CODEWITHSUNDEEP
recursionprologtail
John Smith
John Smith

Reputation: 53

Adding a tail in Prolog

I'm having trouble understanding how prolog works with lists. I'm trying to write a function that will add a tail to a given list and returns a new list. but my implementation does not work and I can't understand how to fix it. Here is what I have:

% add_tail(L,M,E) :- L is M with [E] appended.
% I wanna do this without using the append predicate

add_tail([E],[],E).
add_tail(List, [H|T], E):-
    add_tail(List1, T, E),
    List is [H|List1].

to clarify what i want this function to do, here is an example of an output:

?- add_tail(L,[1,2,3],4).
L = [1,2,3,4].

Upvotes: 2

Views: 2199

Answers (1)

Yan Foto
Yan Foto

Reputation: 11378

Check this out:

append([X|Y],Z,[X|W]) :- append(Y,Z,W).
append([],X,X).

The second line (append([],X,X)) simply states that appending an element X to an empty list is equal to that element itself (break condition). The first line states appending Z to a list [X|Y] (X is head, Y rest) results in a new list [X|W], whereas W is the result of appending Z to Y.

Example

?- append([1,2,3],[4],L).
L = [1,2,3,4]

In your case you just need to reorder the parameters!

Upvotes: 1

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