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From: Robert Klemme <shortcutter@googlemail.com>
Newsgroups: comp.lang.ruby
Subject: Re: Making a count of unique elements in an array
Date: Sat, 28 Dec 2013 12:02:41 +0100
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On 26.12.2013 22:44, lukeabergen@gmail.com wrote:
> Another way is the super-useful group_by and map
>
> arr = ["a", "b", "b", "c", "c", "d", "e", "e", "e"]
> grouped = arr.group_by {|x| x}  #=> produces something like {"a" => ["a"], "b" => ["b", "b"], etc...
> result = grouped.map {|el, arr| [el, arr.count]}

Downside of that approach is that it does unnecessary work and uses more 
memory than necessary.  And the result is not a Hash which may make 
retrieval of the counts of an element quite expensive if there are many 
different values.

Btw, the OP spoke of a "2D array".  So the input likely looks different 
than in your example.  We also do not know what he actually wants to 
count.  Is it individual elements?  Or does "distinct elements" refer to 
a single array?

Kind regards

	robert