反转 Prolog 中每隔一个列表的列表

Question

I have a list containing lists and I want to reverse every second list in it. I tried something but if I have odd number of elements in the list the last list element is lost... So the best solution would be to put the odd lists first and the even lists second till every second list is reversed.

I can't use any libraries. I need to do it recursively or split them and append them again. The best thing I made so far was to reverse only the first even list and append the first odd and even list in a new list.

I tried to do this:

reverselist(List, [List]).
reverselist([X,Y|Rest], [SnakeList|Rest2]):-
    append(X, [], Odd),
    reverse(Y, EvenList),
    append(Odd, EvenList, SnakeList),
    reverselist(Rest, Rest2).

And this:

 reverselist(List1, List2).
 reverselist([H|Ts], [Odd|R]):-
    not(0 is H mod 2),
    append(H, [], Odd),
    reverselist(Ts, R).
 reverselist([H|Ts], [Even|R]):-
    0 is H mod 2,
    reverse(H, Even),
    reverselist(Ts, R).

Sample query:

?- reverselist([[a,b,c],[d,a,b],[c,d,o],[b,c,d],[e,e,d]], List).

I want the result to be:

List = [ [a,b,c],[b,a,d],[c,d,o],[d,c,b],[e,e,d] ].

Answer 1

You can also write mutual recursion:

reverselist([],[]).
reverselist([H|T],[H|T1]):-reverselist2(T,T1).

reverselist2([],[]).
reverselist2([H|T],[H1|T1]):-reverse(H,H1), reverselist(T,T1).

Answer 2

We need to create another predicate with one more argument to keep track of odd or even position:

reverselist(InList,OutList):- reverselist(InList,OutList, 0).

reverselist([],[],_). %base case
%case of even position
reverselist([H|T],[H|T1], 0):- reverselist(T,T1,1).
%case of odd position
reverselist([H|T],[H1|T1], 1):- reverse(H1,H), reverselist(T,T1,0).

Answer 3

You were pretty close with your first variant.

Instead of your

reverselist(List, [List]).
reverselist([X,Y|Rest], [SnakeList|Rest2]):-
  append(X, [], Odd),
  reverse(Y, EvenList),
  append(Odd, EvenList, SnakeList),
  reverselist(Rest, Rest2).

just tweak it as

reverselist([],   []).               % additional clause
reverselist([List], [List]).
reverselist([X,Y|Rest], [X,EvenList|Rest2]):-
  reverse(     Y,          EvenList),
  reverselist(   Rest,              Rest2).

All three clauses are mutually exclusive and together they are exhaustive, ie they cover every possibility.

I believe this definition to be the most immediate and close representation of your problem. In Prolog, to formulate the problem means to have the solution for it.