From: Paul Smith <paul@...>
Date: 2009-09-29T20:55:44+09:00
Subject: Re: Group by unique entries of a hash

On Tue, Sep 29, 2009 at 12:43 PM, Ne Scripter
<stuart.clarke@northumbria.ac.uk> wrote:
> I have two data sets loaded into a hash to give the following output
>
> "2efa4ba470",  "00000005"
> "2efa4ba470",  "00000004"
> "02adecfd5c",  "00000002"
> "c0784b5de101",  "00000006"
> "68c4bf10539",  "00000003"
> "c0784b5de101",  "00000001"
>
> My code to get this is as follows:
>
>  source= "C:\\dummyFile.txt"
>  hashMapping = Hash.new
>  ocrIDMapping = Hash.new
>
>  IO.foreach(source.to_s) do |data|
>    fields = data.split(",")
>    hash = fields[0]
>    ocrID = fields[1]
>    hashMapping[ocrID] = hash
>  end
>
>  hashMapping.sort{|a,b| a[1]<=>b[1]}.each { |elem|
>
>  puts "#{elem[1]}, #{elem[0]}"}
>
> I would like to alter my output to group my the first value to give an
> output like this:
>
> "2efa4ba470",  "00000005", "00000004"
> "02adecfd5c",  "00000002"
> "c0784b5de101",  "00000006", "00000001"
> "68c4bf10539",  "00000003"
>
> As you can see now only unique values are shown in the first field
> however a list of the corresponding second field is formed, grouping the
> results. Something like this I could do in SQL however I have never come
> across it in Ruby so does anyone have any pointers?

You want a hash where the key is the element you want to group on, and
the 'item' is an array of all items with the shared key.  A bit like
(untested):

hashMapping = {}

IO.foreach(source.to_s) do |data|
   fields = data.split(",")
   hash = fields[0]
   ocrID = fields[1]

   hashMapping[ocrID] ||= [] #If hashMapping has never seen this key
before, make an empty array

   hashMapping[ocrID] << hash #Add the new element to the array for this key

 end

>
> Many thanks
> --
> Posted via http://www.ruby-forum.com/.
>
>



-- 
Paul Smith
http://www.nomadicfun.co.uk

paul@pollyandpaul.co.uk