Find how many same objects exist

Question

I have this code

color(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?

Do you mean that after calling `count_color(blue, X)`, `X` should be 2? — svick, May 28 '11 at 19:01
This question covers the same ground as [this earlier one](http://stackoverflow.com/questions/6060268/prolog-count-the-number-of-times-a-predicate-is-true), which presents a method of solution not yet described here (assert/retract) besides the two which are. — hardmath, May 31 '11 at 01:25

score 2 · Answer 1 · answered May 30 '11 at 12:27

2

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).

answered May 30 '11 at 12:27

Jerome

score 1 · Answer 2 · edited May 23 '17 at 12:04

1

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

edited May 23 '17 at 12:04

Community

answered May 28 '11 at 19:07

pad

2 Answers2