Undefined function or variable 'dx'.

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Mustafa Abdelfattah 2024-4-11

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https://ww2.mathworks.cn/matlabcentral/answers/2105861-undefined-function-or-variable-dx

回答： Aniket 2024-8-7，14:25

jacobi = [dx(f1,x1,y1) dy(f1,x1,y1);

dx(f2,x1,y1) dy(f2,x1,y1)]; %jacobian matrix that obtains partial dervatives

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John D'Errico 2024-4-11

编辑：John D'Errico 2024-4-11

You cannot just write dx, and hope that MATLAB will be smart enough to differentiate something. The mindreading toolbox is never working when you need it.

I would strongly suggest you first spend some time with the MATLAB Onramp tutororial. Then spend some time in learnign how to use the symbolic toolbox. diff can differentiate an expression.

Torsten 2024-4-11

在 MATLAB Online 中打开

syms x y

f1 = x^2+y^3;

f2 = sin(x) + exp(y);

f = [f1,f2];

jacobian(f)

ans =

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Answer 1

Aniket 2024-8-7，14:25

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/2105861-undefined-function-or-variable-dx#answer_1495979

在 MATLAB Online 中打开

Hi @Mustafa Abdelfattah,

The error is thrown because `dx` is not a predefined function in MATLAB for differentiation.

Here are two ways to calculate the Jacobian matrix:

Using Symbolic Differentiation (`diff`)

% Define symbolic variables
syms x y
% Define the functions
f1 = x^2 + y^2 - 1;
f2 = x^2 - y;
% Compute the Jacobian matrix using symbolic differentiation
jacobi = [diff(f1, x), diff(f1, y); 
    diff(f2, x), diff(f2, y)];

2. Using the Inbuilt `jacobian` Function

% Define symbolic variables
syms x y
% Define the functions
f1 = x^2 + y^2 - 1;
f2 = x^2 - y;
% Create a vector of the functions
F = [f1; f2];
% Compute the Jacobian matrix using the inbuilt function
jacobi = jacobian(F, [x, y]);