From: Dave Burt <dave@...>
Date: 2006-06-22T15:46:04+09:00
Subject: Re: Question about Vector class

Nasir Khan wrote:
> I was using the library Vector class defined in matrix.rb in one of my
> programs, the [] method in the library class does not check for range out of
> bounds and the []= does not even exist. In my program I extended the Vector
> class as follows -
> ...
> class Vector
>   include ExceptionForMatrix

Once a module has been included, you don't need to re-include it when
you re-open the class.

>   # []= not defined in the library class
>   def []= (i, val)
>     Vector.Raise ErrDimensionMismatch if i >= @elements.size
>     @elements[i] = val
>   end

I haven't a great deal of experience with matrix.rb, but from the
quick look I just did, it seems that Vector instances are immutable
(un-changeable), like the Numeric classes. Immutability means you can
pass a vector or number into a method and know it won't be
unexpectedly altered. It means you can think of a vector as a fixed
value, rather than a container.

So your proposed []= method is a bad idea. If you want to be able to
tweak a vector one element at a time, consider using Array (especially
if you're not wanting a mathematical vector) or a method like the
following, to keep Vector immutable:

def with_element_replaced (i, val)
  Vector.Raise ErrDimensionMismatch if i >= @elements.size
  new_vec = self.to_a
  new_vec[i] = val
  Vector[*new_vec]
end

>   # Fix the [] in library Vector class
>   def [](i)
>     Vector.Raise ErrDimensionMismatch if i >= @elements.size
>     @elements[i]
>   end
> end

I don't see a strong argument for this over the existing behaviour.
What you've done isn't wrong, though, and if you find it handy, use
it. You've gained early and descriptive failure, but lost a bit of
flexibility in using negative indexes (vec[-1] for the last element).

Cheers,
Dave