Cumulative Moving Average (lets you calculate average in the middle of a stream of numbers)

cmavg.coffee

avg = (a) ->
  sum = 0
  for v in a
    sum += v
  sum / a.length

cavg = (v, av, c) ->
  av + ((v - av) / c)

console.log '---'
console.log avg [1,2,3,4]
console.log avg [1,2,3,4,5]
console.log cavg 5, 2.5, 5 # notice cavg() == avg() above

# see also:
# https://en.wikipedia.org/wiki/Moving_average#Cumulative_moving_average