Group Ruby array by specific values

Question

I have an array and I'd like to select only the elements between two specified values.

For example, I have an array that looks like this:

a = ["don't want", "don't want", "Start", "want", "want", "Stop", "don't want", "Start", "want", "Stop", "don't want"]

I want to call a method on the a array that captures the elements between "Start" and "Stop" (that includes the "Start" and "Stop" elements). The resulting array should look like this:

[["Start", "want", "want", "Stop"], ["Start", "want", "Stop"]]

I could not find a method like this, so I tried writing one:

class Array
  def group_by_start_and_stop(start = "Start", stop = "Stop")
    main_array = []
    sub_array = []

    group_this_element = false

    each do |e|
      group_this_element = true if e == start

      sub_array << e if group_this_element

      if group_this_element and e == stop
        main_array << sub_array
        sub_array = []
        group_this_element = false
      end
    end

    main_array
  end
end

This works, but it seems perhaps unnecessarily verbose to me. So I have two questions: Does a similar method already exist? If not, are there ways to make my group_by_start_and_stop method better?

Answer 1

That's the perfect example where a flip-flop is useful!

a.select {|i| true if (i=="Start")..(i=="Stop")}.slice_before("Start").to_a

Not super known feature, but pretty cool nonetheless! Use it together with slice_before, and you get exactly what you want!

Answer 2

a.each_with_index.select{|s, i| s.eql?("Start") or s.eql?("Stop")}
                 .map{|s, i| i}
                 .each_slice(2)
                 .map{|s, f| a[s..f]}

Answer 3

a.inject [] do |m, e|
  if e == 'Start'
    @state = true
    m << []
  end
  m.last << e if @state
  @state = false if e == 'Stop'
  m
end