# Matrix class for ruby? -example

#1

On Thu, 21 Dec 2006 09:12:47 +0200, seepee wrote:

Does anyone know of a functional Matrix math class for ruby that could
multiply, add and substract 3x2 or larger matrices.

The matrix.rb that is in Ruby API cannot multiply anything larger than 2x2
matrices, which makes it useless for 2D or 3D graphics (for instance).

SP

Problem:
go to terminal, and type “irb” or “irb1.8”

require ‘matrix’ #or require >rubylibrarypath/matrix.rb
p Matrix[ [25, 93, 33], [0, -1, 66] ] * Matrix[ [1, 53, 33], [1, -21, 77]

ExceptionForMatrix::ErrDimensionMismatch: Matrix dimension mismatch
from /usr/lib/ruby/1.8/matrix.rb:466:in `*’ from (irb):4

??

#2

require ‘matrix’ #or require >rubylibrarypath/matrix.rb
p Matrix[ [25, 93, 33], [0, -1, 66] ] * Matrix[ [1, 53, 33], [1, -21, 77]

ExceptionForMatrix::ErrDimensionMismatch: Matrix dimension mismatch
from /usr/lib/ruby/1.8/matrix.rb:466:in `*’ from (irb):4

??

How exactly would you multiply such two matrices?

Jakub

#3

seepee wrote:

Problem:
go to terminal, and type “irb” or “irb1.8”

require ‘matrix’ #or require >rubylibrarypath/matrix.rb
p Matrix[ [25, 93, 33], [0, -1, 66] ] * Matrix[ [1, 53, 33], [1, -21, 77]

Err… Have you done matrix computations for a long time ? You need
that the number of columns from the left matrix matches the number of
lines from the right matrix. If you try to multiply two 3x2 matrices,
it is rather encouraging that Matrix refuses it…

Try transposing your matrix first, if that is what you need. Are you
dealing with transformation matrices (which are not real matrices) ?

``Vincent``

#4

seepee wrote:

``````    from /usr/lib/ruby/1.8/matrix.rb:466:in `*' from (irb):4
``````

??

Yes, it’s because you cannot multiply a 2x3 matrix with another 2x3
matrix… try:

Matrix[ [25, 93, 33], [0, -1, 66] ] * Matrix[ [1, 53, 33], [1, -21,
77]].transpose

Julien

#5

Hi,

Just a small correction… it is the number of lines from the left
matrix
that must
match the number of columns from the right matrix.

Kind Regards

J. Augusto

#6

On Thu, 21 Dec 2006 09:12:47 +0200, seepee wrote:

Does anyone know of a functional Matrix math class for ruby that could
multiply, add and substract 3x2 or larger matrices.

The matrix.rb that is in Ruby API cannot multiply anything larger than 2x2
matrices, which makes it useless for 2D or 3D graphics (for instance).

SP

others in this thread have corrected the logical problem.

but let me point out a typical additional response - the included
Matrix library in ruby is NOT FAST. There are other libs that do this
on the C level, such as NArray, that will allow much better
performance for some graphical uses.

Cameron