From: Paul Smith <paul@...>
Date: 2009-09-29T23:22:54+09:00
Subject: Re: Group by unique entries of a hash

On Tue, Sep 29, 2009 at 2:38 PM, Robert Klemme
<shortcutter@googlemail.com> wrote:
> 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] = []}

Is this defining a default element for the hash?  I had a vague
recollection you could do this but completely forgot how.

I'd also rename the 'hash' variable to 'key' or something, I think
it's less confusing.  Then Your hashMapping can either be given the
name 'hash', because that's what it is, or a name that's actually
useful for describing what the mystical contents of the hash are.

> ...
> hashMapping[ocrID] << hash
>
> Kind regards
>
> robert
>
> --
> remember.guy do |as, often| as.you_can - without end
> http://blog.rubybestpractices.com/
>
>



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

paul@pollyandpaul.co.uk