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

Another way to count array duplicates is:

arr= [2,2,3,3,2,4,2]

arr.group_by{|x| x}.map{|k,v| [k,v.count] }

result is

[[2, 4], [3, 2], [4, 1]]

Answer 2

Simple.

arr = [2,3,4,3,2,67,2]
repeats = arr.length - arr.uniq.length
puts repeats

Answer 3

Given:

arr = [ 1, 2, 3, 2, 4, 5, 3]

My favourite way of counting elements is:

counts = arr.group_by{|i| i}.map{|k,v| [k, v.count] }

# => [[1, 1], [2, 2], [3, 2], [4, 1], [5, 1]]

If you need a hash instead of an array:

Hash[*counts.flatten]

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

Answer 4

arr = %w( a b c d c b a )
# => ["a", "b", "c", "d", "c", "b", "a"]

arr.count('a')
# => 2

Answer 5

This will yield the duplicate elements as a hash with the number of occurences for each duplicate item. Let the code speak:

#!/usr/bin/env ruby

class Array
  # monkey-patched version
  def dup_hash
    inject(Hash.new(0)) { |h,e| h[e] += 1; h }.select { 
      |k,v| v > 1 }.inject({}) { |r, e| r[e.first] = e.last; r }
  end
end

# unmonkeey'd
def dup_hash(ary)
  ary.inject(Hash.new(0)) { |h,e| h[e] += 1; h }.select { 
    |_k,v| v > 1 }.inject({}) { |r, e| r[e.first] = e.last; r }
end

p dup_hash([1, 2, "a", "a", 4, "a", 2, 1])
# {"a"=>3, 1=>2, 2=>2}

p [1, 2, "Thanks", "You're welcome", "Thanks", 
  "You're welcome", "Thanks", "You're welcome"].dup_hash
# {"You're welcome"=>3, "Thanks"=>3}

Answer 6

Ruby code to get the repeated elements in the array:

numbers = [1,2,3,1,2,0,8,9,0,1,2,3]
similar =  numbers.each_with_object([]) do |n, dups|
    dups << n if seen.include?(n)
    seen << n 
end
print "similar --> ", similar