From: jasa <jasa@...>
Date: 2002-01-25T20:31:55+09:00
Subject: Problem with arrays of arrays (matrices) copying

Hi,

I would like to report a problem while working
with arrays of arrays (matrices) in Ruby 1.6.5,
in Windows 98SE (PragProg installer 1.6.5-2).
I also tried the code in Ruby 1.6.6, compiled with gcc (I think it is
2.95.3)
in a Linux Slackware 8.0 box. The results were exactly the same.

The data in the example is a bit large, sorry.
But, perhaps, there is something happening with the 'internals'
of array manipulations (duplication of data, garbage collection?).

Here it goes.

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.

Sorry, but I never tried to hack the Ruby source, and
I don't know where or what to look for...

Here goes the code and outputs:

                *************  Code: ***************

............................
#
#    Code for reodering lines. Index 0 (zero) is not used
#    either in the arrays or the matrices
#
# newLineOrder is an array with the reordering of lines

newLineOrder = [0,5,1,3,2,4]

# newColOrder is an array with the reordering of columns

newColOrder = [0,1,5,3,2,4]

# Reordering lines. This swaps the 'line-pointers'... Works OK!

$temp = $mat.dup       # DUPLICATE!
for i in (1..dim)      # dim is the dimension of the matrix.
  $mat[i] = $temp[newLineOrder[i]]
end

            *************  END Code: ***************


This the swapped matrix (only shown filled elements). It worked OK.


#----  Matrix $mat after swap lines   ----   OK HERE!

(1,1)         +1

(2,1)      +g_r1
(2,2)      -g_r1
(2,5)         +1

(3,3) +((+c1)/h)
(3,4)         -1

(4,1)      -g_r1
(4,2)      +g_r1
(4,4)         +1

(5,2)         +1
(5,3)         -1
(5,4) +((-l1)/h)

This worked as expected!


Now the code for swapping columns, following the data in
the newColOrder[....] array:


            *************  Code: ***************

# Swapping cols....

  $temp = $mat.dup             # MUST DUPLICATE!

# It was confirmed here that '$temp' is equal to '$mat' after this .dup

  for i in (1..dim)
#                      'RESETS' or 'CLEARS' the line!
#
#   $mat[i] = []    # ****** WHEN THIS IS UNCOMMENTED WORKS WELL!!!!
****
#
    for j in (1..dim)
      $mat[i][j] = $temp[i][newColOrder[j]]
    end
  end

            *************  END Code: ***************


The result of the above code is:


----  Matrix  mat   ----

(1,1)         +1

(2,1)      +g_r1
(2,2)         +1
(2,4)         +1
(2,5)         +1

(3,3) +((+c1)/h)

(4,1)      -g_r1

(5,3)         -1


Some data is lost! In the last 2 columns, I think.

However, after RESETTING or CLEARING each matrix line before reordering
with copying, (see the comment in the code above), with the instruction:


   $mat[i] = []

the following CORRECT column reordered matrix is obtained!

----  Matrix  mat   ----

(1,1)         +1

(2,1)      +g_r1
(2,2)         +1
(2,4)      -g_r1

(3,3) +((+c1)/h)
(3,5)         -1

(4,1)      -g_r1
(4,4)      +g_r1
(4,5)         +1

(5,3)         -1
(5,4)         +1
(5,5) +((-l1)/h)

Any pointers on this behavior? Is this the consequence of a Ruby
feature,
or can it be some kind of bug? I think that dup really duplicates the
structures,
and as the iterators span all the indices of the matrix when copying, it

should work OK...

As I said, the problem was solved by clearing the line,
but I don't understand why...

Thanks in advance!

Best regards

J. Augusto