Variables in matrix not updating from for loop

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Nolan Ames 2021-6-22

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https://ww2.mathworks.cn/matlabcentral/answers/862765-variables-in-matrix-not-updating-from-for-loop

评论： Nolan Ames 2021-6-22

I am currently using a for loop in order to find the maximum eigenvalues of a matrix. The loop is as follows

for k = 0:c
    e = eig(z)
    M = max(e)
    x = [x,M]
    k = k+1
end

The variable k begins at 0, and the loop runs until c, a count of 50. z is the following 4x4 matrix: [k 0 2 0; 0 k 0 2; 3 4 5 0; 4 3 0 5]. What I am attempting to do is extract the maximum eigenvalue from the matrix, add it to an array x , then add 1 to k within the matrix, and continue until c is reached. However, k within the matrix is not updating, so it is simply finding the same eigenvalue 50 times. The k working the counter is performing as expected, but what do I have to do in order to make the k within the matrix update at each step as well?

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Scott MacKenzie 2021-6-22

编辑：Scott MacKenzie 2021-6-22

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It would help if you provided all the code so it can be excuted to produce the error.

But, note that this line

k = k+1;

has no bearing on "k within the matrix".

Furthermore, k is the loop index variable in your for-loop, so it cannot be changed within the loop. So, there are a few issues to resolve.

Nolan Ames 2021-6-22

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That is all the code. If you're reffering to the variables, they are as follows. My assumption based on your response is I should likely change the loop index variable to something besides the matrix k to avoid future confusion.

c = 50
z = [k 0 2 0; 0 k 0 2; 3 4 5 0; 4 3 0 5]

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Answer 1

Scott MacKenzie 2021-6-22

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/862765-variables-in-matrix-not-updating-from-for-loop#answer_730840

编辑：Scott MacKenzie 2021-6-22

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I think this is what you are looking for. It updates k in the z matrix in each iteration of the loop. The max values accumulate in the vector x.

c = 50;
k = 0;
z = [k 0 2 0; 0 k 0 2; 3 4 5 0; 4 3 0 5];
x = [];
for k = 0:c
    e = eig(z);
    M = max(e);
    x = [x,M];
    % update k in z
    z(1,1)=k;
    z(2,2)=k;
end
x % output max values