初始化列表的序言长度将新列表添加到现有列表

Question

puzzle(ListToAdd,ListToAdd1,Res,_,Carry) :-
   length(Res,ResultLength),
   length(Carry,ResultLength),
   domain(Carry,0,1).

First I use length to get the size of the Res list (ResultLength) and then I will use ResultLength to initialize the other list, Carry. However after I do that both ListoAdd and ListToAdd1 now have the aforementioned Carry list in them, which I belive it shouldn´t happen.

Query done: puzzle([D,O,N,A,L,D],[G,E,R,A,L,D],[R,O,B,E,R,T],X,L).


        Before  length(Carry,ResultLength):
        write(ListToAdd):  [_5459,_5499,_5539,_5319]
        write(ListToAdd1): [_5199,_5239,_5279,_5319]

        After  length(Carry,ResultLength):
       write(Carry):      [_12043,_12107,_12171,_12235]
       write(ListToAdd):  [_5459,_5499,_5539, [_12043,_12107,_12171,_12235],_5319]
       write(ListToAdd1): [_5199,_5239,_5279, [_12043,_12107,_12171,_12235],_5319]

I´m using SICStus Prolog 4.3.2.

Does anyone know what's happening?

Answer 1

The reason for the troubles you are experiencing is this:

?- puzzle([D,O,N,A,L,D],[G,E,R,A,L,D],[R,O,B,E,R,T],X,L).

Got it? If not, consider:

?- puzzle([D,O,N,A,,D],[G,E,R,A,,D],[R,O,B,E,R,T],X,).
                   ^             ^                    ^
                   +-------------+--------------------+

Those logical variables:)

HTH!