Let's say I have list(A,X,Y,R) and i want to get X and Y two values from A that are different and all the other A values go to R.

How could I do this ?

Examples

?- list([],X,Y,R). false.
?- list([1],X,Y,R). false.
?- list([1,2],X,Y,R). X=1, Y=2, R=[];
X=2, Y=1, R=[].
?- list([1,1],X,Y,R). X=1, Y=1, R=[];
X=1, Y=1, R=[].
?- list([1,2,3],X,Y,R). X=1, Y=2, R=[3];
X=1, Y=3, R=[2];
X=2, Y=1, R=[3];
X=2, Y=3, R=[1]; X=3, Y=1, R=[2]; X=3, Y=2, R=[1].

CodePudding user response：

Okey, here you have a general solution. I've tried it in SWI-prolog, but if any predicate doesn't work for you at least you can look for an equivalent:

list([A,B|T], X, Y, T) :- 
    permutation([A,B], [X, Y]).
list([A,B|T], X, Y, [D|R]) :- 
    permutation([C, D], [A,B]),
    list([C|T], X, Y, R).

Breakdown:

• Base case: X and Y are the two first elements in any order. R would be the tail of the list.
• Recursive case: From the first two elements, pick one (D) and remove it from the list before trying again, then add it to R.

It passes the examples you've given, although the solutions aren't retrieved in the same order as you wrote.