Save each iteration of a for loop to table

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Emma Stanton 2023-2-2

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https://ww2.mathworks.cn/matlabcentral/answers/1905061-save-each-iteration-of-a-for-loop-to-table

编辑： Stephen23 2023-2-7

I'm trying to calculate the roots of a cubic polynomial for several different values. This is the original polynomial, in case that helps:

I'm not confused about getting the coefficients, that isn't the issue. I'm trying to get the roots of alpha for several values of alpha_0, and I want to save them all in one table or matrix so that I can use the data to generate multiple plots.

for alpha_0 = [-1 -0.5 -0.1 0 0.1 0.5 1]

for alpha = zeros()

p = [1 2*alpha_0, alpha_0^2, -1/2];

r = roots(p);

alpha = [; r];

Roots = array2table(alpha)

end

This is the code I have currently, but it rewrites my output each time. Could anyone advise on how I can save all roots of alpha for all values of alpha_0 in the one matrix and/or table? Thanks so much!

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Answer 1

Stephen23 2023-2-2

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https://ww2.mathworks.cn/matlabcentral/answers/1905061-save-each-iteration-of-a-for-loop-to-table#answer_1161971

在 MATLAB Online 中打开

With MATLAB it is invariably easier to loop over indices, rather than looping over data values directly.

V =  [-1,-0.5,-0.1,0,0.1,0.5,1];
N = numel(V);
R = nan(3,N);
for k = 1:N % loop over indices
    p = [1,2*V(k),V(k)^2,-1/2];
    R(:,k) = roots(p);
end
display(R)
R = 
   1.5652 + 0.0000i   1.1573 + 0.0000i   0.8617 + 0.0000i  -0.3969 + 0.6874i  -0.4642 + 0.6862i  -0.7500 + 0.6614i  -1.1486 + 0.6028i
   0.2174 + 0.5217i  -0.0786 + 0.6526i  -0.3309 + 0.6861i  -0.3969 - 0.6874i  -0.4642 - 0.6862i  -0.7500 - 0.6614i  -1.1486 - 0.6028i
   0.2174 - 0.5217i  -0.0786 - 0.6526i  -0.3309 - 0.6861i   0.7937 + 0.0000i   0.7285 + 0.0000i   0.5000 + 0.0000i   0.2972 + 0.0000i

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Emma Stanton 2023-2-7

Thank you very much!

I'm now trying to separate them into real and imaginary parts. I've tried adding real = real(R) and imag = imag(R) at the end of the code there, but am getting error messages. Do you have any suggestions for this? Your help is much appreciated!

Stephen23 2023-2-7

编辑：Stephen23 2023-2-7

在 MATLAB Online 中打开

"I've tried adding real = real(R) and imag = imag(R) at the end of the code there, but am getting error messages"

Do not use variable names that are the same as the function names.

V =  [-1,-0.5,-0.1,0,0.1,0.5,1];
N = numel(V);
R = nan(3,N);
for k = 1:N % loop over indices
    p = [1,2*V(k),V(k)^2,-1/2];
    R(:,k) = roots(p);
end
R_real = real(R)
R_real = 3×7
    1.5652    1.1573    0.8617   -0.3969   -0.4642   -0.7500   -1.1486
    0.2174   -0.0786   -0.3309   -0.3969   -0.4642   -0.7500   -1.1486
    0.2174   -0.0786   -0.3309    0.7937    0.7285    0.5000    0.2972
R_imag = imag(R)
R_imag = 3×7
         0         0         0    0.6874    0.6862    0.6614    0.6028
    0.5217    0.6526    0.6861   -0.6874   -0.6862   -0.6614   -0.6028
   -0.5217   -0.6526   -0.6861         0         0         0         0