Question

I trying to write a function getNumbers(List,Result) such that List is a list which his elements can be integer or list of lists , for example –

List = [1,[1,2,[3],[4]],2,[4,5]]
List = [1,[1,1,[1],[1]],1,[1,1,[[[[[1]]]]]]]
List = [[4,[[]],2],[[1],[],[1]]]
etc..

And the output should be all the numbers stored in that List , for example –

?- getNumbers([1,[1,2,[3],[4]],2,[4,5]],R).
R = [1,2,3,4,5].
?- getNumbers([1,[1,1,[1],[1]],1,[1,1,[[[[[1]]]]]]],R).
R = [1].
?- getNumbers([],R).
R = [].
?- getNumbers([[4,[[]],2],[[1],[],[1]]],R).
R = [1,2,4].

So far I tried the follow code –

getNumbers([],Result) :- Result=[],!.
getNumbers([H|Rest],Result) :- getNumbers(Rest,NewResultRest),
    ( atomic(H) ->  
      Result = [H|NewResultRest]    
    ; getNumbers(H,NewResultHead),Result = [NewResultHead|NewResultRest]    ).

But it gives wrong result , like –

 getNumbers([[2],5,7,[3,6,5]],Result).
Result = [[2], 5, 7, [3, 6, 5]].

It seems that the function doesn’t exludes 2 from [2] or any other numbers stored in nested list.

How could I fix my implementation ?

Answer 1

you need to append nested lists:

getNumbers([],Result) :- Result=[],!.
getNumbers([H|Rest],Result) :-
    getNumbers(Rest,NewResultRest),
    (  atomic(H)
    -> Result = [H|NewResultRest]
    ;  getNumbers(H,NewResultHead),
       append(NewResultHead, NewResultRest, Result) % only this change
    ).

note that [] is atomic: thus

?- getNumbers([[4,[[]],2],[[1],[],[1]]],R).
R = [4, [], 2, 1, [], 1].

from your description, you should use number/1 to test element’ type. After the change

?- getNumbers([[4,[[]],2],[[1],[],[1]]],R).
R = [4, 2, 1, 1].