Replacing function variables with a vector before taking a gradient

Question

Valeri Aronov 2021-7-30

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/889117-replacing-function-variables-with-a-vector-before-taking-a-gradient

回答： Walter Roberson 2021-7-31

采纳的回答： Walter Roberson

在 MATLAB Online 中打开

I have a symbolic function:

A = 1/((C1*C2*R1*R2*w^2 - 1)^2 + w^2*(C2*R1 + C2*R2)^2)^(1/2)

I would like to replace C1, C2, R1, R2 symbolic variables with an x vector where C1 = C1_0*x(1); C2 = C2_0*x(2); R1 = R1_0*x(3); R2 = R2_0*x(4); and take a gradient of A by x. Whatever I tried failed to produce the result I am seeking.

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Valeri Aronov 2021-7-30

I do it symbolically, if I got your question right.

Valeri Aronov 2021-7-31

编辑：Valeri Aronov 2021-7-31

在 MATLAB Online 中打开

I have found a path (I did not know about str2sym() existence):

        syms x1 x2 x3 x4 real
        x=[x1,x2,x3,x4]
        A = subs(A, C1, str2sym('x1*C1_0'))
        A = subs(A, C2, str2sym('x2*C2_0'))
...
        gradient(A, x)

It may do for me even ideally I would prefer the notation in my question text for x.

Now, can some one advise if I need to remove this question entirely.

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Answer 1

Walter Roberson 2021-7-31

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/889117-replacing-function-variables-with-a-vector-before-taking-a-gradient#answer_757722

在 MATLAB Online 中打开

        syms x1 x2 x3 x4 real
        syms C1_0 C2_0
        x=[x1,x2,x3,x4]
        A = subs(A, [C1, C2, R1, R2],  [C1_0, C2_0, R1_0, R2_0] .*x)
        gradient(A, x)