Prolog Type Error

Question

I\'m working on a prolog algorithm that will perform a \"swap\" on a list.

Example:

Input: [1,2,3,4] -> Output: [3,4,1,2] Input: [1,2,3,4,5] -> Output: [4,5,3,1

Answer 1

Prolog doesn't do inline expression evaluation. Thus, calls such as trim(L2, Length/2 , B) and trim(L, round(Length/2), A) will not work as you expect. Expressions are only evaluated in specific when using certain operators such as is/2, arithmetic comparisons, or their CLP(FD) counterparts. These expressions would need to be done as: L is Length // 2, trim(L2, L, B) and R is round(Length/2), trim(L, R, A) if done literally.

Your solution could be condensed, however, as follows.

swap(L, S) :-
    same_length(L, S),               % L and S are necessarily the same length
    length(L, N),
    M is N // 2,   % integer divide ; half the original list length
    length(Left, M),                 % Left is a list of half the length of L
                                     %   or half minus one if the length is odd
    (   (N mod 2) =:= 1              % If the original length is odd...
    ->  append(Left, [H|Right], L),  % then L is Left + [H|Right]
        append(Right, [H|Left], S)   %   So, S is Right + [H|Left]
    ;   append(Left, Right, L),      % otherwise, L is Left + Right
        append(Right, Left, S)       %   So, S is Right + Left
    ).