How to count only the number elements in a list (PROLOG)

Question

count([],0).
count([_|Tail], N) :- count(Tail, N1), N is N1 + 1.

This count all the elements, but I need to count only the numbers.

With a little searching on SO, you could find [Find Only Numbers In List](https://stackoverflow.com/questions/26618589/find-only-numbers-in-list) — lurker, Jun 13 '17 at 00:07

score 5 · Accepted Answer · answered Jun 12 '17 at 23:26

5

Prolog has an ISO builtin predicate number/1 that checks whether the given parameter is a number.

We can simply use an if-then-else statement that either increments N is N1+1, or sets N = N1, like:

count([],0).
count([H|Tail], N) :-
    count(Tail, N1),
    (  number(H)
    -> N is N1 + 1
    ;  N = N1
    ).

answered Jun 12 '17 at 23:26

Willem Van Onsem

When I try ?- count([1,3,4,5,a,f,v], 0). The answer is false, instead of the count of number elements – Alexandre Rocha Jun 12 '17 at 23:32
@AlexandreRocha: that is correct, the total count is not `0`, it is `4`... You should query `count([1,3,4,5,a,f,v], N).` or `count([1,3,4,5,a,f,v], 4).`. – Willem Van Onsem Jun 12 '17 at 23:35
Ohh... I see, THANK YOU VERY MUCH Wiillem , you really help me. – Alexandre Rocha Jun 12 '17 at 23:38
1

`count([X], 0))`. succeeds, but `X = 1, count([X], 0)` fails. – false Jun 13 '17 at 08:03
See [this](https://stackoverflow.com/a/30600104/772868) for a clean solution – false Jun 13 '17 at 08:12

score 1 · Answer 2 · answered Jun 12 '17 at 23:24

1

You could use number/1 iso built-in predicate:

count([],0).
count([H|Tail], N) :- number(H),count(Tail, N1), N is N1 + 1.
count([H|Tail], N) :- \+number(H),count(Tail, N).

answered Jun 12 '17 at 23:24

coder

When I try ?- count([1,3,4,5,a,f,v], 0). The answer is false, instead of the count of number elements – Alexandre Rocha Jun 12 '17 at 23:33
Yes the answer that prolog gives you is right!! – coder Jun 12 '17 at 23:38
`count([X], 0))`. succeeds, but `X = 1, count([X], 0)` fails. – false Jun 13 '17 at 08:04
@false I don't see anything wrong, `X = 1, count([X], 0) `clearly must fails and the only thought is what should `count([X], 0) ` do ?? (succeeding in last case is not that bad...) – coder Jun 13 '17 at 08:13
1

For a relational solution (that is, a solution which maintains that the definition is a relation) you have at least two options: produce an instantiation error (as with [`number_itype/1`](https://stackoverflow.com/a/30600104/772868)), or use, say `number_type(N) :- freeze(N, number(N))` - but then you would need a `non_number_type(N) :- freeze(N, \+ number(N)).` too. – false Jun 13 '17 at 08:17
1

Another option might be that you loop in that case. Not the best option but better than producing an incorrect result. – false Jun 13 '17 at 08:19
@false ,thanks for the helpful comments !! – coder Jun 13 '17 at 08:23

2 Answers2