From: Lars Haugseth <ruby-talk@...>
Date: 2012-04-20T19:46:07+09:00
Subject: Re: finding duplicates in an array and its index number

On 04/19/2012 11:53 PM, newto ruby wrote:
> Hi, I have following array and I am trying to find if there are 
> duplicates in the array. I also want to find out their index. 

Generate a hash with unique items as keys, and arrays of indices as values,
then select only those with more than one index:

array = [1, 2, 3, 4, 5, 3, 6, 7, 2, 8, 1, 9]

array.each_with_index.reduce({}) { |hash, (item, index)|
   hash[item] = (hash[item] || []) << index
   hash
}.select { |key, value|
   value.size > 1
}

# => {1=>[0, 10], 2=>[1, 8], 3=>[2, 5]}

-- 
Lars Haugseth