From: Rob Biedenharn <Rob@...>
Date: 2008-06-06T07:24:23+09:00
Subject: Re: Histogram

On Jun 5, 2008, at 6:01 PM, Justin To wrote:

> I've been looking around to learn about histograms, and haven't had  
> any
> luck... can someone tell me how a histogram would work for something
> like this:
>
> Name, Age
> Bob, 2
> Jim, 2
> Eric, 4
> Rob, 5

Depending on how you get that data, FasterCSV might be a good option,  
but I'll just create a nested array for illustration.

ages = [ ['Bob', 2], ['Jim',2], ['Eric',4], ['Rob',5] ]

> How would I use a histogram to get a result like this:
>
> Age | Names
> 2   | Bob, Jim
> 4   | Eric
> 5   | Rob
>
> Thanks in advance!!!


So the answer you want is:  (note that I'm using a Hash since the keys  
will be unique anyway)

histogram = { 2 => ['Bob','Jim'], 4 => ['Eric'], 5 => ['Rob'] }


So you could just use Enumerable#group_by (provided by the  
ActiveSupport gem from Rails; yes, you can use it by itself).  Of  
course, it's also a simple matter to group them yourself:

histogram = Hash.new { |h,k| h[k] = [] }   # hash that defaults values  
to empty Arrays
ages.each {|name,age| histogram[age] << name }

If you need to produce that exact output:

puts "Age | Names"
histogram.sort_by {|age,names| age}.each do |age,names|
   puts "%-3d | %s"%[age, names*', ']
end

Age | Names
2   | Bob, Jim
4   | Eric
5   | Rob

And I expect that you have a lot of thumbing through the Pickaxe to  
understand all that ;-)

-Rob

Rob Biedenharn		http://agileconsultingllc.com
Rob@AgileConsultingLLC.com