Overview

 

Question

You are given the following information, but you may prefer to do some research for yourself.

1 Jan 1900 was a Monday.
Thirty days has September,
April, June and November.
All the rest have thirty-one,
Saving February alone,
Which has twenty-eight, rain or shine.
And on leap years, twenty-nine.
A leap year occurs on any year evenly divisible by 4, 
but not on a century unless it is divisible by 400.

How many Sundays fell on the first of the month during the twentieth century (1 Jan 1901 to 31 Dec 2000)?

click for answer

171

Solutions

 

Ruby

 centurySundays.rb https://raw.github.com/addamh/euler/master/019/centurySundays.rb download
#!/usr/bin/env ruby

require 'date'

sunday_count = 0

1.upto(100).with_index.each do |i|
  1.upto(12).with_index.each do |j|
    sunday_count += 1 if Date.new(1900+i,j,1).cwday == 7
  end
end

p sunday_count
$ time ruby centurySundays.rb
real	0m0.452s
user	0m0.430s
sys	0m0.020s