# How to get only the duplicatet items from an array

**URL:** <https://www.ruby-forum.com/t/how-to-get-only-the-duplicatet-items-from-an-array/73883>\
**Category:** Ruby\
**Created:** [October 27, 2006, 7:36pm UTC](https://www.ruby-forum.com/t/how-to-get-only-the-duplicatet-items-from-an-array/73883 "2006-10-27T19:36:08Z")\
**Posts on this page:** 5\
**Page:** 1

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**Author:** ![Nico\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/ad7895/32.png) [@Nico\_L](https://www.ruby-forum.com/u/Nico_L)\
**Post date:** [October 27, 2006, 7:36pm UTC](https://www.ruby-forum.com/t/how-to-get-only-the-duplicatet-items-from-an-array/73883/1 "2006-10-27T19:36:08Z")

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hi all,

is there a oneline to get only the duplicatet item from an array?

i have an array like this  
aTemp = [1, 1, 2, 2, 3, 4]  
and i want to know, which items are duplicatet.

i thought i can do it like  
aTemp - aTemp.uniq

[1, 1, 2, 2, 3, 4] - [1, 2, 3, 4] =\> [1, 2]  
like inside the manual

but i get only an empty array

so i found an bad solution and hope you can help me to get the right  
way.

thanks for your help  
nico

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**Author:** ![Nico\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/ad7895/32.png) [@Nico\_L](https://www.ruby-forum.com/u/Nico_L)\
**Post date:** [October 27, 2006, 7:52pm UTC](https://www.ruby-forum.com/t/how-to-get-only-the-duplicatet-items-from-an-array/73883/2 "2006-10-27T19:52:10Z")

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Nico L. wrote:

> i have an array like this  
> aTemp = [1, 1, 2, 2, 3, 4]  
> and i want to know, which items are duplicatet.

/ …

> so i found an bad solution and hope you can help me to get the right way.

#!/usr/bin/ruby -w

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

dups = []

hash = {}

a.each do |item|  
dups \<\< item if hash[item]  
hash[item] = true  
end

p dups

[1, 1, 1, 1, 2, 2, 2, 9, 9]

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**Author:** ![Nico\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/ad7895/32.png) [@Nico\_L](https://www.ruby-forum.com/u/Nico_L)\
**Post date:** [October 27, 2006, 7:58pm UTC](https://www.ruby-forum.com/t/how-to-get-only-the-duplicatet-items-from-an-array/73883/3 "2006-10-27T19:58:10Z")

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On 10/27/06, Nico L. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote:

> [1, 1, 2, 2, 3, 4] - [1, 2, 3, 4] =\> [1, 2]  
> like inside the manual
> 
> but i get only an empty array
> 
> so i found an bad solution and hope you can help me to get the right way.

Here’s one way:  
irb(main):001:0\> h = Hash.new(0)  
=\> {}  
irb(main):002:0\> array = [1,1,2,2,3,4,1]  
=\> [1, 1, 2, 2, 3, 4, 1]  
irb(main):003:0\> duplicates = array.select {|e| h[e] += 1; h[e] == 1}  
=\> [1, 2, 3, 4]  
irb(main):004:0\>

There are shorter ways, but I find that one to be pretty readable. The  
resulting array doesn’t contain duplicates, just one each of the  
entries that appeared multiple times in the input array.

It also makes it easy to find out how many times something appeared,  
if you need that.

h[1] will return 3, for example.

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<div class="post-metadata">

**Author:** ![Nico\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/ad7895/32.png) [@Nico\_L](https://www.ruby-forum.com/u/Nico_L)\
**Post date:** [October 27, 2006, 8:04pm UTC](https://www.ruby-forum.com/t/how-to-get-only-the-duplicatet-items-from-an-array/73883/4 "2006-10-27T20:04:11Z")

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thanks paul,

looks like my way.  
i hoped there is a smarter way in ruby.

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**Author:** ![Nico\_L](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/n/ad7895/32.png) [@Nico\_L](https://www.ruby-forum.com/u/Nico_L)\
**Post date:** [October 27, 2006, 8:26pm UTC](https://www.ruby-forum.com/t/how-to-get-only-the-duplicatet-items-from-an-array/73883/5 "2006-10-27T20:26:12Z")

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On Oct 27, 2006, at 10:35 AM, Nico L. wrote:

> aTemp = [1, 1, 2, 2, 3, 4]

Here’s a one-liner:

irb\> aTemp.reject { |x| (aTemp - [x]).size == aTemp.size - 1 }.uniq  
=\> [1, 2]

Mike D.  
[http://www.rubywizards.com](http://www.rubywizards.com)