I got a problem with lists. What I need to do is to split one list [1,-2,3,-4], into two lists [1,3] and [-2,-4]. My code looks like the following:
lists([],_,_).
lists([X|Xs],Y,Z):- lists(Xs,Y,Z), X>0 -> append([X],Y,Y) ; append([X],Z,Z).
and I'm getting
Y = [1|Y],
Z = [-2|Z].
What am I doing wrong?
Refer this:
domains
list=integer*
predicates
split(list,list,list)
clauses
split([],[],[]).
split([X|L],[X|L1],L2):-
X>= 0,
!,
split(L,L1,L2).
split([X|L],L1,[X|L2]):-
split(L,L1,L2).
Output :
Goal: split([1,2,-3,4,-5,2],X,Y)
Solution: X=[1,2,4,2], Y=[-3,-5]
See, if that helps.
Just for variety, this can also be done with a DCG, which is easy to read for a problem like this:
split([], []) --> [].
split([X|T], N) --> [X], { X >= 0 }, split(T, N).
split(P, [X|T]) --> [X], { X < 0 }, split(P, T).
split(L, A, B) :-
phrase(split(A, B), L).
As in:
| ?- split([1,2,-4,3,-5], A, B).
A = [1,2,3]
B = [-4,-5] ? ;
no
It also provides all the possible solutions in reverse:
| ?- split(L, [1,2,3], [-4,-5]).
L = [1,2,3,-4,-5] ? ;
L = [1,2,-4,3,-5] ? ;
L = [1,2,-4,-5,3] ? ;
L = [1,-4,2,3,-5] ? ;
L = [1,-4,2,-5,3] ? ;
L = [1,-4,-5,2,3] ? ;
L = [-4,1,2,3,-5] ? ;
L = [-4,1,2,-5,3] ? ;
L = [-4,1,-5,2,3] ? ;
L = [-4,-5,1,2,3] ? ;
(2 ms) no
Gaurav's solution will also do this if the cut is removed and an explicit X < 0 check placed in the third clause of the split/3 predicate.
If your Prolog system offers clpfd you could preserve logical-purity. Want to know how? Read on!
We take the second definition of lists/3 that @CapelliC wrote in
his answer as a starting point, and replace partition/4 by tpartition/4 and (<)/2 by (#<)/3:
lists(A,B,C) :- tpartition(#<(0),A,B,C).
Let's run a sample query!
?- As = [0,1,2,-2,3,4,-4,5,6,7,0], lists(As,Bs,Cs). As = [0,1,2,-2,3,4,-4,5,6,7,0], Bs = [ 1,2, 3,4, 5,6,7 ], Cs = [0, -2, -4, 0].
As we use monotone code, we get logically sound answers for more general queries:
?- As = [X,Y], lists(As,Bs,Cs).
As = [X,Y], Bs = [X,Y], Cs = [ ], X in 1..sup, Y in 1..sup ;
As = [X,Y], Bs = [X ], Cs = [ Y], X in 1..sup, Y in inf..0 ;
As = [X,Y], Bs = [ Y], Cs = [X ], X in inf..0 , Y in 1..sup ;
As = [X,Y], Bs = [ ], Cs = [X,Y], X in inf..0 , Y in inf..0 .
There are several corrections to be done in your code. If you enjoy compact (as readable) code, a possibility is
lists([],[],[]).
lists([X|Xs],Y,Z) :-
( X>0 -> (Y,Z)=([X|Ys],Zs) ; (Y,Z)=(Ys,[X|Zs]) ), lists(Xs,Ys,Zs).
But since (SWI)Prolog offers libraries to handle common list processing tasks, could be as easy as
lists(A,B,C) :- partition(<(0),A,B,C).
Here you have. It splits a list, and does not matter if have odd or even items number.
div(L, A, B) :-
append(A, B, L),
length(A, N),
length(B, N).
div(L, A, B) :-
append(A, B, L),
length(A, N),
N1 is N + 1,
length(B, N1).
div(L, A, B) :-
append(A, B, L),
length(A, N),
N1 is N - 1,
length(B, N1).
