From: Phrogz <gavin@...>
Date: 2007-02-13T06:25:08+09:00
Subject: Re: What's the correct and fast way to determine if a (gig) number is a perfect square?

On Feb 12, 2:07 pm, "Sam Kong" <sam.s.k...@gmail.com> wrote:
> On Feb 12, 12:12 pm, Joel VanderWerf <v...@path.berkeley.edu> wrote:
> > Wrong how?
>
> > irb(main):001:0> x = 55833579873437812
> > => 55833579873437812
> > irb(main):002:0> sqrt = Math::sqrt(x)
> > => 236291303.0
> > irb(main):003:0> sqrt.floor**2 - x
> > => -3
>
> I think even if you use #round, the problem won't go away.
> Float type cannot generate correct result due to its limited
> precision.

For example:
irb(main):004:0>
x=981723462487562983749812734972342334123435635465656432452
=> 981723462487562983749812734972342334123435635465656432452

irb(main):005:0> y=x**2
=>
963780956798569484937549463180492938080866374138542452022484415498543617865176348383792466015930367123924038732304

irb(main):006:0> z=Math.sqrt(y)
=> 9.81723462487563e+056

irb(main):007:0> z==x
=> true

irb(main):010:0> z==(x+1000)
=> true

irb(main):011:0> z==(x+10000)
=> true