MATLAB Central Newsreader - matrix manipulation

matrix manipulation

Ross — Fri, 28 Nov 2008 21:02:47 -0500

Hi there,

I have a question regarding matrix manipulation. I am using a 'brute force' method, but I guess there might be a straightforward way to do this that I am not aware of.

Assume you have array A, s.t. [N1 N2 N3] = size(A).
Also, assume you have a vector p, s.t. N1 = length(p), whose entries are integers between 1 and N3.

You want to obtain a new matrix B, N1-by-N2 where the rows are selected using p.

Example:
A = [1 2;3 4];
A(:,:,2) = [10 20;30 40]
p = [1 2];

B = [1 2; 30 40];

Thanks!
Ross

Re: matrix manipulation

Bruno Luong — Fri, 28 Nov 2008 21:30:20 -0500

Ross <fed.rossi@gmail.com> wrote in message <26885583.1227906197694.JavaMail.jakarta@nitrogen.mathforum.org>...
> Hi there,
>
> I have a question regarding matrix manipulation. I am using a 'brute force' method, but I guess there might be a straightforward way to do this that I am not aware of.
>
> Assume you have array A, s.t. [N1 N2 N3] = size(A).
> Also, assume you have a vector p, s.t. N1 = length(p), whose entries are integers between 1 and N3.
>
> You want to obtain a new matrix B, N1-by-N2 where the rows are selected using p.
>
> Example:
> A = [1 2;3 4];
> A(:,:,2) = [10 20;30 40]
> p = [1 2];
>
> B = [1 2; 30 40];
>
>

% Data
A = [1 2;
     3 4;
     5 6];
A(:,:,2) = [10 20;
            30 40;
            50 60]

[N1 N2 N3]=size(A);

p = [1 2 2];

% Engine 1
[I J]=ndgrid(1:N1,1:N2);
K=repmat(p(:),1,N2);
B = A(sub2ind(size(A),I,J,K))

% Engine 2
Ap=permute(A,[2 1 3]);
B=Ap(:,sub2ind([N1 N3],1:N1,p)).'

% Bruno

Re: matrix manipulation

Walter Roberson — Fri, 28 Nov 2008 21:42:11 -0500

Ross wrote:
> Assume you have array A, s.t. [N1 N2 N3] = size(A).
> Also, assume you have a vector p, s.t. N1 = length(p), whose entries are integers between 1 and N3.
>
> You want to obtain a new matrix B, N1-by-N2 where the rows are selected using p.
>
> Example:
> A = [1 2;3 4];
> A(:,:,2) = [10 20;30 40]
> p = [1 2];
>
> B = [1 2; 30 40];
>

That's pretty much the same task as was explored by a previous poster; you should be
able to adapt the solution I gave in

http://groups.google.ca/group/comp.soft-sys.matlab/browse_thread/thread/2316f0c1c23f97b4

--
.signature note: I am now avoiding replying to unclear or ambiguous postings.
Please review questions before posting them. Be specific. Use examples of what you mean,
of what you don't mean. Specify boundary conditions, and data classes and value
relationships -- what if we scrambled your data or used -Inf, NaN, or complex(rand,rand)?

Re: matrix manipulation

Ross — Fri, 28 Nov 2008 22:22:34 -0500

Thanks!

Re: matrix manipulation

Ross — Fri, 28 Nov 2008 22:28:59 -0500

ok, thanks