Ruby: How to group a Ruby array?

Question

I have a Ruby array

> list = Request.find_all_by_artist("Metallica").map(&:song)
=> ["Nothing else Matters", "Enter sandman", "Enter Sandman", "Master of Puppets", "Master of Puppets", "Master of Puppets"]

and I want a list with the counts like this:

{"Nothing Else Matters" => 1,
 "Enter Sandman" => 2,
 "Master of Puppets" => 3}

So ideally I want a hash that will give me the count and notice how I have Enter Sandman and enter sandman so I need it case insensitive. I am pretty sure I can loop through it but is there a cleaner way?

Paweł Gościcki · Accepted Answer · 2016-02-02 09:57:14Z

101

list.group_by(&:capitalize).map {|k,v| [k, v.length]}
#=> [["Master of puppets", 3], ["Enter sandman", 2], ["Nothing else matters", 1]]

The group by creates a hash from the capitalized version of an album name to an array containing all the strings in list that match it (e.g. "Enter sandman" => ["Enter Sandman", "Enter sandman"]). The map then replaces each array with its length, so you get e.g. ["Enter sandman", 2] for "Enter sandman".

If you need the result to be a hash, you can call to_h on the result or wrap a Hash[ ] around it.

edited Feb 2, 2016 at 9:57

Paweł Gościcki

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answered Oct 7, 2010 at 18:53

sepp2k

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Comments

BrunoF · Accepted Answer · 2020-02-21 20:57:42Z

15

As of Ruby 2.7, you can use Enumerable#tally.

list.tally
# => {"Nothing else Matters"=>1, "Enter sandman"=>1, "Enter Sandman"=>1, "Master of Puppets"=>3}

answered Feb 21, 2020 at 20:57

BrunoF

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1 Comment

Alexey Zalyotov Over a year ago

I think this answer should be chosen as the best answer!

ghostdog74 · Accepted Answer · 2010-10-08 06:38:38Z

11

list.inject(Hash.new(0)){|h,k| k.downcase!; h[k.capitalize] += 1;h}

answered Oct 8, 2010 at 6:38

ghostdog74

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Comments

glenn jackman · Accepted Answer · 2010-10-07 19:41:46Z

9

Another take:

h = Hash.new {|hash, key| hash[key] = 0}
list.each {|song| h[song.downcase] += 1}
p h  # => {"nothing else matters"=>1, "enter sandman"=>2, "master of puppets"=>3}

As I commented, you might prefer titlecase

answered Oct 7, 2010 at 19:41

glenn jackman

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1 Comment

sepp2k Over a year ago

In this case you don't need to use the block form of Hash.new. So you can just do h = Hash.new(0).

Harish Shetty · Accepted Answer · 2014-06-25 21:11:09Z

5

Grouping and sorting of a data set of unknown size in Ruby should be a choice of last resort. This is a chore best left to DB. Typically problems like yours is solved using a combination of COUNT, GROUP BY, HAVING and ORDER BY clauses. Fortunately, rails provides a count method for such use cases.

song_counts= Request.count(
              :select => "LOWER(song) AS song"
              :group => :song, :order=> :song,
              :conditions => {:artist => "Metallica"})

song_counts.each do |song, count|
  p "#{song.titleize} : #{count}"
end

edited Jun 25, 2014 at 21:11

answered Oct 8, 2010 at 6:27

Harish Shetty

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Comments

ray · Accepted Answer · 2018-10-27 12:37:35Z

0

Late but clean answer I have,

l = list.group_by(&:titleize)
l.merge(l) { |k,v| l[k] = v.count }

Note: If we do want unique keys i.e. without titleize, then replace it with itself

answered Oct 27, 2018 at 12:37

ray

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Comments

Tyler2P · Accepted Answer · 2022-10-11 17:08:06Z

0

list = ["Nothing else Matters", "Enter sandman", "Enter Sandman", "Master of Puppets", "Master of Puppets", "Master of Puppets"]

list.each_with_object(Hash.new(0)){|k,v| v[k.downcase] += 1}

edited Oct 11, 2022 at 17:08

Tyler2P

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answered Oct 6, 2022 at 12:07

Amith T G

1

1 Comment

Tyler2P Over a year ago

Your answer could be improved by adding more information on what the code does and how it helps the OP.

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