Ruby code optimization: select from array

Question

Let's suppose that we class SomeClass.

class SomeClass < SomeClassThatAssignAttributesOnInitialization
  attr_accessor :group
  attr_accessor :name
end

And array of it instances

arr = [SomeClass.new(group:0, name:'one'),
       SomeClass.new(group:0, name:'two'),
       SomeClass.new(group:1, name:'three'),
       SomeClass.new(group:1, name:'four')]

I need to put first element of each group into array.

Now it works next way

current_group = nil
first_elements = arr.map do |instance|
  if current_group != instance.group
    current_group = instance.group
    instance.name
  end
end

This works fine but i think that there is some kind of "Ruby" way to do it.

Can anybody help me?

Thanks

"some kind of 'Ruby' way" questions should be asked on codereview.stackexchange.com these days. — the Tin Man
– the Tin Man, Commented Dec 18, 2012 at 17:26

Chowlett · Accepted Answer · 2012-12-18 14:56:19Z

3

Ruby has a handy group_by method on Enumerable object, which returns a Hash:

arr.group_by(&:group)
# => {0=>[#<SomeClass group=0, name="one">, #<SomeClass group=0, name="two">], 1=>[#<SomeClass group=1, name="three">, #<SomeClass group=1, name="four">]}

So just chain a few methods on to the end of that:

arr.group_by(&:group).values.map {|vs| vs[0].name}
# => ["one", "three"]

answered Dec 18, 2012 at 14:56

Chowlett

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zed_0xff · Accepted Answer · 2012-12-18 14:56:50Z

2

arr.group_by(&:group).values.map(&:first)

answered Dec 18, 2012 at 14:56

zed_0xff

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Comments

mpapis · Accepted Answer · 2012-12-18 14:52:59Z

1

why not just:

first_elements = arr.map(&:name)

answered Dec 18, 2012 at 14:52

mpapis

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2 Comments

mibon Over a year ago

I need to take name from first element of each group. You example will take names from all elements

mpapis Over a year ago

yeah sorry for that - I have noticed this after writing my answer.

Jörg W Mittag · Accepted Answer · 2012-12-18 15:01:20Z

1

SomeClass = Struct.new(:group, :name)

arr = [
  SomeClass.new(0, 'one'),
  SomeClass.new(0, 'two'),
  SomeClass.new(1, 'three'),
  SomeClass.new(1, 'four')
]

arr.group_by(&:group).values.map(&:first)
# => [
#   #<struct SomeClass group=0, name="one">,
#   #<struct SomeClass group=1, name="three">
# ]

Or to get just the names:

arr.group_by(&:group).values.map(&:first).map(&:name)

answered Dec 18, 2012 at 15:01

Jörg W Mittag

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