MATLAB Central Newsreader - divide and conquer matrix

divide and conquer matrix

Diego Lass — Thu, 18 Jun 2009 20:46:02 -0400

Hi
I have a problem with a large matrix, what I want to do is to divide the matrix into 4 submatrices, do some operation then reassemble the submatrix together. A toy example

A = [1 2 3 ; 3 4 5 ; 2 3 4; 5 1 2 ]
A =

     1 2 3
     3 4 5
     2 3 4
     5 1 2
I want to divide A into ANY approximately equal sized partitions, for example
1 2
3 4

3
5

2 3
5 1

4
2

then do some operation, say add 2 to each element. Then reassemble them together, to get

     3 4 5
     5 6 7
     4 5 6
     7 3 4
What is the most efficient way of doing this?
Thanks
Diego

Re: divide and conquer matrix

Nicolaie Popescu-Bodorin — Fri, 19 Jun 2009 00:59:02 -0400

-use powers of two as much as possible;
-see blkproc (caution with implicit padding);
-if the matrix is huge, consider mex using;
-column is the "fast dimension"; this could be helpful;

Bodorin

"Diego Lass" <dlISCool@gmail.com> wrote in message <h1e92a$bd7$1@fred.mathworks.com>...
> Hi
> I have a problem with a large matrix, what I want to do is to divide the matrix into 4 submatrices, do some operation then reassemble the submatrix together. A toy example
>
> A = [1 2 3 ; 3 4 5 ; 2 3 4; 5 1 2 ]
> A =
>
> 1 2 3
> 3 4 5
> 2 3 4
> 5 1 2
> I want to divide A into ANY approximately equal sized partitions, for example
> 1 2
> 3 4
>
> 3
> 5
>
> 2 3
> 5 1
>
> 4
> 2
>
> then do some operation, say add 2 to each element. Then reassemble them together, to get
>
> 3 4 5
> 5 6 7
> 4 5 6
> 7 3 4
> What is the most efficient way of doing this?
> Thanks
> Diego

Re: divide and conquer matrix

Jos — Fri, 19 Jun 2009 09:40:18 -0400

"Diego Lass" <dlISCool@gmail.com> wrote in message <h1e92a$bd7$1@fred.mathworks.com>...
> Hi
> I have a problem with a large matrix, what I want to do is to divide the matrix into 4 submatrices, do some operation then reassemble the submatrix together. A toy example
>
> A = [1 2 3 ; 3 4 5 ; 2 3 4; 5 1 2 ]
> A =
>
> 1 2 3
> 3 4 5
> 2 3 4
> 5 1 2
> I want to divide A into ANY approximately equal sized partitions, for example
> 1 2
> 3 4
>
> 3
> 5
>
> 2 3
> 5 1
>
> 4
> 2
>
> then do some operation, say add 2 to each element. Then reassemble them together, to get
>
> 3 4 5
> 5 6 7
> 4 5 6
> 7 3 4
> What is the most efficient way of doing this?
> Thanks
> Diego

The definition of "efficient" is open to many interpretations ... Here is an example that is not particularly fast, but quite flexible:

% your data
  A = [1 2 3 ; 3 4 5 ; 2 3 4; 5 1 2 ]
  myfun = @(X) 1110 * X(1) + X ;

% engine
  C = mat2cell(A,[2 2],[2 1]) ;
  C = cellfun(myfun, C,'uniformoutput',false) ;
  B = cell2mat(C)

hth
Jos