From: Boris Schmid <boris@...>
Date: 2008-06-06T16:25:05+09:00
Subject: Re: Histogram

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

Assuming that you have a file that looks exactly like the one above, 
this program does exactly what you asked.

# Reads in the datafile, and drops the 'Name, Age' line
data = IO.readlines("nameofdatafile")
data.shift

# Store the data in a hash
hash = Hash.new
data.each {|entry|
  name = entry.split(",")[0]
  age = entry.split(",")[1].to_i

  hash[age] = [] unless hash.has_key?(age) # make sure there's a hash 
key
  hash[age] << name
}

# Pretty print it for each entry in the hash
puts "Age | Names"
hash.keys.each {|age|
  printf("%3i | %s\n",age,hash[age].sort.join(", "))
}


For how histograms work: the general idea is that you create a number of 
categories, and assign every single datapoint to its corresponding 
category. One way to do this in ruby is to create a hash, in which each 
key of the hash is a category, and the corresponding value of the hash 
is an empty array. Then you proceed to fill these empty arrays with your 
data, by distributing every datapoint to its corresponding array.

(Hopefully this is a general enough answer, it is in essence the same as 
Rob's answer.)
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