How to Count Duplicate Elements in a Ruby Array

How to count duplicate elements in a Ruby array

The following code prints what you asked for. I'll let you decide on how to actually use to generate the hash you are looking for:

# sample array
a=["aa","bb","cc","bb","bb","cc"]

# make the hash default to 0 so that += will work correctly
b = Hash.new(0)

# iterate over the array, counting duplicate entries
a.each do |v|
  b[v] += 1
end

b.each do |k, v|
  puts "#{k} appears #{v} times"
end

Note: I just noticed you said the array is already sorted. The above code does not require sorting. Using that property may produce faster code.

How to count duplicates in Ruby Arrays

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}

Counting the number of repeated elements in an array

I would expect that something like this might work:

# in the controller (this returns a hash)
@locations = Country.joins(:user).order(:name).group(:name).count

# in your view
- @locations.each do |name, count|
  %li
    = name
    = "(#{count})" if count > 1

Ruby count adjacent duplicate elements in array

%w[a b c a a b b c c c].chunk{|e| e}.map{|_, v| v.length}.max #=> 3

How to count identical string elements in a Ruby array

names = ["Jason", "Jason", "Teresa", "Judah", "Michelle", "Judah", "Judah", "Allison"]
counts = Hash.new(0)
names.each { |name| counts[name] += 1 }
# => {"Jason" => 2, "Teresa" => 1, ....

Find a Duplicate in an array Ruby

Array#difference comes to the rescue yet again. (I confess that @user123's answer is more straightforward, unless you pretend that Array#difference is already a built-in method. Array#difference is probably the more efficient of the two, as it avoids the repeated invocations of count.) See my answer here for a description of the method and links to its use.
In a nutshell, it differs from Array#- as illustrated in the following example:

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

a - b          #=> [1]
a.difference b #=> [1, 3, 2, 2]

One day I'd like to see it as a built-in.

For the present problem, if:

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

the duplicate elements are given by:

arr.difference(arr.uniq).uniq
  #=> [3, 4]

Array Count of repeating variable with integer output in ruby

So by the wording of the question, we only care about the number of occurrences for the string cat:

array.count { |x| x == 'cat' }
=> 3

How do you count the amout of duplicate characters in a string ruby

Possible solution:

string = "chineedne"
string.chars.uniq.count { |char| string.count(char) > 1 }
#=> 2

or without uniq method to count total amount of duplicated characters:

string = "chineedne"
string.chars.count { |char| string.count(char) > 1 }
#=> 5

In order to get away from N**2 complexity, you also can use group_by method for creating hash with character -> array that include all of this character from string and than just use this hash to get any data that you want:

duplicates = string.chars.group_by { |char| char }.select { |key, value| value.size > 1 } 
# or, for Ruby version >= 2.2.1 - string.chars.group_by(&:itself).select { |key, value| value.size > 1 }

than:

> duplicates.keys.size # .keys => ['n', 'e']
#=> 2

and

> duplicates.values.flatten.size # .values.flatten => ["n", "n", "e", "e", "e"]
#=> 5

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