From: Run Paint Run Run <redmine@...>
Date: 2009-05-26T05:24:06+09:00
Subject: [ruby-core:23556] [Bug #1515] [PATCH] Teach Matrix#square? About Non-Square Matrices; Fix Crasher

Bug #1515: [PATCH] Teach Matrix#square? About Non-Square Matrices; Fix Crasher
http://redmine.ruby-lang.org/issues/show/1515

Author: Run Paint Run Run
Status: Open, Priority: Normal
ruby -v: ruby 1.8.7 (2008-08-11 patchlevel 72) [i486-linux] / ruby 1.9.0 (2008-06-20 revision 17482) [i486-linux]

The .square? method of the Matrix class regards a matrix as square if it contains the same number of rows as it does columns in the first row. Thus, Matrix[ [1,2],[1] ].square? returns true. Not only is this wrong, as admitted in the source code, but it also breaks other methods that rely upon it. For example:

irb(main):003:0> Matrix[ [1,2],[1] ].determinant
NoMethodError: undefined method `-' for nil:NilClass
	from /usr/lib/ruby/1.9.0/matrix.rb:715:in `block (2 levels) in determinant'
	from /usr/lib/ruby/1.9.0/matrix.rb:713:in `upto'
	from /usr/lib/ruby/1.9.0/matrix.rb:713:in `block in determinant'
	from /usr/lib/ruby/1.9.0/matrix.rb:710:in `upto'
	from /usr/lib/ruby/1.9.0/matrix.rb:710:in `determinant'
	from (irb):3
	from /usr/bin/irb1.9:12:in `<main>'

This occurs on the following versions:
* ruby 1.9.0 (2008-06-20 revision 17482) [i486-linux]
* ruby 1.8.7 (2008-08-11 patchlevel 72) [i486-linux]
And it's been confirmed on 1.9.1, too.

There are a host of issues with this class, some of which I'll report separately, but this can be fixed with a trivial change to the .square? method: "@rows.all?{|r| r.size == @rows.size}". With this patch:

>> Matrix[ [1,2],[1] ].determinant
=> 0
>> Matrix[ [1,2],[1] ].square?
=> false

This is consistent with the documentation which states that non-square matrices will have a determinant of 0.


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