计算 Prolog 中给定列表的正数列表

Question

I tried resolving it by myself in the following way:

list_of_positives(L1, L2) :- 
   list_of_positives(L1, L2, []).
list_of_positives([], L, L).
list_of_positives([H|T], L2, L3) :- 
   (   H > 0 
   ->  list_of_positives(T,L2,[H|L3])
   ;   list_of_positives(T,L2,L3) 
   ).

The problem with this solution is that I get as response a reversed list of positive numbers. Can someone help me to find a way to get the list in the "correct order"?

Answer 1

You can solve the problem as follows:

positives([], []).

positives([H|T], P) :-
   (   H > 0
   ->  P = [H|R]     % desired order!
   ;   P = R),
   positives(T, R) .

Example:

?- positives([2,-3,6,-7,1,4,-9], P).
P = [2, 6, 1, 4].

Answer 2

You want to use a difference list , a non-closed, or open list . So, something like this:

positives( []     , []     ) .  % An empty list has not positives, and closes the list.
positives( [N|Ns] , [N|Rs] ) :- % For a non-empty list, we prepend N to the result list
  N > 0,                        % - if N is positive
  positives(Ns,Rs)              % - and recurse down.
  .                             %
positives( [N|Ns] , Rs     ) :- % For non-empty lists, we discard N
  N =< 0,                       % - if N is non-positive
  positives(Ns,Rs)              % - and recurse down.
  .                             %