MATLAB Central Newsreader - -extract 120X9 matrix from 1000X9 matrix randomly

-extract 120X9 matrix from 1000X9 matrix randomly

muk — Tue, 09 Feb 2010 16:04:04 -0500

Hello all,

Now, i have 1000X9 matrix which is quality parameters. And, i want to extract 120X9 matris randomly from 1000x9 matrix.
How can i do that?

Sincerely,

Re: -extract 120X9 matrix from 1000X9 matrix randomly

Oleg Komarov — Tue, 09 Feb 2010 16:35:19 -0500

"muk " wrote in message
> Hello all,
>
> Now, i have 1000X9 matrix which is quality parameters. And, i want to extract 120X9 matris randomly from 1000x9 matrix.
> How can i do that?
>
> Sincerely,

help rand

Example:
A = rand(1000,9);

rows = ceil(120*rand(120,1));
cols = ceil(9*rand(9,1));

randMat = A(rows,cols);

Oleg

Re: -extract 120X9 matrix from 1000X9 matrix randomly

Walter Roberson — Tue, 09 Feb 2010 16:35:38 -0500

muk wrote:

> Now, i have 1000X9 matrix which is quality parameters. And, i want to
> extract 120X9 matris randomly from 1000x9 matrix.
> How can i do that?

randperm(1000) and take the first 120 output values.

Re: -extract 120X9 matrix from 1000X9 matrix randomly

Cygnine — Tue, 09 Feb 2010 16:38:04 -0500

"muk " <hungocan@hotmail.com> wrote in message
> Now, i have 1000X9 matrix which is quality parameters. And, i want to extract 120X9 matris randomly from 1000x9 matrix.

Do you mean:
a.) You want to randomly choose a 120x9 BLOCK from the bigger matrix, or
b.) You want to randomly choose 120*9 = 1080 elements from the big matrix?

For a:

>> big_matrix = ones([1000 9]); % or whatever data
>> starting_row = ceil(881*rand);
>> ending_row = starting_row + 119;
>> small_matrix = big_matrix(starting_row:ending_row,:);

For b:

>> big_matrix = ones([1000 9]);
>> small_matrix = reshape(big_matrix(ceil(1000*rand([1080 1]))), [120 9]);

You should of course worry about seeding the random number generator.

Re: -extract 120X9 matrix from 1000X9 matrix randomly

Cygnine — Tue, 09 Feb 2010 16:45:05 -0500

"Cygnine " <cygnine@remove.this.gmail.com> wrote in message
> Lots of stuff

Sorry, I realized I made a mistake, part b should be

>> small_matrix = reshape(big_matrix(ceil(9000*rand([1080 1]))), [120 9]);

And the other answers provided seem as good, if not simpler.

Re: -extract 120X9 matrix from 1000X9 matrix randomly

ade77 — Tue, 09 Feb 2010 16:45:05 -0500

Re: -extract 120X9 matrix from 1000X9 matrix randomly

us — Tue, 09 Feb 2010 17:23:04 -0500

"muk " <hungocan@hotmail.com> wrote in message <hks11k$ti$1@fred.mathworks.com>...
> Hello all,
>
> Now, i have 1000X9 matrix which is quality parameters. And, i want to extract 120X9 matris randomly from 1000x9 matrix.
> How can i do that?
>
> Sincerely,

as others have said: it is not quite clear what you mean by a randomly selected submatrix...
assuming you want to carve out a randomly selected sub-block from your main mat,...

one of the solutions

% the data
     m=ceil(10*rand(5,4)); % <- main mat
     sr=2; % <- #rows of submat
     sc=3; % <- #cols of submat
% the engine
     [nr,nc]=size(m);
     [x,y]=meshgrid(1:nr:sc*nr,0:sr-1);
     seg=x+y; % <- SEG: indices into block at top/left
     off=nr*(nc-sc+1)-sr; % <- OFF: max movement of SEG
     roff=ceil(off*rand); % <- select random OFF
% the result
     disp(m);
     disp(m(seg+0)); % <- top/left submat
     disp(m(seg+off)); % <- last possible submat
     disp(roff); % <- random SEG
     disp(m(seg+roff)); % <- randomly selected submat
%{
% the mat
     2 2 6 6
     7 1 9 2
     6 5 1 6
     1 9 10 1
     1 7 2 8
% top/left
     2 2 6
     7 1 9
% last possible block given SR/SC
     9 10 1
     7 2 8
% random OFF
     7
% submat at random OFF
     5 1 6
     9 10 1
%}

us

Re: -extract 120X9 matrix from 1000X9 matrix randomly

us — Tue, 09 Feb 2010 17:32:04 -0500

"us "
...slight modification to the random OFF generator, which must be ]0:OFF[

% the data
     m=ceil(10*rand(5,4)); % <- main mat
     sr=2; % <- #rows of submat
     sc=3; % <- #cols of submat
% the engine
     [nr,nc]=size(m);
     [x,y]=meshgrid(1:nr:sc*nr,0:sr-1);
     seg=x+y; % <- SEG: indices into block at top/left
     off=nr*(nc-sc+1)-sr; % <- OFF: max movement of SEG
     roff=ceil((off+1)*rand)-1; % <- select random OFF
% the result
     disp(m);
     disp(m(seg+0)); % <- top/left submat
     disp(m(seg+off)); % <- last possible submat
     disp(roff); % <- random SEG
     disp(m(seg+roff)); % <- randomly selected submat
%{
% the mat
     2 2 6 6
     7 1 9 2
     6 5 1 6
     1 9 10 1
     1 7 2 8
% top/left: OFF = 0
     2 2 6
     7 1 9
% last possible block given SR/SC
     9 10 1
     7 2 8
% random OFF ]0:OFF[
     7
% submat at random OFF
     5 1 6
     9 10 1
%}

us

Re: -extract 120X9 matrix from 1000X9 matrix randomly

muk — Tue, 09 Feb 2010 19:05:21 -0500

"ade77 " <ade100a@gmail.com> wrote in message <hks3eh$46u$1@fred.mathworks.com>...
> "muk " <hungocan@hotmail.com> wrote in message <hks11k$ti$1@fred.mathworks.com>...
> > Hello all,
> >
> > Now, i have 1000X9 matrix which is quality parameters. And, i want to extract 120X9 matris randomly from 1000x9 matrix.
> > How can i do that?
> >
> > Sincerely,
>
> Like Walter suggested, randperm seems to be your easiest way.
>
> your_matrix = 1000 by 9
> h = randperm(1000);
> g = h(1:120);
> rand_array = your_matrix(g,:);

Thank you Everyone!

That is the best choice and fast choice. There are many view to solve this problem.

> your_matrix = 1000 by 9
> h = randperm(1000);
> g = h(1:120);
> rand_array = your_matrix(g,:);

That is choice which is independent in data. May be, distributions of data may be important. But, there is not a problem. So, as i said ,this is the best choice, and clever choice.

Thank you all thank you "ade77" my friend.