CODEWITHSUNDEEP
prologpredicate
AndroidDev
AndroidDev

Reputation: 67

Split a list in prolog to atoms and integers

I am new to prolog and trying to make a predicate to separate a list in to atoms and integers but I have been try different ways for a while now but no where is there is an example on how this would be done. To me this seems like a basic thing to know in proleg yet there is no example that I can find on how this would be done.

eg: separate([3,t,8,l,0,a,5,g,2],Atoms,Integers). Atoms = [a,g,l,t], Integers = [0,2,5,3,8]

Upvotes: 0

Views: 400

Answers (2)

Nicholas Carey
Nicholas Carey

Reputation: 74277

Especially if you're learning, roll your own. This also allows you to handle the edge cases. This, for instance, simply discards things other than atoms and integers:

atoms_and_ints( []     , []        , []       ) .   % The empty list doesn't have any atoms or ints.
atoms_and_ints( [X|Xs] , [X|Atoms] , Ints     ) :-  % For a non-empty list,
  atom(X),                                          % - is the head an atom?
  !,                                                % - eliminate the choice point
  atoms_and_ints(Xs,Atoms,Ints).                    % - add it to the atoms and recurse down.
atoms_and_ints( [X|Xs] ,    Atoms  , [X|Ints] ) :-  % Similarly,
  integer(X),                                       % - is the head an integer?
  !,                                                % - eliminate the choice point
  atoms_and_ints(Xs,Atoms,Ints).                    % - if so, add it to the Ints and recurse down.
atoms_and_ints( [_|Xs], Atoms, Ints ) :-            % If the head of the list is something else...
  atoms_and_ints(Xs,Atoms,Ints).                    % - discard it and recurse down.

Upvotes: 0

gusbro
gusbro

Reputation: 22585

In swi-prolog, using partition/4:

separate(List, Atoms, Integers):-
  partition(integer, List, Integers, Atoms).

This assumes that every item in the list is either an integer or an atom

Sample run:

?- separate([3,t,8,l,0,a,5,g,2],Atoms,Integers).
Atoms = [t, l, a, g],
Integers = [3, 8, 0, 5, 2].

Upvotes: 1

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