From: Robert Klemme <shortcutter@...>
Date: 2010-09-14T18:26:49+09:00
Subject: Re: Converting from one class to another in Ruby

On Tue, Sep 14, 2010 at 4:35 AM, Harry Spier <harryspier@hotmail.com> wrote:

> Many thanks for your reply.

You're welcome.

> I have simplified somewhat.  Its an OCR application where Binary_matrix
> is a black and white pixel representation of a page of text(or line of
> text, or letter or part of a letter etc.).  There are many different
> objects which are to be converted into Binary_matrix such as the above
> portions of an existing Binary_matrix object, or Rmagick ImageList
> objects, or graphic objects processed by other packages than RMagick.
> This list will definitely grow in the future so for that reason I wanted
> the conversion to Binary_matrix logic to reside in the class being
> converted to Binary_matrix not in the Binary_matrix class.

That's perfectly OK.  Just the way you coded your logic is weird IMHO.
 Just do this for your classes:

class A
  def initialize( what goes here? )
    # what do you want to do here?
  end

  def to_binary_matrix
    bm = BinaryMatrix.new
    # fill bm
    bm
  end
end

> I.e. I didn't want a list of statements in Binary_matrix initialize
> method like:
> if arg.class == ImageList then ImageList_conversion end
> if arg.class == JPEG then jpeg_conversion end
> etc. etc.

I would not want it that way either nor did I suggest this.

> Rather I wanted my Binary_matrix object to check if the object to be
> converted to a Binary_matrix had a to_Binary_matrix method and then to
> call it.

Just call it (see below).

> I also require all the Array methods for Binary_matrix because of the
> complex traversing and manipulation of the pixel data.  To give a
> trivial example: does a black pixel in line n touch a black pixel in
> line n+1? What are the array ranges of a contiguous group of black
> pixels etc. Change a contiguous range of pixels to white (i.e. clear up
> noise) etc.

These are the exact operations that I would place in a class
BinaryMatrix (btw. note the naming conventions for Ruby constants).
An Array is just not a matrix.  By exposing all Array methods client
code (i.e. classes that use BinaryMatrix) needs to know about the
internal representation of a BinaryMatrix which is about the worst
thing you can do in an OO application from a design perspective.

> Again because of complexity of the application I wanted to make the code
> as humanly readable as possible.  So I want to be able to say
> page_in_pixels = page_image.to_Binary_matrix
> as well as
> page_in_pixels = Binary_matrix(page_image)

For that to work you only need

def BinaryMatrix(x)
  x.to_binary_matrix
end

and implement #to_binary_matrix in all your classes properly.

> (I'm not saying its necessarily a good solution and I really wish I
> could make it less clunky. if there was some way to tell whether method
> to_Binary_matrix was being called within the context of Binary_matrix
> initialize method or outside it then the flag -from_initialize_method-
> wouldn't be necessary)

Why and what do you want to convert to BM in #initialize?  This hasn't
become clear yet.

Cheers

robert

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