CODEWITHSUNDEEP
prolog
GelarehNo
GelarehNo

Reputation: 19

how to remove the first two elements and the last two elements of a list?

the following code is not possible, because it can only remove the first two or last two.

delements(L,L1):- append([_,_],L1,L),append(L1,[_,_],L).

Upvotes: 1

Views: 88

Answers (2)

madeofmistake
madeofmistake

Reputation: 528

trim(X,Y) :-
    append([[_,_],Y,[_,_]],X).

Upvotes: 2

willeM_ Van Onsem
willeM_ Van Onsem

Reputation: 476669

You should not reuse L1 and L in the same list. You need an extra variable here:

delelements(L, R) :-
    append(M, [_, _], L),
    append([_, _], R, M).

So here M is a list that contains the elements of L, except for the last two. R is a variant of M, except that the first elements are removed.

This then give us:

?- delelements([1,4,1,3,0,2,2,5], R).
R = [1, 3, 0, 2] ;
false.

Furthermore we do not need to use append/3 [swi-doc] to remove a fixed number of elements from the head. We can use unification for this:

delelements(L, R) :-
    append([_, _ | R], [_, _], L).

Upvotes: 3

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