unique_xml.rb

require 'nokogiri'

# retrive unique values of category in an XML document 
# doc is an Nokogiri::XML::Document
# (http://nokogiri.org/Nokogiri/XML/Document.html)
@doc.css('category').to_a.map { |c| c.content }.uniq


# css('category') # get all <category> nodes
# to_a # convert to an array
# map # invoke block on each element to pull
#     the xml node's content out
# uniq # remove duplicate categories