forming a matrix with given entries

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MADHVI 2023-11-4

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https://ww2.mathworks.cn/matlabcentral/answers/2042847-forming-a-matrix-with-given-entries

评论： MADHVI 2023-12-20

correct the use of if condition in the following code:

function [A,B] = fdm1(H,xi,a,b,N)

H = 1; xi = 0.1;

a= -1; b = 1; N=5;

h = (b-a)/N; h2 = h*h;

A = zeros(20*(N-1),20*(N-1));

B = zeros(20*(N-1),20*(N-1));

K = 10;

for k = 1:K;

A(2*(k-1)*(N-1)+1,2*(k-1)*(N-1)+1:2*(k-1)*(N-1)+2) = [-(2+(k*pi*h/H)^2) 1];

A(2*(k-1)*(N-1)+N,2*(k-1)*(N-1)+N:2*(k-1)*(N-1)+N+1) = [-(2+(k*pi*h/H)^2) 1];

A(2*(k-1)*(N-1)+N,2*(k-1)*(N-1)+2) = [h/2]

for n = 1:K;

if (n~=k) && any(n+k == 1:2:K) %ismember(n+k,1:2:K)

x = sum((2 * H * h * n * k * ((-1)^(n + k) - 1)) / ((n^2 - k^2)^2 * pi^2 * xi));

A(2*(k-1)*(N-1)+N,2*(n-1)*(N-1)+2) = [x]; end

B(2*(k-1)*(N-1)+1,2*(k-1)*(N-1)+2) = [h/2];

end

Thanks in advance.

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Dyuman Joshi 2023-11-4

编辑：Dyuman Joshi 2023-11-4

"correct the use of if condition in the following code:"

How would we know what is the correct use?

You will need to specify more than just simply showing the code.

What is the objective here? What is supposed to be the "correct" use?

What is the expected output?

MADHVI 2023-11-4

_doc.pdf

Thank you for the response.

The objective is attached in the file.

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Answer 1

prabhat kumar sharma 2023-12-20

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在 MATLAB Online 中打开

Hi Madhvi,

I assume that you are trying to construct matrices `A` and `B` for a finite difference method (FDM) and are having trouble with the summation part of the second equation within the `if` condition. The code provided seems to have a few issues that need to be corrected, including the placement of the `if` block and the summation logic.

1. The `if` block should be properly nested within the `for` loops.

2. The `sum` function is not being used correctly. If you're trying to compute a sum based on the condition, you should accumulate the sum manually within the loop.

3.It's not clear what the indices for `A` and `B` should be in the summation part, but I'll assume you're trying to update the `(k,n)` element based on the condition.

function [A,B] = fdm1(H,xi,a,b,N) 
% H = 1; xi = 0.1;  
% a= -1; b = 1;  N=5; 
h = (b-a)/N;  h2 = h*h; 
A = zeros(20*(N-1),20*(N-1)); 
B = zeros(20*(N-1),20*(N-1)); 
K = 10;  
 
for k = 1:K 
    A(2*(k-1)*(N-1)+1,2*(k-1)*(N-1)+1:2*(k-1)*(N-1)+2) = [-(2+(k*pi*h/H)^2) 1]; 
    A(2*(k-1)*(N-1)+N,2*(k-1)*(N-1)+N:2*(k-1)*(N-1)+N+1) = [-(2+(k*pi*h/H)^2) 1]; 
    A(2*(k-1)*(N-1)+N,2*(k-1)*(N-1)+2) = h/2;  % Removed brackets for scalar assignment 
    B(2*(k-1)*(N-1)+1,2*(k-1)*(N-1)+2) = h/2;  % Moved outside of the inner loop 
 
    for n = 1:K 
        if (n ~= k) && any(n+k == 1:2:K) % ismember(n+k,1:2:K) 
            x = (2 * H * h * n * k * ((-1)^(n + k) - 1)) / ((n^2 - k^2)^2 * pi^2 * xi);  
            A(2*(k-1)*(N-1)+N,2*(n-1)*(N-1)+2) = x; % Corrected the assignment 
        end 
    end 
end 
end 