Advanced Rails - Building Industrial-Strength Web Apps in Record Time

Enumerable

Enumerable | 65

1.month # => 1 month

1.month.ago # => Mon Sep 24 23:57:04 -0500 2007

1.month.since(1.year.ago) # => Fri Nov 24 23:57:04 -0600 2006

The addition ofActiveSupport::Durationfixes many subtle problems with time calcu-

lations, because it stores durations as exact values (years, months, days, hours, min-

utes, and seconds). Prior to its addition, errors could easily accumulate, as some

duration functions assumed 30-day months and 365.25-day years.

• Time#advance adds the specified (exact) values to the given time:
  t = Time.now # => Wed Oct 24 23:50:11 -0500 2007
  t.advance :days => 1 # => Thu Oct 25 23:50:11 -0500 2007
  t.advance :months => 1 # => Sat Nov 24 23:50:11 -0600 2007
  t.advance :years => 1 # => Fri Oct 24 23:50:11 -0500 2008


• Convenient shorthand methods, listed below (all are instance methods ofTimethat
  take no arguments):
  — last_year andnext_year
  — last_month andnext_month
  — beginning_of_week (aliased asmonday andat_beginning_of_week) andnext_week
  — beginning_of_day(aliased asmidnight,at_midnight, andat_beginning_of_day) and
  end_of_day
  — beginning_of_month(alsoat_beginning_of_month) andend_of_month(alsoatend
  of_month)
  — beginning_of_quarter (at_beginning_of_quarter)
  — beginning_of_year (at_beginning_of_year)
  — yesterday,tomorrow

Enumerable core_ext/enumerable.rb

• Enumerable#group_by groups the values into a hash based on the result of a block.
  (1..5).to_a.group_by {|x| x%3} # => {0=>[3], 1=>[1, 4], 2=>[2, 5]}


• Enumerable#index_byindexes values based on the result of a block. It differs from
  group_by in that it only keeps one value per index key.
  (1..5).to_a.index_by {|x| x%3} # => {0=>3, 1=>4, 2=>5}


• Enumerable#sumreturns the sum of the values (or the result of mapping the given block
  over the values, if a block is given). The optional first argument provides a value to use
  if the enumerable is empty.
  (1..5).to_a.sum # => 15
  (1..5).to_a.sum {|x| x ** 2} # => 55
  This makes statistical calculations easy:
  module Enumerable
  def mean
  (sum.to_f) / length
  end

def variance

m = mean