Hey guys, Hello Martyr2!

I am trying to put the finishing touches on my Matrix project, but my calculation for multiplying matrices appears to be wrong.

Here is my code first of all, then I will explain:



When multiplying Matrices, we take the sum of the multiplication of the first row's cells and the first column's cells.

An example of this would be:

[1 2] [5 6] = [19 22]
[3 4] [7 8] = [43 50]

To find the first # of matrix 3, we do:
(1*5) + (2*7) = 19
Next value (to the right of 19) would be:
(1*6) + (2*8) = 22
Completing the Result Matrix:
(3*5) + (4*7) = 43
(3*6) + (4*8) = 50

I hope I'm doing this correctly.

Can you all please help me derive the correct formula for computing this?

It involves a bunch of nasty for loops, and I THOUGHT I had it but I don't appear to be correct.

Thank you so much!

Here is my code for working matrix multiplication.


Now I have to divide Matrices. How do I do this?

This post has been edited by Sonastylol: 15 Oct, 2008 - 11:07 AM

Bump.

I need to know the formula for dividing matrices.

Thanks!

Matrix division in general is not so simple but if you're only about to define matrix division for a 2x2 matrix there are ways to simplify the problem.

If you have Matrices A and B and wish to calculate Matrix C = A/B = A x Inverse of B (which is also a 2x2 matrix).
You already have a way to solve AxB so all you need is a way to calculate the inverse of B.

Which is done as follows:
Let B be the matrix:

[a b]
[c d]

Calculate e = 1 / ((a * d) - (b * c))

(ad-bc) is the determinant of the matrix and has to be nonzero if the matrix is to have an inverse (it's a good idea to test this in the beginning of your division function).

multiply e by the matrix:

[d -b]
[-c a]

ex:
Let a = 2, b = 1, c = 4, d = 3 gives the following matrix B

[2 1]
[4 3]

ad-bc = 6 - 4 = 2 => e = 1/2

Gives the inverse of B:

[1.5 -0.5]
[-2.0 1.0]

The rest is just to perform matrix Multiplication with matrix A and then you have you result matrix C.

I hope this wasn't too complicated.

This post has been edited by Gloin: 16 Oct, 2008 - 11:43 AM

oh wow, seems pretty complicated.

well my matrices are 3x3, and what about if they were even larger than this?

Seems awful.

I havent been able to find any source code of the matter. darn

Are you really supposed to perform division of one matrix by another or are you only supposed to do division by some constant like A/k??

If you have to do Matrix division of 3x3 matrices then you'll have to do Gauss-elimination (to find the inverse). It's a bit more difficult but definitely doable. Althogh if you haven't seen it before, I guess you're in trouble..

This post has been edited by Gloin: 16 Oct, 2008 - 05:17 PM