Delete from Array and return deleted elements in Ruby

Question

How can I delete some elements from an array and select them?

For example:

class Foo 
  def initialize
    @a = [1,2,3,4,5,6,7,8,9]
  end

  def get_a
    return @a
  end
end

foo = Foo.new
b = foo.get_a.sth{ |e| e < 4 }
p b # => [1,2,3]
p foo.get_a # => [4,5,6,7,8,9,10]

What I can use instead of foo.get_a.sth?

Answer 1

If you don't need to retain the object id of a:

a = [1,2,3,4,5,6,7,8,9,10]
b, a = a.partition{|e| e < 4}
b # => [1, 2, 3]
a # => [4, 5, 6, 7, 8, 9, 10]

If you do need to retain the object id of a, then use a temporal array c:

a = [1,2,3,4,5,6,7,8,9,10]
b, c = a.partition{|e| e < 4}
a.replace(c)

Answer 2

Rails 6 now has this:

a = [1, 2, 3]
#=> [1, 2, 3]
a.extract! { |n| n.even? }
#=> [2]
a
#=> [1, 3]