The filter Method

filter() calls a function on every element and keeps only the elements for which it returns true. It returns them in a new array, in their original order:

func is_even(n):
	return n % 2 == 0

var numbers = [1, 2, 3, 4, 5, 6]
print(numbers.filter(is_even))

[2, 4, 6]

is_even(1) returns false, so 1 is dropped. is_even(2) returns true, so 2 is kept, and so on.

With a lambda, the condition sits right in the call:

var words = ["apple", "banana", "cherry", "date"]
print(words.filter(func(w): return w.length() > 5))

["banana", "cherry"]

The result can be shorter than the original, or even empty, and the original array does not change. On a typed array such as Array[String], filter() returns an array of the same type.

filter() and map() both return arrays, so you can chain them. This keeps the even numbers and then squares them:

print(numbers.filter(is_even).map(func(n): return n * n))

[4, 16, 36]