one pass through records (for mean and variance); annoyingly proper definition of median

stats.rb

# Run the given block +num+ times and then print out the mean, median, min,
# max, and stddev of the run. For example:
#
# irb> stats(10) { sleep(rand / 100) }
# mean:   5.99ms
# median: 6.76ms
# min:    1.49ms
# max:    9.28ms
# stddev: 2.54ms

def stats(num)
  records = []
  num.times do
    t0 = Time.now
    yield
    records << (Time.new-t0)
  end
  srecords = records.sort 
  num = records.size 
  n, mean, m2 = records.inject([0,0.0,0.0]) do
    |acc, r|
    n = acc[0]+1
    delta = r-acc[1]
    mean = acc[1] + (delta/n)
    m2 = acc[2] + (delta * (r-mean))
    [n, mean, m2]
  end
  var = m2/n
  
  puts "mean:   %1.2fms" % (mean * 1000)  
  puts "median: %1.2fms" % ((num % 2 == 1) ? (srecords[num/2] * 1000) : ((srecords[num/2]+(srecords[num/2-1])/2) * 1000))
  puts "min:    %1.2fms" % (srecords.first * 1000)
  puts "max:    %1.2fms" % (srecords.last * 1000)
  puts "stddev: %1.2fms" % (Math.sqrt(var) * 1000)
end