counting the same elements in the list.

[metalazo]

Hi everybody, I'm a new user...and I hope that this forum can help me!!!
I've got a list composed by the following elements: (a,b,c,d,e,f,g,h,e)...(the 'e' char is present two times)
ok now I have to verify if the 'e' char belongs to the list. The code is very simple:
member(X,[X|_]).
member(X,[_|Y]):-member(X,Y).
The question to the prolog compiler is :
?-member(e,[a,b,c,d,e,f,g,e]).
In this way I can verify that the 'e' char belongs to the list.
My question is: How can I count the number of 'e' char in the list???

Thanks to all.
Andrea

 

[kahleen]

You will have to write a suitable predicate, like member, only for counting occurences of elements within lists:

count(X, List, Occurrences) ...

For example, it will work like this, in your example:

count(e, [a,b,c,d,e,f,g,e], Occurences).
Occurences = 2 ...

The code for it is something like this:

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

 

[kahleen]

sorry, there is a mistake in my code ... below is the correct code:

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

 

[metalazo]

Oh wow, you are great !!!!

thanks a lof, it works fine !!!

bye bye Andrea