From: "Jan E." <lists@...>
Date: 2012-04-20T07:49:49+09:00
Subject: Re: finding duplicates in an array and its index number

Hi,

As always, there are many ways to do this.

A highlevel approach: (short but not really inefficient)

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

a = x.each_with_index.group_by(&:first).inject({}) do |result, (val, 
group)|
  next result if group.length == 1
  result.merge val => group.map {|pair| pair[1]}
end

p a
####

A more efficient lowlevel solution would be to iterate over the array, 
compare the values and save the indices in case of duplicates.

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