CODEWITHSUNDEEP
ruby-on-railsrubyruby-on-rails-3group-by
ben
ben

Reputation: 29817

How can I group this array of hashes?

I have this array of hashes:

- :name: Ben
  :age: 18
- :name: David
  :age: 19
- :name: Sam
  :age: 18

I need to group them by age, so they end up like this:

18:
- :name: Ben
  :age: 18
- :name: Sam
  :age: 18
19:
- :name: David
  :age: 19

I tried doing it this way:

array = array.group_by &:age

but I get this error:

NoMethodError (undefined method `age' for {:name=>"Ben", :age=>18}:Hash):

What am I doing wrong? I'm using Rails 3.0.1 and Ruby 1.9.2

Upvotes: 46

Views: 51972

Answers (3)

KARASZI István
KARASZI István

Reputation: 31477

The &:age means that the group_by method should call the age method on the array items to get the group by data. This age method is not defined on the items that are Hashes in your case.

This should work:

array.group_by { |d| d[:age] }

Also with newer Ruby versions you can use the following shortcut:

array.group_by { _1[:age] }

_1 here reflects the first positional argument of the block passed to the #group_by method.

Upvotes: 116

gunn
gunn

Reputation: 9175

As others have pointed out ruby's Symbol#to_proc method is invoked and calls the age method on each hash in the array. The problem here is that the hashes do not respond to an age method.

Now we could define one for the Hash class, but we probably don't want it for every hash instance in the program. Instead we can simply define the age method on each hash in the array like so:

array.each do |hash|
  class << hash
    def age
      self[:age]
    end
  end
end

And then we can use group_by just as you were before:

array = array.group_by &:age

Upvotes: 0

Michael Johnston
Michael Johnston

Reputation: 5330

out = {}
array_of_hashes.each do |a_hash|
  out[a_hash[:age]] ||= []
  out[a_hash[:age]] << a_hash
end

or 

array.group_by {|item| item[:age]}

Upvotes: 1

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