benchmark for this SO question: http://stackoverflow.com/questions/19118688/ruby-array-map-and-join-in-one-loop

benchmark.rb

require 'benchmark'
iterations = 10_000
arr = Array.new(1000) { Array.new(2) << (rand()*100).to_s}
 
def use_map_join(arr)
  arr.map {|a| a[2]}.join ', '
end
 
def use_inject(arr)
  join_str = ', '
  arr.inject(nil) {|str, arr| str ? (str << join_str << arr[2]) : arr[2].dup}
end
 
Benchmark.bm do |bm|
  bm.report('map/join') do
    iterations.times do
      use_map_join arr
    end
  end
   
  bm.report('inject') do
    iterations.times do
      use_inject arr
    end
  end
end

result

              user     system      total        real
map/join  1.440000   0.000000   1.440000 (  1.441858)
inject    2.220000   0.000000   2.220000 (  2.234554)