How do you append an element to a list in place in Prolog?

Question

If I have a list in Prolog such as X = [1, 2, 3, 4], how do I add the element 5 to the end of the list to have X = [1, 2, 3, 4, 5]?

The append function needs two lists

Answer 1

You can't modify lists in Prolog, but you can create a list with an unspecified length:

main :-
    A = [1,2,3,4|_].

Then, you can insert an element using nth0/3 in SWI-Prolog:

:- initialization(main).

main :-
    A = [1,2,3,4|_],
    nth0(4,A,5),
    writeln(A).

After this element is inserted, A = [1,2,3,4,5|_].

You can also define a function that appends an item to the end of a list in-place, and then use it like this:

:- initialization(main).

append_to_list(List,Item) :-
    List = [Start|[To_add|Rest]],
    nonvar(Start),
    (var(To_add),To_add=Item;append_to_list([To_add|Rest],Item)).

main :-
    A = [1,2,3|_],
    append_to_list(A,4),
    append_to_list(A,4),
    writeln(A).

In this example, A = [1,2,3,4,4|_] after these two items are appended.