For loop assignment error

Hi all,

I have crated a set of equations assigned to a letter and contains a variable that goes until 14002, to fill a matrix with this computations I am trying the following:

Axz = zeros(14002,4);

for i=1:14002
    
    Axz(:,i) = [dxx,dxx_,dxz,dxz_;dzx,dzx_,dzz,dzz_];
    
end

The problem comes with the followinf Error:

Unable to perform assignment because the size of the left side is 14002-by-1 and the size of the right side is 14002-by-4.

Error in Test (line 51)
    Axz(:,i) = [dxx,dxx_,dxz,dxz_;dzx,dzx_,dzz,dzz_];

How can I set the left side to be 14002 by 4?

Thanks for you help!

Hugo

Answer 1

[Rahul]

Hi Hugo,

The issue seems be to be due to a mismatch in the dimensions of the following, while assigning values to the 2D matrix Axz inside the loop:

[dxx,dxx_,dxz,dxz_;dzx,dzx_,dzz,dzz_] % Size (2, 4)
Axz(i, :) % Size (1, 4)

I've solved it using two methods as decribed below:

Method 1: Pre-allocate additional rows

Size of the matrix Axz can be increased during pre-allocation, while keeping it as 2-dimensional, to accommodate twice as many rows as follows:

% Pre-Allocation of Axz
Axz = zeros(2 * 14002, 4);

for i = 1:2:2*14002
    Axz([i i + 1],:) = [dxx, dxx_, dxz, dxz_;dzx, dzx_, dzz, dzz_];
end

This will assign every 2 consecutive rows of Axz, with the desired value, at each iteration.

Method 2: Convert to a 3D Matrix

Using zeros function, pre-allocation of Axz can be done to include an extra dimension as follows:

% Pre-Allocation of Axz
Axz = zeros(14002, 4, 2);

for i = 1:14002
    Axz(i, :, 1) = [dxx, dxx_, dxz, dxz_];
    Axz(i, :, 2) = [dzx, dzx_, dzz, dzz_];
end

For each row i, matrix Axz now contains two 4x1 vectors Axz(i, :, 1) and Axz(i, :, 2) which contain the desired value.

For further information regarding pre-allocation of matrices, you can refer to the documentation of zeros function:

https://www.mathworks.com/help/matlab/ref/zeros.html#d126e1907886

Hope this helps!