From: Wilson Bilkovich <wilsonb@...>
Date: 2006-11-08T05:51:51+09:00
Subject: Re: Help with a program to determin leap years.

On 11/7/06, dblack@wobblini.net <dblack@wobblini.net> wrote:
> Hi --
>
> On Wed, 8 Nov 2006, Shiloh Madsen wrote:
>
> > So, I'm trying to go through the Teach Yourself Programming book by
> > Pragmatic Press and I am encountering a few hurdles. The chapter I am
> > working on now is asking me to create a program which will ask for a
> > start and end year and then calculate all leap years in that range.
> > The logic behind leap years (for those who need a refresher) is all
> > years divisible by for are leap years EXCEPT those that are divisible
> > by 100 UNLESS they are also divisible by 400. I am somewhat at a loss
> > for how to handle the logic for this...finding all numbers that are
> > divisible by 4 and removing those divisible by 100 is easy. Its adding
> > in that third condition which adds some of the removed numbers back
> > into the "true" group that I am having trouble with...or maybe I am
> > just not wrapping my mind around the problem well
> > enough...suggestions?
>
> require 'date'
> Date.leap?(year)   # :-)
>
> See Morton's implementation.  Here, just for fun, is another:
>
>    def leap?(year)
>      year % 4 == 0 unless (year % 100 == 0 unless year % 400 == 0)
>    end
>
> It returns nil/true rather than false/true, so it's a bit
> non-slick.  But I thought the semantics might be interesting.
>

You can fix that with the fun !! trick to convert nils back to false:
def leap?(year)
  !!(year % 4 == 0 unless (year % 100 == 0 unless year % 400 == 0))
end