From: Phrogz <phrogz@...>
Date: 2009-12-24T12:20:17+09:00
Subject: Re: Date.gregorian_leap? code

On Dec 23, 2:42 pm, Colin Bartlett <colin...@googlemail.com> wrote:
> from date.rb
>   # Is a year a leap year in the Gregorian calendar?
>   # All years divisible by 4 are leap years in the Gregorian calendar,
>   # except for years divisible by 100 and not by 400.
>   def self.gregorian_leap? (y) y % 4 == 0 && y % 100 != 0 || y % 400 == 0 end
>
> Because of the precedence of && and || this is presumably parsed as
>      ( y % 4 == 0 && y % 100 != 0 ) || y % 400 == 0
> and I'm trying to think of reasons why it is coded as it is instead of, say:
>      y % 4 == 0 && ( y % 100 != 0 || y % 400 == 0 )

I suppose because either one works:

irb> LEAP = [ 1796, 1804, 1896, 1904, 1992, 1996, 2000, 2004, 2008 ]
irb> NOLEAP = [ 1799, 1800, 1801, 1802, 1803, 1805, 1899, 1900, 1901 ]
irb> def test_leap
irb>    LEAP.each{ |y| puts "BAD! #{y} must leap" unless yield y }
irb>    NOLEAP.each{ |y| puts "BAD! #{y} musn't leap" if yield y }
irb> end
irb> test_leap{ |y| y % 4 == 0 && y % 100 != 0 || y % 400 == 0 }
irb> test_leap{ |y| ( y % 4 == 0 && y % 100 != 0 ) || y % 400 == 0 }
irb> test_leap{ |y| y % 4 == 0 && ( y % 100 != 0 || y % 400 == 0 ) }