How to convert nested array to hash or JSON with values as array

Question

I have this nested array

array = [["Colorado", "Adams County"], ["Colorado", "Jefferson County"], ["California", "Amador"], ["California", "Tulare"]]

I am expecting this

{"Colorado" => ["Adams County", "Jefferson County"], "California" => ["Amador", "Tulare"]}

I tried these:

array.to_h
#=> {"Colorado"=>"Jefferson County", "California"=>"Tulare"}

b = Hash.new
array.uniq.map{|k,v| b[k] = v}
#=> {"Colorado"=>"Jefferson County", "California"=>"Tulare"}

array.uniq.map{|k,v| b[k] << v}
#=> Error

array.map{|k,v| b[k] = [v]}
#=> {"Colorado"=>["Jefferson CountyAdams CountyJefferson CountyAdams County"], "California"=>["TulareAmadorTulareAmador"]}`

How can I get the values as an array?

What is the difference between array and c? (What is array in the first place?) What is b? — sawa
– sawa, Commented Nov 27, 2017 at 9:43
sorry i was working something on console, i have edited variables now. it should be more clear — Sachin srinivasan
– Sachin srinivasan, Commented Nov 27, 2017 at 9:52

Simple Lime · Accepted Answer · 2017-12-11 22:03:18Z

4

You can first call group_by and then transform_values (Ruby 2.4+):

hash = array.group_by(&:first)
# => {"Colorado"=>[["Colorado", "Adams County"], ["Colorado", "Jefferson County"]], "California"=>[["California", "Amador"], ["California", "Tulare"]]}
hash.transform_values! { |value_list| value_list.map(&:last) }
# => {"Colorado"=>["Adams County", "Jefferson County"], "California"=>["Amador", "Tulare"]}

If your ruby doesn't have transform_values, you can just map!:

hash = array.group_by(&:first)
hash.each_value do |value_list|
  value_list.map!(&:last)
end
# => {"Colorado"=>["Adams County", "Jefferson County"], "California"=>["Amador", "Tulare"]}

edited Dec 11, 2017 at 22:03

answered Nov 27, 2017 at 9:13

Simple Lime

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1 Comment

Stefan Pochmann Over a year ago

How about each_value?

mwp · Accepted Answer · 2017-11-27 09:11:35Z

1

It looks like you want Enumerable#group_by to regroup the array of arrays by the first element.

array
  .group_by { |k, _| k }
  .each_value { |a| a.map! { |_, v| v } } # => {"Colorado"=>["Adams County", "Jefferson County"], "California"=>["Amador", "Tulare"]}

answered Nov 27, 2017 at 9:11

mwp

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Comments

Aleksei Matiushkin · Accepted Answer · 2017-11-27 09:18:16Z

1

[["Colorado", "Adams County"], ["Colorado", "Jefferson County"],
 ["California", "Amador"], ["California", "Tulare"]].
   each_with_object(Hash.new { |h, k| h[k] = [] }) do |(k, v), h|
     h[k] << v
   end
#⇒ {
#  "California" => [
#    [0] "Amador",
#    [1] "Tulare"
#  ],
#    "Colorado" => [
#    [0] "Adams County",
#    [1] "Jefferson County"
#  ]
# }

Or, for Ruby 2.4+:

[["Colorado", "Adams County"], ["Colorado", "Jefferson County"], 
 ["California", "Amador"], ["California", "Tulare"]].
   group_by(&:shift).transform_values(&:flatten)

answered Nov 27, 2017 at 9:18

Aleksei Matiushkin

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4 Comments

Stefan Pochmann Over a year ago

Oh, I like the shift one. But how about array.group_by(&:shift).each_value(&:flatten!)? Works in earlier Ruby versions as well and is a bit shorter.

Sachin srinivasan Over a year ago

cool.. .each_value is also great. Just wondering, is it possible to sort array value with .sort along with flatten ?

Stefan Pochmann Over a year ago

Speaking of sorting... is the output shown above really what you got from that code? Did it really make "California" come before "Colorado"? Doesn't that violate the "key insertion order"?

Aleksei Matiushkin Over a year ago

@StefanPochmann it’s how pry outputs the result: it sorts hashes.

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