Question

I’m learning ruby and have a few questions about some code I wrote for a newbie challenge. Purpose of challenge is to find country with largest population from an xml document.

I’ve included my code below. Questions I have are:

1. Is there a way to avoid having to initialize the @max_pop variable (@max_pop=0)?
2. Is there shorthand for combining the entire conditional block into 1 line?
3. Do I have to use instance vars @max_pop, @max_pop_country? Got error without them.
4. Which is more efficient:
   1. Loop through each country and check if pop > max_pop (approach in code below)
   2. Create pop hash (pop[:country]) and then find country with highest pop
5. Is there hash method to return key value pair for largest element in hash (to do 4.1)?

Source Code:

@max_pop=0
doc.elements.each("cia/country") do |country|
 if country.attributes["population"].to_i > @max_pop
  @max_pop=country.attributes["population"].to_i
  @max_pop_country=country.attributes["name"]
 end
end 
puts "country with largest pop is #{@max_pop_country} with pop of #{@max_pop}

Answer 1

I am not familiar with rexml, but you ought to be able to simplify everything to something like this:

max_pop_elem = doc.elements.enum_for(:each, "cia/country").max_by { |c| c.attributes["population"].to_i }
max_pop_country = max_pop_elem.attributes["name"]
max_pop = max_pop_elem.attributes["population"].to_i

1. Yes, see above.
2. Yes, see above.
3. No. You should use local variables instead of instance variables when possible.
4. Don’t worry about efficiency of CPU time until you have a slow program. Then use ruby-prof. Until then, just worry about the efficiency of coding time (do things the easy way).
5. Yes, just do key, value = hash.max_by{|k,v| v}.

In general, if you are going to be iterating over things you should learn about Ruby’s Enumerable module. I made a reference sheet for it here.