Reshape 2D matrix to 3D using time as 3rd dimension

Hi,

I've got a A = n x n matrix and I want to convert this 2D matrix to a 3D one using time as the third dimension. So the 3D should show how the value of all the elements in the n x n matrix changes with respect to time.

I tried using the reshape function but to no avail.

Thanks!

Answer 1

[Thorsten]

If i is your related to your time and takes values 1,2,3, etc, and A is the n x n matrix at time point i, you can use:

B(:,:,i) = A;

[iijoclu]

Ok so my time function is basically

[kgrid.t_array, dt] = makeTime(kgrid, medium.sound_speed);

When I try to plug kgrid.t_array in the formula above, I get an error saying that the subscripts should be real positive integers or logical.

Size of my kgrid.t_array is [1 5243].

[Thorsten]

You cannot use the values of kgrid.t_array, but instead 1:numel(kgrid.t_array). Your code should look something like below, assuming that you have a function that computes your NxN matrix for every timestep:

 for i=1:numel(kgrid.t_array)

   t = kgrid.t_array(i); % get the ith time value
   A = computeNxNmatrix(t, more parameters); %
   B(:,:,i) = A;

end

Answer 2

[Matt J]

I tried using the reshape function but to no avail.

Be more persistent. RESHAPE is the appropriate thing to use.

[lmaree]

"Beyond the second dimension, the output, B, does not reflect trailing dimensions with a size of 1. For example, reshape(A,3,2,1,1) produces a 3-by-2 matrix." from the reshape documentation.

In my case, this is exactly what I need to multiply a spacial 2D matrix with a time vector. However, reshape doesn't have the functionality it seems...

[Matt J]

It is not needed. There is no difference between what you can do with a 3x2 matrix and with a 3x2x1x1 matrix.

A=rand(3,2);
t=rand(1,1,1,4);

A.*t

ans = 
ans(:,:,1,1) =

    0.2134    0.1069
    0.0927    0.5706
    0.2873    0.2506


ans(:,:,1,2) =

    0.1807    0.0905
    0.0785    0.4833
    0.2433    0.2123


ans(:,:,1,3) =

    0.3407    0.1707
    0.1481    0.9111
    0.4586    0.4002


ans(:,:,1,4) =

    0.0462    0.0232
    0.0201    0.1236
    0.0622    0.0543