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listrecursionprologsublistvariable-length

Prolog Length of Evens in a List

I am a Prolog beginner, and I have the following length function:

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

I would like to know if there's an easy way to query the interpreter to calculate the length of the list of evens in a list, and if not, how I would go about incrementing N only on an even number. Thanks.

Solution

  • Something similar to this is what you might be looking for

    mylen([H|Lc],N) :- 
X is mod(H,2),
X == 0,
mylen(Lc,M),
N is M+1.

mylen([],0).

mylen([H|Lc],N) :-
mylen(Lc, M),
N = M.