For loop assignment error

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Hugo Hernández Hernández
回答: Rahul 2024-8-8
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

回答(1 个)

Rahul
Rahul 2024-8-8
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:
Hope this helps!

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