Find how many same objects exist

I have this code

co开发者_Python百科lor(blue).
color(red).
color(blue).
color(green).

I want to make a rule that will count how many times the X color exists. For this case count_color(X) should return 2.

Is that possible in this way or i have to make a list with the colors?

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aggregate/3 does not exist in ISO prolog, so it's not available in all implementations. But you can get the same result using findall/3, as in:

count_color(Color, N) :- findall(_, color(Color), List), length(List, N).

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It is possible by using the aggregate/3 predicate:

count_color(Color, N) :- aggregate(count, color(Color), N).

A pointer for using aggregate/3: aggregate/3 in swi-prolog