parser.rb

require 'rexml/document'

xml = File.read('cia-1996.xml')
doc = REXML::Document.new(xml)
continents = []
countries = []
doc.elements.each('cia/continent') do |continent|
  continents << {:name => continent.attribute('name').to_s}
end

doc.elements.each('cia/country') do |country|
  countries << {:name => country.attribute('name').to_s, :inflation => country.attribute('inflation').to_s.to_i, :population => country.attribute('population').to_s.to_i, :continent => country.attribute('continent').to_s}
end

countries.sort!{|a,b| b[:population]<=>a[:population]}

p "1. " << [countries.first[:name], countries.first[:population]].join(' with ')

countries.sort!{|a,b| b[:inflation]<=>a[:inflation]}

p "2. "
0.upto(4){ |rank| p [countries[rank][:name], countries[rank][:inflation]].join(' with ')}

p "3. "

continents.sort{ |a,b| a[:name] <=> b[:name] }.each{ |continent|
  p "=====> Continent: #{continent[:name]} <========"
  countries.select{ |country| country[:continent] == continent[:name] }.sort{ |a,b| a[:name] <=> b[:name] }.each{ |country| p country[:name] }
}