Get every element besides the last from a list in Prolog using append

Question

How do you make a rule that returns all elements besides the last in another list in Prolog using append? I've made the equivalent without append but I can't figure out the append solution

remlast([_],[]).
remlast([H1|T1],[H2|T2]):-
  H2=H1,
  remlast(T1,T2),
  !.

I can get the last element from a list with append with this

mylastAppend(X,List):-
  append(_,[X],List),!.

But I can't figure out how to use that in the above example

`append(Beginning, [_], List)` means Beginning must be all the elements of List except the last one. — TessellatingHeckler, Aug 18 '22 at 23:38

score 0 · Accepted Answer · answered Aug 19 '22 at 06:05

0

Use:

list_front(List, Front) :-
    append(Front, [_], List).

This will split the list in Front of the list and a one-element list from the back of List.

answered Aug 19 '22 at 06:05

TA_intern

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score 0 · Answer 2 · answered Aug 19 '22 at 10:03

Your "equivalent without append" is not optimal. This is better: https://stackoverflow.com/a/51291825/

Here is a variation which results in a difference list (which can then be used for e.g. an efficient append):

list_without_last_dl(Lst, LstWithout, LstWithoutTail) :-
    Lst = [H|T],
    list_without_last_dl_(T, H, LstWithout, LstWithoutTail).

list_without_last_dl_([], _H, W, W).
list_without_last_dl_([H|T], H2, [H2|W], WT) :-
    list_without_last_dl_(T, H, W, WT).

Result in swi-prolog:

?- list_without_last_dl([a,b,c], W, WT).
W = [a,b|WT].

Get every element besides the last from a list in Prolog using append

2 Answers2