CODEWITHSUNDEEP
prolog
Chuckles
Chuckles

Reputation: 41

Prolog rotate list N times in either direction

So my objective here is to write a predicate that rotates a list to the right N times if N>0 or the left N times if N<0. Here is my code so far, the output is false unless N == 0 and I am not sure where my issue is.

rotateleft([L|R], N) :-
   append(R, [L], N).

rotateright(L, R) :-
   rotateleft(R, L).

rotate(L,N,R) :-
   (  N = 0 ->
      R = L
   ;  N > 0 ->
      rotateright(L,R),
      N is N-1,
      rotate(L,N,R)
   ;  N < 0,
      rotateleft(L,R),
      N is N+1,
      rotate(L,N,R)
   ).

Upvotes: 3

Views: 988

Answers (1)

false
false

Reputation: 10102

list_rotated(N, Xs, Ys) :-
   maplist(\_^_^true, Xs, Ys),  % or same_length(Xs, Ys)
   length(Xs, M),
   R is -N mod M,
   length(As, R),
   append(As,Bs, Xs),
   append(Bs,As, Ys).

Please note that this definition has much better termination behavior. It terminates iff the length of one of the lists is known.

Upvotes: 3

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