test.rb

=begin
~ ruby test.rb
n = 10000
       user     system      total        real
   0.010000   0.000000   0.010000 (  0.002855)
   0.000000   0.000000   0.000000 (  0.005343)
--
n = 20000
       user     system      total        real
   0.010000   0.000000   0.010000 (  0.005923)
   0.010000   0.000000   0.010000 (  0.009862)
--
n = 30000
       user     system      total        real
   0.010000   0.000000   0.010000 (  0.008921)
   0.010000   0.000000   0.010000 (  0.014062)
--
n = 40000
       user     system      total        real
   0.010000   0.000000   0.010000 (  0.013030)
   0.020000   0.000000   0.020000 (  0.018478)
--
n = 50000
       user     system      total        real
   0.010000   0.000000   0.010000 (  0.013829)
   0.030000   0.000000   0.030000 (  0.025704)
=end

require "benchmark"

c = ARGV[0] ? ARGV[0].to_i : 10000

(1..5).each do | nn |
  n = c * nn
  puts "n = #{n}"
  Benchmark.bm do |x|
    x.report {
      (0..n).inject { |sum, i|
        sum + ((i = i + 1) % 2 == 0 ?  i - 1 : 0)
      }
    }

    x.report {
      (0..n).map { |i| i + 1 }.
      select { |j| j % 2 == 0 }.
      map { |k| k - 1 }.
      reduce(&:+)
    }
    puts "--"
  end
end