CODEWITHSUNDEEP
prolog
DoubleOseven
DoubleOseven

Reputation: 271

List manipulation with prolog

I am trying to create a query that gives back the following:

sky([1,2,3,4,5,6],X). 
X = [1,3,5,2,6,4]

That is, it takes out every other element of a list, and does the same on the remaining list and puts everything together.

This is my code so far.

sky([X|Y], Skied):-
    split([X|Y],Z1),
    split(X,Z2),
    sky(Z2,Z3),
    append(Z1,Z3,Skied).

sky([],[]).

split([X,_|T], [X|R]):-
    split(T,R).

split([X|[]], [X]).
split([],[]).

Can someone explaint o me why it wont work and the process behind it, like a visual guide. Thanks!

Upvotes: 1

Views: 553

Answers (1)

coder
coder

Reputation: 12972

Your code is almost right, instead of split(X,Z2) you need to write split(Y,Z2). That's because X is a single element and split(X,Z2) will return [X], which if I understood correctly isn't what you want. You need to write split(Y,Z2) to take the even elements (or in your description every other element) and call sky(Y,Z3) to do the same recursively. So the new version is:

sky([X|Y], Skied):-
    split([X|Y],Z1),
    split(Y,Z2),
    sky(Z2,Z3),
    append(Z1,Z3,Skied).

sky([],[]).

split([X,_|T], [X|R]):-
    split(T,R).

split([X|[]], [X]).
split([],[]).

Some examples and output:

?- sky([1,2,3,4,5,6],X).
X = [1, 3, 5, 2, 6, 4] ;
false.
?- sky([1,2,3,4,5,6,7,8,9],X).
X = [1, 3, 5, 7, 9, 2, 6, 4, 8] ;
false.

Upvotes: 1

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