Finding all duplicates of a Ruby Array

Question

I am looking for an efficient way to give all the duplicate values for an array.

Example:

a = [1,2,2,3,4,7,4,4]
dups(a)

#[[2,2],[4,4,4]]

I've tried using brute force it by doing:

def dups(array)
  dups = []
  array.uniq.each do |i|
    found = array.select{|s| s.eql?(i)} 
    dups = dups.push found if found.size > 1
  end
  return dups
end

But it is pretty slow if you small percent of duplicate, since you are looping through the entire array.

Not exactly the desired output, but pretty efficient: a.tally.select{|k,v| v>1 } — steenslag
– steenslag, Commented May 18, 2022 at 21:32
@steenslag simple enough to modify though a.tally.filter_map {|k,v| [k] * v if v > 1} — engineersmnky
– engineersmnky, Commented May 18, 2022 at 22:04

Nathan · Accepted Answer · 2022-05-20 02:11:47Z

3

def dup_group_by(array)
  array.group_by(&:itself).filter_map { |_,v| v if v.size > 1 }
end

The key was group_by method which was in the Enumerable module, I was only looking in the Array class.

edited May 20, 2022 at 2:11

answered May 18, 2022 at 20:20

Nathan

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4 Comments

Nathan Over a year ago

@engineersmnky thanks! - I copy my old method and didn't clean it up; I updated it with your suggestions

Cary Swoveland Over a year ago

Consider array.group_by(&:itself).filter_map { |_,v| v if v.size > 1 } #=> [[2, 2], [4, 4, 4]]. @engineersmnky, I expected that whatever you suggested in would include Enumerable#filter_map.

engineersmnky Over a year ago

@CarySwoveland agreed just wasn't trying to disagree with a viable solution from a new comer. See my comment under the original post 😀

Cary Swoveland Over a year ago

@engineersmnky, that comment prompted mine.

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