MATLAB: How to get two matrices with different element values from a mother matrix

matrix

I have this matrix:

A=[2 0 1; 1 2 1; 2 2 1]; %main matrix

I want a code that can extract two matrices, one with 1's elements and anything else is zero and the second matrix with 2's elements and anything else is zero. So, I want to get them as following:

 a1=[0 0 1; 1 0 1; 0 0 1] % sub matrix with 1's elements only
 a2=[2 0 0; 0 2 0; 2 2 0] % sub matrix with 2's elements only

Can anyone helps?

Best Answer

a1 = zeros(size(A));
a1(A == 1) = A(A == 1);

Do the same for a2 obviously replacing the A == 1 by A == 2.

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MATLAB: Problem with Matrix indexing using vector

This is a job for sub2ind:

a        = blkdiag(ones(3),ones(3));
[r,c]    = find(a);
b        = zeros(size(a));
index    = sub2ind(size(a), r, c);
b(index) = 1;
isequal(a, b)  % TRUE

Maybe using logical indexing is more efficient than find():

mask    = (a == 1);
b       = zeros(size(a));
b(mask) = 1;

MATLAB: How to replace the following code, to get the matrix

You really just need to make a sparse diagonal matrix. This can very easily be done with spdiags. Use the last syntax: A = spdiags(B,d,m,n), which you would be calling as: A = spdiags(B,d,n^2,n^2);. The issue now is only in setting up B and d which really just requires you to replace a few values in each of the columns of B, but this can be done by finding the appropriate indices to replace (note that not all values in B will be used, so start your counting from the bottom as this function will omit unnecessary values at the top of the matrix B). Put your head to the grindstone and I'm sure you can find the patterns necessary to do this for arbitrary values of n, without loops or cell arrays. I will do it for just for the lowest diagonal:

n = 5; % Change it to 4 and verify the code works in both cases.
B = 0.298*ones(n^2,1); % Use the calculation of wherever 0.298 came from if you wish
d = -(n+1); % The diagonal where the 1st vector of B should appear
B(end-n:-n:1) = 0;
A = spdiags(B,d,n^2,n^2); % This has the lower diagonal
Q = full(A); % For visualizing the correct results.
open Q

Now, just add the columns for the other diagonals to B and add the diagonal identifier to d. Also, never use the line of code:

if true
   %code
end

true is always true so the code always runs.

Best Answer

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MATLAB: Problem with Matrix indexing using vector

MATLAB: How to replace the following code, to get the matrix

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