Getting frequency of nested elements, with relationship retained

Question

I have an array of

shop objects
- which belong to city objects
  - which belong to prefecture objects

I'd like to end up with a hash listed by prefecture, then city, then frequency...

I came up with this, but it feels really un-rubylike..

city_by_prefecture = shop_list.reduce({}){ |h,e|
  if h[e.prefecture.name].nil?
    h[e.prefecture.name] = {e.city.name => 1}
  elsif h[e.prefecture.name][e.city.name].nil?
    h[e.prefecture.name][e.city.name] = 1
  else
    h[e.prefecture.name][e.city.name] += 1
  end
  h
}

There must be a DRY-er way to do this !

possible duplicate of [How to assign hash\["a"\]\["b"\]= "c" if hash\["a"\] doesn't exist?](http://stackoverflow.com/questions/5878529/how-to-assign-hashab-c-if-hasha-doesnt-exist) — Andrew Grimm, Aug 02 '11 at 08:00
You may want to use `h.has_key?(e.prefecture.name)` rather than `h[e.prefecture.name].nil?`, because that way it's more obvious what you're asking. Also, use `each_with_object` rather than `reduce`, so you don't have to put `h` at the end of the block. — Andrew Grimm, Aug 02 '11 at 08:04

score 1 · Accepted Answer · answered Aug 02 '11 at 08:40

1

city_by_prefecture = shop_list.each_with_object({}){ |e,h|
  h[e.prefecture.name] ||= Hash.new(0)
  h[e.prefecture.name][e.city.name] += 1
}

answered Aug 02 '11 at 08:40

Mladen Jablanović

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score 0 · Answer 2 · answered Aug 02 '11 at 08:49

shops = [
  OpenStruct.new(:prefacture => "pre1", :city => "city1"), 
  OpenStruct.new(:prefacture => "pre1", :city => "city1"), 
  OpenStruct.new(:prefacture => "pre1", :city => "city2"), 
  OpenStruct.new(:prefacture => "pre2", :city => "city3"),
]

counts = Hash[shops.group_by(&:prefacture).map do |prefacture, shops_in_prefacture| 
  [prefacture, Hash[shops_in_prefacture.group_by(&:city).map do |city, shops_in_city| 
    [city, shops_in_city.size]
   end]] 
end]
# {"pre1"=>{"city1"=>2, "city2"=>1}, "pre2"=>{"city3"=>1}}

Getting frequency of nested elements, with relationship retained

2 Answers2