From: Max Williams <toastkid.williams@...>
Date: 2008-09-09T02:04:21+09:00
Subject: Is this a sensible implementation for Array#group_by ?

I was looking for a method to split an array into smaller arrays based
on some property of the members (set with a code block).  I came up with
the following and wanted to know what you guys think about a) whether
it's sensible b) whether it's been done already:

class Array
  def group_by
    result_array = []
    self.each do |element|
      key = yield element
      if (found = result_array.assoc(key))
        found[1] << element
      else
        result_array << [key, [element]]
      end
    end
    return result_array.collect{|a| a[1]}
  end
end

>> arr = %w(apple banana pear plum nectarine orange melon)
=> ["apple", "banana", "pear", "plum", "nectarine", "orange", "melon"]
#group by length of name
>> arr.group_by{|s| s.length}
=> [["apple", "melon"], ["banana", "orange"], ["pear", "plum"],
["nectarine"]]
#group by whether it has an e
>> arr.group_by{|s| (s =~ /e/) == nil}
=> [["apple", "pear", "nectarine", "orange", "melon"], ["banana",
"plum"]]

One thing i haven't done is make it sort: since the grouping key is just
an object and most objects aren't sortable.  I could get round this but
not without slowing it down and the user can always sort the results by
comparing the first member of each subarray.

I'm just after some feedback really.  I'm guessing i just couldn't find
the  good implementation of it :)
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