From: Ne Scripter <stuart.clarke@...>
Date: 2009-09-29T20:43:40+09:00
Subject: Group by unique entries of a hash

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?

Many thanks
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