How to transform array of hashes to hash object with keys from values?

Question

How to create hash object with keys created from values and grouped by them?

cars = [{
  id: 1,
  properties: {
    name: "audi",
    type: "petrol"
  },
},{
  id: 1,
  properties: {
    name: "ford",
    type: "petrol"
  }
},{
  id: 1,
  properties: {
    name: "tesla",
    type: "electric"
  }
}]

Desired effect:

{
  petrol: [{name: "audi"}, {name: "ford"}],
  electric: [{name: "tesla"}]
}

My current function gives desired effect but it is too long, How can I get the same effect with shorter code?

cars.map { |c| Hash[c[:properties][:type], c[:properties][:name]] }.group_by{|h| h.keys.first}.each_value{|a| a.map!{|h| h.values.first}}

Magnuss · Accepted Answer · 2015-10-09 08:39:51Z

4

I came up with something like this. The grouped_cars variable should be extracted to a separate method.

grouped_cars = cars.inject({}) do |result, car|
  result[car[:properties][:type]] ||= []
  result[car[:properties][:type]] << { name: car[:properties][:name] }
  result
end

{ cars: grouped_cars }

answered Oct 9, 2015 at 8:39

Magnuss

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1 Comment

luzny Over a year ago

thanks for your answer, now I know how to use inject({})

nsave · Accepted Answer · 2015-10-09 09:01:06Z

2

My variant:

{
  cars: cars.inject({}) do |hash, data|
      type, name = data[:properties].values_at(:type, :name)
      hash[type] ||= []
      hash[type] << {name: name}
      hash
    end
}

And even shorter:

{
  cars: cars.inject(Hash.new([])) do |hash, car|
      type, name = car[:properties].values_at(:type, :name);
      hash[type] += [{ name: name }];
      hash
    end
}

edited Oct 9, 2015 at 9:01

answered Oct 9, 2015 at 8:49

nsave

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Comments

pangpang · Accepted Answer · 2015-10-09 09:01:44Z

2

inject method have to return the memo each time, I think each_with_object is better.

cars.each_with_object({}) do |item, hash|
  (hash[item[:properties][:type]] ||= []) << { name: item[:properties][:name] }
end

=> {"petrol"=>[{:name=>"audi"}, {:name=>"ford"}], "electric"=>[{:name=>"tesla"}]}

answered Oct 9, 2015 at 9:01

pangpang

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Comments

Cary Swoveland · Accepted Answer · 2015-10-10 08:25:02Z

I suggest the following:

cars.map { |h| h[:properties] }.each_with_object({}) { |g,h|
  h.update(g[:type]=>[{ name: g[:name] }]) { |_,o,n| {name: o+n } } }
  #=> {"petrol"=>{:name=>[{:name=>"audi"}, {:name=>"ford"}]},
  #    "electric"=>[{:name=>"tesla"}]}

Firstly, we obtain:

cars.map { |h| h[:properties] }
  #=> [{:name=>"audi",  :type=>"petrol"},
  #    {:name=>"ford",  :type=>"petrol"},
  #    {:name=>"tesla", :type=>"electric"}]

We then use the form of Hash#update (aka merge!) that uses the block:

{ |_,o,n| { name: o+n } }

to determine the values of keys that are present in both hashes being merged.

Initially, the hash:

{ "petrol"=>[{ :name=>"audi"] }] }

is merged into an empty hash represented by the block variable h. This merge does not make use of the block because h is empty and therefore does not have a key "petrol".

Next, the hash:

{ "petrol"=>[{ :name=>"ford" }] }

is merged into h. Since both this hash and h have keys "petrol", the value of "petrol" is given by the block:

{ |_,o,n| { name: o+n } }
  #=> {|_,[{ :name=>"audi" }] , [{ :name=>"ford" }]| 
  #      [{ name: "audi" }] + [{ name: "ford" }] }
  #=> [{ name: "audi" }, { name: "ford" }]

Lastly, the hash:

{ "electric"=>{ :name=>"tesla" } }

is merged into h. Since h does not have a key "electric", the block is not not needed to determine the value of that key.

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