring) writes:

%  I have a list and need to recursively pick two elements out of it.  I
%  need every pair of elements in every order, however the order of the
%  pair itself is not important.  It should also be possible to pick an
%  element and the [] as a pair

The following code will do what you want.  The final requirement strikes
me as unusual; could you not extract single elements with member/2
(below) and if none of them suits you, get pairs with the first clause
only of pair/3 (below) ?

I have assumed that you don't want pairs such as (a,a) unless there are
actually two or more `a's in the list. 

Sample invocation

| ?- pair([a,b,c],A,B).

     A=a
     B=b ;

     A=a
     B=c ;

     A=b
     B=a ;

     A=b
     B=c ;

     A=c
     B=a ;

     A=c
     B=b ;

     A=a
     B=[] ;

     A=b
     B=[] ;

     A=c
     B=[] ;
no
| ?-

% ------------------------------------------------------------------------
%  Code: Ken Johnson, November 1992.  You may use and distribute this code
%  freely, but if you sell copies at a profit, I want a share. 

pair(L,A,B) :-
	member(A,L,L1),
	member(B,L1).
pair(L,A,[]) :-
        member(A,L).


member(X,[X|_]).
member(X,[_|T]) :-
	member(X,T).

member(X,[X|T],T).
member(X,[H|T],[H|U]) :-
	member(X,T,U).


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