Unable to perform assignment because the size of the left side is 8-by-1 and the size of the right side is 8-by-1001.

I am getting this error where I am sure the size of the left-side = the right side in this code, what else can be causing this problem?

Unable to perform assignment because the size of the left side is 8-by-1 and the size of the right side is 8-by-1001.

Error in RK4 
    U_RK(:,j) = U_T(:,j)-(dt./dx)*a(k)*(Res);

function [ U_RK ] = RK4( U_T,Res, dx, dt, N,k )

U_RK = U_T;
a = zeros(4,1);
a(1) = 0.25;
a(2) = 1.0/3.0;
a(3) = 0.5;
a(4) = 1.0;
        
for j = 2:N-2
    U_RK(:,j) = U_T(:,j)-(dt./dx)*a(k)*(Res); % error here 
end
end

But U_Rk(:,j) is initailly set to = U_T, and is initizalize this way:

L = 1; %domain length 
x = 0:dx:L;
N = length(x);
U_T= zeros(8,N);

So how is the LHS does not equal the RHS??

Answer 1

[KSSV]

The error is clear, you are trying to save more number of elements in the initilaized matrix.

Example:

A = zeros(3,3) ;   % initialize A to 3x3 zeros matrix, to save data later 
A(:,1) = rand(1,3) ;   % no error, because column should have three elements and we have three here 
A(:,2) = rand(1,5) ;   % error, because column should have three elements, but you are saving five elements, This is not allowed 

So, similiarly in your case, U_RK is a array with 8 rows, where you should save 8*1 array; but the RHS is giving you an array of size 8*1001.

Check the dimensions of your code and then initialize the matrix properly.

[Jamie Al]

But, everything is initialized for 8xN or 8x1001 and now if I change Res to be

    U_RK(:,j) = U_T(:,j)-(dt./dx)*a(k)*(Res(:,j)); % error here 
    

Now, I am getting the error:

    Index in position 2 exceeds array bounds (must not exceed 1)

I truly don't understand, since I can use the break points in my code and see the sizes of these arrays to be 8x1001 correctly.

[KSSV]

What is dimension of U_T, Res?

We cannot help unless the complete data and code is shown.

[Jamie Al]

Oh sorry, I see the problem now!!