Array.extract

array_extensions.rb

# We need to create Array.extract if it doesn't already exist
class Array
  # Delete some of our items and return (or "extract") a new array holding the deleted items.
  # Very similar to partition, but instead of returning copies, it modifies the original list
  def extract
    extracted = []
    # reverse our deletion so indexes don't get messed up while we're deleting
    (length - 1).downto(0) do |idx|
      item = self[idx]
      extracted << delete_at(idx) if yield(item)
    end
    extracted.reverse
  end
end

#require_relative "../../../lib/extensions/array_extensions.rb"
describe Array do
  context "extract" do
    it 'should modify the existing list and return a separate non-overlapping list' do
      ar = [1,2,3,4,5]
      odds = ar.extract { |item| item.odd? }
      odds.should == [1,3,5]
      ar.should == [2,4]
    end

    it 'should allow an empty source list' do
      ar = []
      odds = ar.extract { |item| item.odd? }
      odds.should == []
      ar.should == []
    end
    it 'should allow an empty destination list' do
      ar = [1,2,3,4,5]
      none = ar.extract { |item| false }
      none.should == []
      ar.should == [1,2,3,4,5]
    end
    it 'should allow everything to be copied' do
      ar = [1,2,3,4,5]
      under_ten = ar.extract { |item| item < 10 }
      puts "ar: #{ar.inspect}."
      puts "under_ten: #{under_ten.inspect}."
      under_ten.should == [1,2,3,4,5]
      ar.should == []
    end
  end
end