Converting 3D matrix to a bigger 2D matrix efficiently

Hello,

Let's say A is a 10x10x4 matrix. A(1,1,:) = a,b,c,d

I want to convert A to B a 20x20 matrix so that: B (1:2,1:2) = [a b;c d];

Is there an efficient way to create the B matrix without using loops?

Cheers!

Edit1: I wrote A as a 10x10x4 matrix, although in reality the matrix's size is around 20 000 x 20 000 x 4.

Edit 2: I reformulated the question:

1. A is a huge M x N x 4 matrix

Example:

A = zeros(M,N,4);
A(1,1,:) = [1 1 2 2]; 
A(2,1,:) = [3 3 4 4];
A(1,2,:) = [5 5 6 6];
A(2,2,:) = [7 7 8 8];

2. Convert A to a regular 2*M x 2*N matrix so that:

Example:

%Desired output:
B = 

1  1  5  5  .  .
2  2  6  6  .  .
3  3  7  7  .  .
4  4  8  8  .  .
.  .  .  .
.  .  .  .

[Daniel M]

I don't see the transformation. What happens to elements A(2,1,:) ? What goes in B(1,3:end)?

[Nicolas B.]

Have you checked whether the function reshape() could apply to your case?

Answer 1

[Matt J]

[ma,na,~]=size(A);

B=permute(reshape(A,ma,na,2,2), [4,1,3,2]);

B=reshape(B,2*ma,2*na)

[Walter Roberson]

I have to admit that this is one of those times when a simple for loop can be much clearer.

[seb leaf]

Thank you very much! It's exactly what I wanted.

Answer 2

[Matt J]

Is there an efficient way to create the B matrix without using loops?

There is nothing inefficient about using loops in this case. Only 4 loop iterations are required.

B=nan(20);

[k,j]=ind2sub([2,2],1:4);

for i=1:4  
    B( j(i) + (0:2:end-1), k(i) + (0:2:end-1) )=A(:,:,i);
end

[James Tursa]

Also, since the values are being rearranged in memory, a deep data copy is going to be required no matter which method is used.