FILTER - Filter a list based on a condition

filter.red

Red [
	title: "FILTER - Filter a list based on a condition"
	author: "hinjolicious"
	usage: {
		a: [1 2 3 4 5 6 7 8 9 10]
		print mold filter a [[x] x % 2 = 0]
		
		a: [[1 2] [3 6] [5 6] [7 14] [9 10]]
		print mold filter a [[x y] (x * 2) = y]
		print mold a || [[x y] (x * 2) = y]
	}
	resource: "Gemini AI"
]

; version 4
FILTER: function [x [series!] f [block!]] [
	a: f/1 ; arguments must present and at least has one
	ff: function a next f ; construct a func
	
	either (length? a) = 1 [ ; one arg
		collect [foreach e x [
			if ff e [keep/only e]
		]]
	][ ; multiple args
		collect [foreach e x [
			if apply :ff e [keep/only e]
		]]
	]
]

||: make op! :FILTER

comment {
; test:

a: [1 2 3 4 5 6 7 8 9 10]
print mold filter a [[x] x % 2 = 0]

a: [[1 2] [3 6] [5 6] [7 14] [9 10]]
print mold filter a [[x y] (x * 2) = y]

print mold a || [[x y] (x * 2) = y]
}

Example code using map and filter:

#include %pipe-map.red
#include %filter.red

data: [[1 2] [3 4] [5 6] [7 8]]

result: data 
    || [[a b] a + b > 5]            ; 1. Filter out matrices summing <= 5
    ||> [[a b] reduce [b a]]        ; 2. Swap the columns of the survivors

probe result
; == [[4 3] [6 5] [8 7]]