CODEWITHSUNDEEP
listprologbetween
user3090164
user3090164

Reputation: 65

Prolog: between/3 with a list at the end

my question comes up because of this question Can you write between/3 in pure prolog?

would it be possible to make between/3 and the third argument is a list so if you ask

between(2,6,X).

it comes

X=[2,3,4,5,6]

and not like

X=2
X=3
X=4
....

I can’t figure out how this must work (all my solutions don’t work..) I’m a Prolog beginner so I have no idea..

sorry for the bad English..

Thanks for your help :)

Upvotes: 5

Views: 8311

Answers (1)

user1812457
user1812457

Reputation:

Start by going to the library and getting a good book, for example "The Art of Prolog" by Sterling and Shapiro.

Two ways:

?- findall(X, between(2, 6, X), Xs).
Xs = [2, 3, 4, 5, 6].

You should also take a look at bagof/3 and setof/3.

For a direct way, numlist/3, see for example the SWI-Prolog implementation. Without argument checking it comes down to:

numlist(U, U, List) :- !,
    List = [U].
numlist(L, U, [L|Ns]) :-
    L2 is L+1,
    numlist(L2, U, Ns).

There are several ways to break the predicate as it stands.

?- numlist(1,0,L).

will not terminate. You need to either check the arguments before passing them to this particular version of numlist/3:

must_be(integer, L),
must_be(integer, U),
L =< U

These checks are incorporated in the library predicate numlist/3 from the linked SWI-Prolog implementation.

Upvotes: 8

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