From: Ryan Davis <ryand-ruby@...>
Date: 2012-04-21T08:47:41+09:00
Subject: Re: finding duplicates in an array and its index number


On Apr 20, 2012, at 03:46 , Lars Haugseth wrote:

> 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]}

Don't use reduce/inject for non-reductive applications. Use something more appropriate, like a plain each.

Also use Hash to its full capabilities:

hash = Hash.new { |h,k| h[k] = [] }

then it is as clean as:

array.each_with_index do |val, idx|
  hash[val] << idx
end

Notice how you're not constantly re-assigning hash for no good reason in my version? That adds up, but more importantly it obfuscates the original intent.