Find a unique element in a compound array

Question

I am trying to solve a problem where I need to find the airport code in an array of arrays of that represents the starting point of a multi-city flight plan. For example: Given

Answer 1

You could do this:

legs = [['LAX', 'BWI'], ['BOS', 'SEA'], ['HNL', 'LAX'], ['SEA', 'HNL']]

airports = legs.flatten 
  #=> ["LAX", "BWI", "BOS", "SEA", "HNL", "LAX", "SEA", "HNL"] 
legs.map(&:first).find { |ap| airports.count(ap) == 1 }
  #=> "BOS"

This could instead be written using Array#transpose (as has been done in other answers) or Enumerable#zip since:

legs.map(&:first)
  #=> ["LAX", "BOS", "HNL", "SEA"] 
legs.transpose.first
  #=> ["LAX", "BOS", "HNL", "SEA"] 
legs.first.zip(*legs[1..-1]).first
  #=> ["LAX", "BOS", "HNL", "SEA"] 

My main reason for answering, however, is to make a plug for the method Array#difference, which I'd like to see in some future Ruby version. It is defined here.

With it, we can write:

airports = legs.flatten 
  #=> ["LAX", "BWI", "BOS", "SEA", "HNL", "LAX", "SEA", "HNL"] 
(legs.map(&:first) - airports.difference(airports.uniq)).first
  #=> "BOS" 

Note that:

airports.difference(airports.uniq)
  #=> ["LAX", "SEA", "HNL"]

contains all airports that appear more than once in the legs; that is, all all the "intermediate" airports.