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?

What about nested delimiters (['Start', 'want', 'Start', 'want', 'Stop', 'Stop'])? Is that a Start/Stop inside another Start/Stop, overlapping intervals, or invalid? — mu is too short
– mu is too short, Commented Nov 10, 2012 at 19:22
In that case, I think I'd want everything between the "Start" and the first "Stop". Then it would start again. So, given your example, I'd want the result to be: [["Start", "want", "Start", "want", "Stop"]] — Glenn
– Glenn, Commented Nov 10, 2012 at 19:30

Jean-Louis Giordano · Accepted Answer · 2012-11-10 20:40:04Z

7

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!

edited Nov 10, 2012 at 20:40

answered Nov 10, 2012 at 20:17

Jean-Louis Giordano

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4 Comments

Konstantin Ilchenko Over a year ago

Greate decision! a.select {|i| true if (i=="Start")..(i=="Stop") } would be enough

ChuckE Over a year ago

I like that condition, didn't know that was possible! still, the result is not the expected result, You still have to present it as an array of arrays.

Jean-Louis Giordano Over a year ago

OK! I fixed that with slice_before.

Glenn Over a year ago

Thanks for all of the suggestions. I found that this one worked very well.

ChuckE · Accepted Answer · 2012-11-10 20:26:26Z

1

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]}

edited Nov 10, 2012 at 20:26

answered Nov 10, 2012 at 19:43

ChuckE

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Comments

DigitalRoss · Accepted Answer · 2012-11-12 07:00:49Z

1

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

edited Nov 12, 2012 at 7:00

answered Nov 10, 2012 at 19:51

DigitalRoss

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Comments

megas · Accepted Answer · 2012-11-10 20:35:33Z

0

a.join('^').scan(/Start.*?Stop/).map { |x| x.split('^') }
=> [["Start", "want", "want", "Stop"], ["Start", "want", "Stop"]]

The symbol ^ seems to be rare so maybe it won't mix with another original symbols.

answered Nov 10, 2012 at 20:35

megas

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Comments

sawa · Accepted Answer · 2012-11-10 20:46:55Z

0

:dummy below is to ensure that the first chunk of the return from slice_before is what you do not want.

p [pres = :dummy, *a].slice_before{|e|
  prev, pres = pres, e
  prev == "Stop" or pres == "Start"
}
.select.with_index{|_, i| i.odd?}
# => => [["Start", "want", "want", "Stop"], ["Start", "want", "Stop"]]

edited Nov 10, 2012 at 20:46

answered Nov 10, 2012 at 20:38

sawa

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