How to count duplicates in Ruby Arrays

Question

How do you count duplicates in a ruby array?

For example, if my array had three a\'s, how could I count that

Answer 1

I don't think there's a built-in method. If all you need is the total count of duplicates, you could take a.length - a.uniq.length. If you're looking for the count of a single particular element, try
a.select {|e| e == my_element}.length.

Answer 2

arr = [1, 2, "a", "a", 4, "a", 2, 1]

arr.group_by(&:itself).transform_values(&:size)
#=> {1=>2, 2=>2, "a"=>3, 4=>1}

Answer 3

Another version of a hash with a key for each element in your array and value for the count of each element

a = [ 1, 2, 3, 3, 4, 3]
h = Hash.new(0)
a.each { | v | h.store(v, h[v]+1) }

# h = { 3=>3, 2=>1, 1=>1, 4=>1 } 

Answer 4

requires 1.8.7+ for group_by

ary = %w{a b c d a e f g a h i b}
ary.group_by{|elem| elem}.select{|key,val| val.length > 1}.map{|key,val| key}
# => ["a", "b"]

with 1.9+ this can be slightly simplified because Hash#select will return a hash.

ary.group_by{|elem| elem}.select{|key,val| val.length > 1}.keys
# => ["a", "b"]

Answer 5

What about a grep?

arr = [1, 2, "Thanks", "You're welcome", "Thanks", "You're welcome", "Thanks", "You're welcome"]

arr.grep('Thanks').size # => 3

Answer 6

Another way to do it is to use each_with_object:

a = [ 1, 2, 3, 3, 4, 3]

hash = a.each_with_object({}) {|v, h|
  h[v] ||= 0
  h[v] += 1
}

# hash = { 3=>3, 2=>1, 1=>1, 4=>1 } 

This way, calling a non-existing key such as hash[5] will return nil instead of 0 with Kim's solution.