From: Matthew Moss <matthew.moss.coder@...>
Date: 2006-01-27T06:41:31+09:00
Subject: Re: Need a ruby math genius - potential ruby bug.

I think (offhand, didn't actually run your code) your problem is not
math but pass-by-reference vs. pass-by-value issues.  Look at your
Vector.normalize function:

>   def Vector.normalize(v)
>     divisor = 0
>     v.elements.each do |elem|
>       divisor += elem*elem
>     end
>
>     if divisor != 0
>       divisor = Math.sqrt(divisor);
>       i = 0
>       v.elements.each do |elem|
>         v.elements[i] /= divisor
>         i += 1
>       end
>     end
>     v
>   end

v is not a copy of the parameter passed in but a reference to it.  And
you are changing the elements of v directly.

> l = Vector.normalize(l)
> a = Vector.normalize(a)

In these two cases here, you don't notice the destructive behaviour,
since you assign the result to the parameter.  But here:

> b2 = Vector.normalize(b1)

You are assigning the result to a different variable, but you've
actually changed b1 as well because it's not pass-by-value.


I would write a Vector normalize like such:

class Vector
   def normalize
      div = inner_product(self)
      if div.zero?
         self
      else
         self * (1 / Math.sqrt(div))
      end
   end

   def Vector.normalize(v)
     v.normalize
   end
end