From: Robert Klemme <shortcutter@...>
Date: 2009-09-29T22:38:37+09:00
Subject: Re: Group by unique entries of a hash

2009/9/29 Paul Smith <paul@pollyandpaul.co.uk>:
> 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

It is slightly more efficient to do it in one step:

(hashMapping[ocrID] ||= []) << hash

Even nicer

hashMapping = Hash.new {|h,k| h[k] = []}
...
hashMapping[ocrID] << hash

Kind regards

robert

-- 
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