From: "Jesús Gabriel y Galán" <jgabrielygalan@...>
Date: 2010-02-17T03:40:48+09:00
Subject: Re: ruby to_i problem

On Tue, Feb 16, 2010 at 7:16 PM, Phil Cooper-king <phil@cooperking.net> wrote:
> I'm having an issue with a float being converted to an int,
>
> The maths is part of a vector rotation of a little video game, which x,y
> coords are rotated at a given angle.
>
> For what ever reason the first instance gives an error, and every
> instance there after works as it should
>
> This cant be right?
>
>
> ruby 1.8.7
> [irb]
> # This works the way I would think
>>> 1.0.o_i
> => 1
>
>
> # This doesn't
>>> x = -1
>>> y = -1
>>> angle = 90 * Math::PI / 180
>>> n_x = (Math::cos(angle) * x) - (Math::sin(angle) * y)
> => 1.0
>
>
> # this is where its werid
>>> n_x.to_i
> => 0
> [/irb]
>
> any further 1.0.to_i works

You are facing a problem about the inaccuracy inherent to floating
point numbers:


irb(main):001:0> 0.9999.to_i
=> 0
irb(main):002:0> x = -1
=> -1
irb(main):003:0> y = -1
=> -1
irb(main):004:0> angle = 90 * Math::PI / 180
=> 1.5707963267949
irb(main):005:0> n_x = (Math::cos(angle) * x) - (Math::sin(angle) * y)
=> 1.0
irb(main):009:0> "%.20f" % n_x
=> "0.99999999999999988898"
irb(main):010:0>
irb(main):010:0> n_x
=> 1.0

n_x is very close to 1.0 but it's not. to_i rounds the number down, so
you get 0. You can check the archives for a more thorough explanation
about this, there's been plenty of discussion in the past.

Jesus.