Converting a 2d array into a 3d array

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Vinay Killamsetty
Vinay Killamsetty el 28 de Nov. de 2021
Respondida: Turlough Hughes el 28 de Nov. de 2021
Hi,
I am having a 2d array with size mxn
I want to convert each column into a seperate 2d diagonal matrix--- a total of 'n' diagonal matrices of size mxm
Then combining all the 2d diagonal matrices as a single 3d matrix.---size mxmxn
I tried to do this with employing a for-loop but it is taking lot of time
Can this be done without using a loop and at a faster speed?
  2 comentarios
Turlough Hughes
Turlough Hughes el 28 de Nov. de 2021
Can you give an example of the code you have implemented?
Vinay Killamsetty
Vinay Killamsetty el 28 de Nov. de 2021
Editada: Vinay Killamsetty el 28 de Nov. de 2021
size of Z_int is mxn
Z = nan(m,m,n)
for i = 1 : size( Z_int , 2)
Z( : , : , i ) = diag( (Z_int( :, i ) ) ;
end

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Respuestas (2)

Chunru
Chunru el 28 de Nov. de 2021
"it is taking lot of time" for what size of array?
m=200; n=100;
Z_int = rand(m, n);
Z = nan(m,m,n);
tic
for i = 1 : size( Z_int , 2)
Z( : , : , i ) = diag( Z_int( :, i ) );
end
toc
Elapsed time is 0.009776 seconds.

Turlough Hughes
Turlough Hughes el 28 de Nov. de 2021
You could write a linear index for the diagonal elements as follows:
[m,n] = size(Z_int)
Z = zeros(m,m,n);
idx = repmat((1:m+1:m^2).',1,n) + m^2*(0:n-1);
Z(idx(:)) = Z_int(:)
This is 2-4x faster for m >= 400 on my computer (n=500), however, on the matlab servers its a slightly slower. Whether it's faster or not for you will depend on your hardware, the size of your arrays and probably the version of matlab that you're using too.

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