I have an assignment where I need to rotate a list once to the right. I also have a constraint where I can only use one predicate. It seems that shifting to the left is very easy:
([H|T], R) :- append(T, [H], R).
Though, it seems to be harder to rotate right AND keep the constraint. This is my attempt:
rotate([H], H).
rotate([H|T], F) :- rotate(T,F1), append(F1,T,F).
In my head, it works perfectly, though I only get false as an output. Any help solving this would be much appreciated!
If you have already a predicate, say rotl(L, Rotated) that rotates one to the left, you can use that very same predicate to rotate to the right. Simply put the list into the second argument and keep the first a variable!
That is the deep idea of relations: You can use them in more than one manner!
So to summarize:
rotleft([E|T], R) :-
append(T,[E],R).
rotright(L, R) :-
rotleft(R, L).
And here is the most general query that shows you all answers that contain all possible solutions:
?- rotright(L,R).
L = [_A], R = [_A]
; L = [_A,_B], R = [_B,_A]
; L = [_A,_B,_C], R = [_C,_A,_B]
; L = [_A,_B,_C,_D], R = [_D,_A,_B,_C]
; L = [_A,_B,_C,_D,_E], R = [_E,_A,_B,_C,_D]
; L = [_A,_B,_C,_D,_E,_F], R = [_F,_A,_B,_C,_D,_E]
; L = [_A,_B,_C,_D,_E,_F,_G], R = [_G,_A,_B,_C,_D,_E,_F]
; L = [_A,_B,_C,_D,_E,_F,_G,_H], R = [_H,_A,_B,_C,_D,_E,_F,_G]
; L = [_A,_B,_C,_D,_E,_F,_G,_H,_I], R = [_I,_A,_B,_C,_D,_E,_F,_G,_H]
; ... .
Do you see the pattern? For each length of a list (except for the empty list), there is one answer that contains all possible solutions. Look how in one answer the variables are the same. I will take one answer to illustrate this:
; L = [_A,_B,_C,_D,_E,_F,_G],
% \ \ \ \ \ \ \____
% ___ \ \ \ \ \ \
% \ \ \ \ \ \ \
R = [_G,_A,_B,_C,_D,_E,_F]
And here is the most interesting question:
How do lists look like that are the same as the list rotated to the left/right?
Try to formulate that query and look at the answers in detail.