From: David Alan Black <dblack@...>
Date: 2002-01-25T21:15:58+09:00
Subject: Re: Problem with arrays of arrays (matrices) copying

Hello --

On Fri, 25 Jan 2002, jasa wrote:

> I have one global matrix '$mat' and I want to reorder the columns
> and lines of it. I make a duplicate, twice, in matrix '$temp' and
> I reorder the lines and columns from there.
> I know the algorithm I use is not optimal, but that is not the issue
> B-).
> It works OK with reordering lines, but has problems with reordering
> columns.
>
> The problem I report has to do with matrix copying.
> It has also to do with the method 'Object.dup'.
> I solved the problem, as I will show, but I don't know why.

The problem, I think, is that when you dup your matrix, you're getting
a shallow dup -- meaning, the actual row objects inside it are the
same as the originals, and all you have is a new way to refer to them.

Here's a small example where that happens.  Note that the id's of
t's elements and m's elements are the same:

  irb(main):002:0> m = [ [1,2,3], [4,5,6] ]
  [[1, 2, 3], [4, 5, 6]]
  irb(main):003:0> t = m.dup
  [[1, 2, 3], [4, 5, 6]]
  irb(main):004:0> t[0].id
  537724350
  irb(main):005:0> m[0].id
  537724350


Instead of duping, you could do this or something fancier based on this:

  temp = mat.map {|row| row.dup}  # not using globals, hint hint :-)

Also, don't forget the Matrix class that comes with the Ruby
distribution (though I know that's not the same as working out the
problem yourself :-)


David

-- 
David Alan Black
home: dblack@candle.superlink.net
work: blackdav@shu.edu
Web:  http://pirate.shu.edu/~blackdav