I cannot remove 0 elements of a symbolic array

Question

Mohammad Shojaei Arani 2022-2-1

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https://ww2.mathworks.cn/matlabcentral/answers/1640355-i-cannot-remove-0-elements-of-a-symbolic-array

评论： Mohammad Shojaei Arani 2022-2-1

采纳的回答： Walter Roberson

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Hello friends!

Consider the following commands

syms x sigma(x)
par=sym('par%d', [1 2],'positive');
sigma(x)=par(1)+par(2)*x;
D_sigma=[sigma diff(sigma) diff(sigma,2) diff(sigma,3)];
D_sigma
nonzeros(D_sigma)

D_sigma(x) =

[par1 + par2*x, par2, 0, 0]

ans(x) =

[par1 + par2*x, par2, 0, 0]

I really have no idea why this does not work!!!!!!!

Any help!

Thanks in advance!

Babak

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Answer 1

Walter Roberson 2022-2-1

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https://ww2.mathworks.cn/matlabcentral/answers/1640355-i-cannot-remove-0-elements-of-a-symbolic-array#answer_886055

在 MATLAB Online 中打开

syms x sigma(x)

par=sym('par%d', [1 2],'positive');

sigma(x)=par(1)+par(2)*x;

D_sigma=[sigma diff(sigma) diff(sigma,2) diff(sigma,3)];

D_sigma

D_sigma(x) =

DSX = D_sigma(x);

nonzeros(DSX)

ans =

The reason you had a problem is that your D_sigma is a symbolic function, not a vector.

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Walter Roberson 2022-2-1

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Your D_sigma was a single function that returns a vector. The function itself is not zero, so nonzeros() does not remove the function.

You should not think of your D_sigma as being a vector of functions. It cannot be indexed into. It is a 1 x 1 object that happens to return a vector (whos size depends upon the input). [Huh, it can return a cell array???)

syms x sigma(x)

par=sym('par%d', [1 2],'positive');

sigma(x)=par(1)+par(2)*x;

D_sigma=[sigma diff(sigma) diff(sigma,2) diff(sigma,3)]

D_sigma(x) =

D_sigma([-1 1])

ans = 1×4 cell array

{[par1 - par2 par1 + par2]} {[par2 par2]} {[0 0]} {[0 0]}

Mohammad Shojaei Arani 2022-2-1