How to count identical string elements in a Ruby array

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Ruby v2.7+ (latest)

As of ruby v2.7.0 (released December 2019), the core language now includes Enumerable#tally – a new method, designed specifically for this problem:

names = ["Jason", "Jason", "Teresa", "Judah", "Michelle", "Judah", "Judah", "Allison"]

names.tally
#=> {"Jason"=>2, "Teresa"=>1, "Judah"=>3, "Michelle"=>1, "Allison"=>1}

Ruby v2.4+ (currently supported, but older)

The following code was not possible in standard ruby when this question was first asked (February 2011), as it uses:

These modern additions to Ruby enable the following implementation:

names = ["Jason", "Jason", "Teresa", "Judah", "Michelle", "Judah", "Judah", "Allison"]

names.group_by(&:itself).transform_values(&:count)
#=> {"Jason"=>2, "Teresa"=>1, "Judah"=>3, "Michelle"=>1, "Allison"=>1}

Ruby v2.2+ (deprecated)

If using an older ruby version, without access to the above mentioned Hash#transform_values method, you could instead use Array#to_h, which was added to Ruby v2.1.0 (released December 2013):

names.group_by(&:itself).map { |k,v| [k, v.length] }.to_h
#=> {"Jason"=>2, "Teresa"=>1, "Judah"=>3, "Michelle"=>1, "Allison"=>1}

For even older ruby versions (<= 2.1), there are several ways to solve this, but (in my opinion) there is no clear-cut “best” way. See the other answers to this post.

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