Duplicate Elements of Array in Ruby

How to find and return a duplicate value in array

a = ["A", "B", "C", "B", "A"]
a.detect{ |e| a.count(e) > 1 }

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I know this isn't very elegant answer, but I love it. It's beautiful one liner code. And works perfectly fine unless you need to process huge data set.

Looking for faster solution? Here you go!

def find_one_using_hash_map(array)
  map = {}
  dup = nil
  array.each do |v|
    map[v] = (map[v] || 0 ) + 1

    if map[v] > 1
      dup = v
      break
    end
  end

  return dup
end

It's linear, O(n), but now needs to manage multiple lines-of-code, needs test cases, etc.

If you need an even faster solution, maybe try C instead.

And here is the gist comparing different solutions: https://gist.github.com/naveed-ahmad/8f0b926ffccf5fbd206a1cc58ce9743e

Duplicate array elements in Ruby

How about

rangers.zip(rangers).flatten

using Array#zip and Array#flatten?

A solution that might generalize a bit better for your second request might be:

rangers.flat_map { |ranger| [ranger] * 2 }

using Enumerable#flat_map.
Here you can just replace the 2 with any value or variable.

Find a Duplicate in an array Ruby

How I wish we had a built-in method Array#difference:

class Array
  def difference(other)
    h = other.tally
    reject { |e| h[e] > 0 && h[e] -= 1 }
  end
end

though @user123's answer is more straightforward. (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]

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]

Getting the indexes of duplicate elements in arrays (Ruby)

Taking a page from procedural languages, we could write:

fruits.each_index.select { |i| fruits[i]=="apples" }
  #=> [0, 2, 4]

How to split an array on duplicate elements in Ruby

Enumerable#slice_when does that.

arr = ['A', 'B', 'C', 'C', 'D', 'D', 'F']
p arr.slice_when{|a,b| a==b}.to_a

# =>[["A", "B", "C"], ["C", "D"], ["D", "F"]]

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 do I detect duplicate values within an array in Ruby?

You can create a hash to store number of times any element is repeated. Thus iterating over array just once.

h = Hash.new(0)
['a','b','b','c'].each{ |e| h[e] += 1 }

Should result

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

Remove duplicate elements from array in Ruby

array = array.uniq

uniq removes all duplicate elements and retains all unique elements in the array.

This is one of many beauties of the Ruby language.

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