CODEWITHSUNDEEP
prolog
Blair
Blair

Reputation: 3761

Counting Sublist Elements in Prolog

How can I count nested list elements in Prolog? I have the following predicates defined, which will count a nested list as one element:

length([ ], 0).
length([H|T],N) :- length(T,M), N  is  M+1.

Usage:

?- length([a,b,c],Out).
Out = 3

This works, but I would like to count nested elements as well i.e.

length([a,b,[c,d,e],f],Output).
?- length([a,b,[c,d,e],f],Output).

Output = 6

Upvotes: 4

Views: 5192

Answers (1)

Volodymyr Gubarkov
Volodymyr Gubarkov

Reputation: 2173

len([H|T],N) :-
    len(H, LH),
    len(T, LT),
    !,
    N is LH + LT.
len([], 0):-!.
len(_, 1):-!.

Test:

?- len([a,b,[c,d,e],f],Output).
Output = 6.

Upvotes: 3

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