Trying to display only duplicate values

Question

I'm trying to display only the duplicate value in an array which just holds names.

So for instance, my code is:

  <%= s= arrayOfStuff %> 
  <%= t= arrayOfStuff.uniq %>

which displays

["UK01USV005", "NJ08APP516", "NJ08MHF001", "UK01USV505", "NY01MHF0006", "UK01USV525", "UK01USV005", "NJ08APP515", "NJ08MHF002"]
["UK01USV005", "NJ08APP516", "NJ08MHF001", "UK01USV505", "NY01MHF0006", "UK01USV525", "NJ08APP515", "NJ08MHF002"]

so theortically when I do s-t it should give me the duplicate value which in this case is UK01USV005, however the results I get is an empty array which obviously looks like this: [].

Any ideas why that could be?

Answer 1

arrayOfStuff.group_by {|e| e}.select { |k,v| v.size > 1}.keys

should work fine.