Calculating the number of weeks in a year with Ruby

Question

Is there a way in Ruby to calculate the number of weeks(ISO 8601) for a given year? I'm currently using a lookup table and I'd like to stop using it.

Answer 1

def num_weeks(year = Date.today.year)
  Date.new(year, 12, 28).cweek # magick date!
end

long_iso_years = (2000..2400).select{|year| num_weeks(year) == 53} 

Yields the same list as wikipedia

Answer 2

require 'date'
def num_weeks(year = Date.today.year)
  # all years starting with Thursday, and leap years starting with Wednesday have 53 weeks
  # http://en.wikipedia.org/wiki/ISO_week_date#Last_week
  d = Date.new(year, 1, 1)
  return 53 if d.wday == 4
  return 53 if d.leap? and d.wday == 3
  52
end

Answer 3

You can do the following:

require 'date'
@year = 2001 #year you want to count the number of weeks
d = Date.new @year, 12, 30 # as in Date.new 
d.cweek # returns the commercial week number for the last week of the year, in this case, 52

if that's what you're looking for :)

PS: that only works for commercial year though, so in 2001, the 31th of December was actually commercial week 1