From: Robert Klemme <shortcutter@...>
Date: 2008-03-22T22:34:53+09:00
Subject: Re: Cloning arrays

On 22.03.2008 14:10, Len Lawrence wrote:
> Another newbie question.  I have started recoding some of my homegrown
> Tcl/Tk applications in Ruby/rubytk and have hit an unexpected snag when
> attempting to clone arrays.  The following code is snipped verbatim from
> the program:
> 
>   # Calculate coordinates for the Postscript page canvas and the
>   # label icons canvas in the setupInkjet window.
> 
>   def User.calcxy ( disposition, f )
> 
>     xygrid = { }
> 
>     # Split the disposition string into x and y counts.    
>     xy = Typefaces::Disposition[disposition]
>     # Obtain the label dimensions.
>     labelsize = Typefaces::Labelsizes[disposition]
>     # Populate the coordinate data structure.
>     xygrid['x']  = nx = xy[0]
>     xygrid['y']  = ny = xy[1]
>     xygrid['dx'] = dx = labelsize[0] * f
>     xygrid['dy'] = dy = labelsize[1] * f
>     # Obtain an array of 4 element arrays from the Typefaces module.
>     # These represent opposing corners of the address spaces on the
>     # Postscript canvas. 
>     xygrid['page'] = Typefaces.labelgrid( disposition )
>     puts 'page coordinates'
>     puts xygrid['page'][0]
>     puts '______________'
>     # Copy the page label coordinates.
>     e = xygrid['page'].clone
>     # Shrink the label icon coordinates.
>     e.each { |z| z.collect! { |d| d *= f } }
>     # Shrink the icons further to make separations visible.
>     e.each { |z| z[2] -= 2.0; z[3] -= 2.0 }
>     # Save the label icon coordinates.
>     xygrid['boxes'] = e
>     puts 'page again'
>     puts xygrid['page'][0]
>     puts 'boxes'
>     puts e[0]
>     puts '=============='
>     xygrid['max'] = nx * ny
>     return xygrid
> 
>   end
> 
> The diagnostics at 'page again' and 'boxes' show that the copy of the
> original array has been successfully processed but the original array now
> reflects the same values.  The 'page' and 'boxes' arrays are to be used in
> different circumstances.  
> 
> What am I missing?  It is bound to be something simple or something
> fundamental.  I tried Object#dup also - same result:

Your issue is caused by the fact that dup and clone to shallow copies 
only, i.e. both copies of a clone Array contain the exact same 
instances.  Now you modify those instances and of course changes are 
seen via both arrays:

# Shrink the label icon coordinates.
e.each { |z| z.collect! { |d| d *= f } }
# Shrink the icons further to make separations visible.
e.each { |z| z[2] -= 2.0; z[3] -= 2.0 }

A solution would be to do

e = xygrid['page'].map do |z|
   z = z.map { |d| d *= f }
   z[2] -= 2.0; z[3] -= 2.0
   z
end

Or you can do a deep copy via the usual Marshal idiom

e = Marshal.load(Marshal.dump(xygrid['page']))

Kind regards

	robert