How to count duplicate elements in a Ruby array

You can do this very succinctly (one line) by using inject:

a = ['FATAL <error title="Request timed out.">',
      'FATAL <error title="Request timed out.">',
      'FATAL <error title="There is insufficient ...">']

b = a.inject(Hash.new(0)) {|h,i| h[i] += 1; h }

b.to_a.each {|error,count| puts "#{count}: #{error}" }

Will produce:

1: FATAL <error title="There is insufficient ...">
2: FATAL <error title="Request timed out.">

Using Enumerable#tally

["a", "b", "c", "b"].tally 

#=> { "a" => 1, "b" => 2, "c" => 1 }

Note: Only for Ruby versions >= 2.7

If you have array like this:

words = ["aa","bb","cc","bb","bb","cc"]

where you need to count duplicate elements, a one line solution is:

result = words.each_with_object(Hash.new(0)) { |word,counts| counts[word] += 1 }

A different approach to the answers above, using Enumerable#group_by.

[1, 2, 2, 3, 3, 3, 4].group_by(&:itself).map { |k,v| [k, v.count] }.to_h
# {1=>1, 2=>2, 3=>3, 4=>1}

Breaking that into its different method calls:

a = [1, 2, 2, 3, 3, 3, 4]
a = a.group_by(&:itself) # {1=>[1], 2=>[2, 2], 3=>[3, 3, 3], 4=>[4]}
a = a.map { |k,v| [k, v.count] } # [[1, 1], [2, 2], [3, 3], [4, 1]]
a = a.to_h # {1=>1, 2=>2, 3=>3, 4=>1}

Enumerable#group_by was added in Ruby 1.8.7.

How about the following:

things = [1, 2, 2, 3, 3, 3, 4]
things.uniq.map{|t| [t,things.count(t)]}.to_h

It sort of feels cleaner and more descriptive of what we're actually trying to do.

I suspect it would also perform better with large collections than the ones that iterate over each value.

Benchmark Performance test:

a = (1...1000000).map { rand(100)}
                       user     system      total        real
inject                 7.670000   0.010000   7.680000 (  7.985289)
array count            0.040000   0.000000   0.040000 (  0.036650)
each_with_object       0.210000   0.000000   0.210000 (  0.214731)
group_by               0.220000   0.000000   0.220000 (  0.218581)

So it is quite a bit faster.

From Ruby >= 2.2 you can use itself: array.group_by(&:itself).transform_values(&:count)

With some more detail:

array = [
  'FATAL <error title="Request timed out.">',
  'FATAL <error title="Request timed out.">',
  'FATAL <error title="There is insufficient system memory to run this query.">'
];

array.group_by(&:itself).transform_values(&:count)
 => { "FATAL <error title=\"Request timed out.\">"=>2,
      "FATAL <error title=\"There is insufficient system memory to run this query.\">"=>1 }

Personally I would do it this way:

# myprogram.rb
a = ['FATAL <error title="Request timed out.">',
'FATAL <error title="Request timed out.">',
'FATAL <error title="There is insufficient system memory to run this query.">']
puts a

Then run the program and pipe it to uniq -c:

ruby myprogram.rb | uniq -c

Output:

 2 FATAL <error title="Request timed out.">
 1 FATAL <error title="There is insufficient system memory to run this query.">

a = [1,1,1,2,2,3]
a.uniq.inject([]){|r, i| r << { :error => i, :count => a.select{ |b| b == i }.size } }
=> [{:count=>3, :error=>1}, {:count=>2, :error=>2}, {:count=>1, :error=>3}]

If you want to use this often I suggest to do this:

# lib/core_extensions/array/duplicates_counter
module CoreExtensions
  module Array
    module DuplicatesCounter
      def count_duplicates
        self.each_with_object(Hash.new(0)) { |element, counter| counter[element] += 1 }.sort_by{|k,v| -v}.to_h
      end
    end
  end
end

Load it with

Array.include CoreExtensions::Array::DuplicatesCounter

And then use from anywhere with just:

the_ar = %w(a a a a a a a  chao chao chao hola hola mundo hola chao cachacho hola)
the_ar.duplicates_counter
{
           "a" => 7,
        "chao" => 4,
        "hola" => 4,
       "mundo" => 1,
    "cachacho" => 1
}

Since #tally is for 2.7 and up, and I'm not there yet, it's easy to use the #count method on the array. Use #uniq on the array to get one copy of each member of the array, and then find #count for that member in the array:

counts=Hash.new
arr.uniq.each {|name| counts[name]=arr.count(name) }

Example:

arr = [ 1, 2, 2, 3, 3, 3, 3, 3, 4, 4, 5]
arr.uniq => [1, 2, 3, 4, 5]
counts=Hash.new; arr.uniq.each {|name| counts[name]=arr.count(name) }

gives us

counts => {1=>1, 2=>2, 3=>5, 4=>2, 5=>1} 

Simple implementation:

(errors_hash = {}).default = 0
array_of_errors.each { |error| errors_hash[error] += 1 }

Here is the sample array:

a=["aa","bb","cc","bb","bb","cc"]

1. Select all the unique keys.
2. For each key, we'll accumulate them into a hash to get something like this: {'bb' => ['bb', 'bb']}

    res = a.uniq.inject({}) {|accu, uni| accu.merge({ uni => a.select{|i| i == uni } })}
    {"aa"=>["aa"], "bb"=>["bb", "bb", "bb"], "cc"=>["cc", "cc"]}

Now you are able to do things like:

res['aa'].size