I have a matrix 'm' of oder 4 by6by960. I want to extract 1st and 3rd dimensions, i.e 4by960. how to do this?

m=rand(4,960);

m = [m; zeros(20,960)];

m=permute(reshape(m, 4,6,960), [1 2 3]); % need this 3d matrix for some operation

m1=squeeze(m(:,6,:));

when i use above line to get my requirement. it returns a matrix m1 of 4by960 with all zero enteries but i need to extract non-zero elements in 4by960 dimensions. kindly help to resolve the issue

Answer 1

[Stephan]

Hi,

all the non-zero elements are in column 1 - not in column 6:

m=rand(4,960);
m = [m; zeros(20,960)];
m=permute(reshape(m, 4,6,960), [1 2 3]);      % need this 3d matrix for some operation
m1=squeeze(m(:,1,:));

Sometimes for debugging it is helpful to have a look to the results that your code produces - See here for m after performing the permute and reshape:

val(:,:,1) =

    0.5108         0         0         0         0         0
    0.5300         0         0         0         0         0
    0.1545         0         0         0         0         0
    0.2341         0         0         0         0         0


val(:,:,2) =

    0.6191         0         0         0         0         0
    0.3090         0         0         0         0         0
    0.2135         0         0         0         0         0
    0.2826         0         0         0         0         0


val(:,:,3) =

    0.5438         0         0         0         0         0
    0.7191         0         0         0         0         0
    0.0393         0         0         0         0         0
    0.6480         0         0         0         0         0

You can continue this up to 960 - but the result keeps the same. So now you know the reason for your problem, but what do you want to achieve with permute? Note that:

>> A = [1 2 3; 4 5 6; 7 8 9]

A =

     1     2     3
     4     5     6
     7     8     9

>> B = permute(A, [1 2 3])

B =

     1     2     3
     4     5     6
     7     8     9

In other words - your permute does not do anything - if you leave the permute command away the resulting matrix m keeps the same:

m1 = permute(reshape(m, 4,6,960), [1 2 3]);
m2 = reshape(m, 4,6,960);
sum(sum(sum(m1 == m2))) == 4*6*960

ans =

  logical

   1

This seems to be the original problem you have - your permute call does not change your matrix the way that you expect it to do.

Best regards

Stephan

[Sajid Sarwar]

thanks. i got it