问题
I've been trying to create a predicate in Prolog which splits a list of integers into a list of positive integers and into a list of negative integers.
Sample query with expected result:
?- split([1,-2,3,4,-8],X,Y).
X = [1,3,4],
Y = [-2,-8].
This is the code I got so far:
split([], [], []).
split([Head|Tail], List1, List2) :- split(Tail, [Head|List1], List2), Head>=0.
split([Head|Tail], List1, List2) :- split(Tail, List1, [Head|List2]), Head<0.
I can't seem to figure out what I'm doing wrong.
回答1:
The recursive part is not quite correct.
split([], [], []).
split([Head|Tail], [Head|List1], List2) :- Head>=0, split(Tail, List1, List2).
split([Head|Tail], List1, [Head|List2]) :- Head<0, split(Tail, List1, List2).
The Head
should be added to the positive list if Head >= 0
and to the negative list when Head < 0
.
Moreover, checking the sign of Head
at the beginning is better, because it will prevent unnecessary recursive calls.
回答2:
In SWI-Prolog you can use the predicate partition/4
(which is typically autoloaded from the apply
module):
?- partition(=<(0), [1,-2,3,4,-8,0], X, Y).
X = [1, 3, 4, 0],
Y = [-2, -8].
回答3:
Here is a definition using constraints. In this case, it is library(clpfd)
of SWI and YAP (maybe also XSB). This library is so general that it subsumes regular integer uses.
:- use_module(library(clpfd)).
Using reification:
split([], [], []).
split([E|Es], Poss, Negs) :-
E #>= 0 #<==> B,
i_split(B, E, Es, Poss, Negs).
i_split(1, E, Es, [E|Poss], Negs) :-
split(Es, Poss, Negs).
i_split(0, E, Es, Poss, [E|Negs]) :-
split(Es, Poss, Negs).
Alternatively, you can use zcompare/3
, I prefer that version:
split([], [], []).
split([E|Es], Poss, Negs) :-
zcompare(Order, E, 0),
c_split(Order, E, Es, Poss, Negs).
c_split(>, E, Es, [E|Poss], Negs) :-
split(Es, Poss, Negs).
c_split(=, E, Es, [E|Poss], Negs) :-
split(Es, Poss, Negs).
c_split(<, E, Es, Poss, [E|Negs]) :-
split(Es, Poss, Negs).
You can use it for the regular queries, and for more general ones, like
?- split(Es,[A],[]).
Es = [A],
A in 1..sup ;
Es = [0],
A = 0 ;
false.
回答4:
Stay logically pure and efficient. How? By using meta-predicate tpartition/4 and (#=<)/3
!
First, let us define (#=<)/3
, the reified version of (#=<)/2
, based
on bool01_t/2.
For the sake of completeness, let's also define (#<)/3
, (#>)/3
, and (#>=)/3
!
#=<(X,Y,Truth) :- X #=< Y #<==> B, bool01_t(B,Truth).
#<( X,Y,Truth) :- X #< Y #<==> B, bool01_t(B,Truth).
#>( X,Y,Truth) :- X #> Y #<==> B, bool01_t(B,Truth).
#>=(X,Y,Truth) :- X #>= Y #<==> B, bool01_t(B,Truth).
That's it! Now, let's do the split the OP wanted:
?- tpartition(#=<(0),[1,-2,3,4,-8,0],Ts,Fs).
Ts = [1,3,4,0], Fs = [-2,-8]. % succeeds deterministically
This is monotone, so we get sound answers even when using general, non-ground terms:
?- tpartition(#=<(0),[A,B,C],Ts,Fs).
Ts = [ ], Fs = [A,B,C], A in inf.. -1, B in inf.. -1, C in inf.. -1 ;
Ts = [ C], Fs = [A,B ], A in inf.. -1, B in inf.. -1, C in 0..sup ;
Ts = [ B ], Fs = [A, C], A in inf.. -1, B in 0..sup, C in inf.. -1 ;
Ts = [ B,C], Fs = [A ], A in inf.. -1, B in 0..sup, C in 0..sup ;
Ts = [A ], Fs = [ B,C], A in 0..sup, B in inf.. -1, C in inf.. -1 ;
Ts = [A, C], Fs = [ B ], A in 0..sup, B in inf.. -1, C in 0..sup ;
Ts = [A,B ], Fs = [ C], A in 0..sup, B in 0..sup, C in inf.. -1 ;
Ts = [A,B,C], Fs = [ ], A in 0..sup, B in 0..sup, C in 0..sup .
Edit 2015-06-02
What if we used the SWI-Prolog library predicate partition/4 in above query?
?- partition(#=<(0),[A,B,C],Ts,Fs).
Ts = [A,B,C], Fs = [], A in 0..sup, B in 0..sup, C in 0..sup.
We would lose 7 out of 8 solutions because partition/4
is not monotone!
来源:https://stackoverflow.com/questions/9547062/splitting-a-list-of-integers-into-a-list-of-positive-integers-and-a-list-of-nega