How to extract factors of variables in an equation?

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Robert Roos 2019-4-5

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https://ww2.mathworks.cn/matlabcentral/answers/454737-how-to-extract-factors-of-variables-in-an-equation

回答： Robert Roos 2019-4-24

采纳的回答： Robert Roos

I am using the symbolic toolbox to generate a set of three very big equations:

The result should be an equation of motion, so of this form:

Here q, f , τ and N are vectors of three elements and M and C are 3x3.

If I have a variable with the vector 'f', is there a way of find the matrices M and C?

I found the `collect` function, which orders an equation into polynomials with respect to a variable and this can be easily used to manually find those matrices, however, the rest of the equation remains, so it cannot be automated.

Example, say the function is

I want to extract the factors 10, 7 and -1 for the three states respectively as seperate variables.

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madhan ravi 2019-4-6

Have you seen coeffs()?

Robert Roos 2019-4-6

I did not. And it seems it's exactly what I was looking for! I'll come back with a working example.

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Answer 1

Robert Roos 2019-4-24

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https://ww2.mathworks.cn/matlabcentral/answers/454737-how-to-extract-factors-of-variables-in-an-equation#answer_372096

在 MATLAB Online 中打开

I ended up accomplishing this using coeffs: (Thanks to madhan ravi)

for i = 1:N
    for j = 1:N
        
        % M matrix
        
        cM = coeffs(eom(i), ddq(j), 'All');
        
        if length(cM) > 2
            error('Polynomial in mass-matrix count not equal to 2, equation is not linear to mass');
        end
        
        if length(cM) == 2
            M(i,j) = cM(1);
        else
            M(i,j) = 0;
        end
    end
end