CODEWITHSUNDEEP
prolog
Guilherme Moreira
Guilherme Moreira

Reputation: 121

Prolog - Appending to list recursively

I wrote this function where I have as a parameter a set of coordinates and I want to create a new list with all the coordinates to the left of them (so basically X-1) but at the end it doesn't ask me if the option given is correct, it just prints yes. Here is the code:

all_valid_moves([], L):-
    write('------------------------'),
    nl,
    write(L),
    nl,
    write('------------------------'),
    nl,
    !.
all_valid_moves([Head|Tail], All_Valid):-
    nth0(0, Head, X),
    nth0(1, Head, Y),
    NewX is X - 1,
    append([[NewX, Y]], All_Valid, New_All_Valid),
    write(All_Valid),
    nl,
    all_valid_moves(Tail, New_All_Valid).

Test case all_valid_moves([[1,1],[2,2],[3,3]], X).. Should return [[0,1],[1,2],[2,3]].

If anyone can point out the mistake(s) I made, please do so! Thanks alot for your time, Have a good one

Upvotes: 0

Views: 53

Answers (2)

Nicholas Carey
Nicholas Carey

Reputation: 74227

Simplify, simplify, simplify. No need for nth0/3. No need for append/3. Just this:

all_valid_moves( []         , []          ) .
all_valid_moves( [[X,Y]|As] , [[X1,Y]|Bs] ) :- X1 is X-1, all_valid_moves(As,Bs).

Upvotes: 0

Guilherme Moreira
Guilherme Moreira

Reputation: 121

Final answer

I added a new variable to the function so I could return it in a new list, this was the final result:

all_valid_moves(Board, [], L, New):-
    New = L.
all_valid_moves(Board, [Head|Tail], Previous, All_Valid):-
    nth0(0, Head, X),
    nth0(1, Head, Y),
    NewX is X - 1,
    append([[NewX, Y]], All_Valid, New_All_Valid),
    all_valid_moves(Board, Tail, New_Prev3, New_All_Valid),
    All_Valid = New_All_Valid.

Upvotes: 1

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