I cannot remove 0 elements of a symbolic array

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Mohammad Shojaei Arani
评论： Mohammad Shojaei Arani 2022-2-1
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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Walter Roberson
Walter Roberson 2022-2-1
Ran in:
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
Walter Roberson 2022-2-1
Ran in:
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
Mohammad Shojaei Arani 2022-2-1
Thanks Walter for your kins explanation!
Although I am a mathematician I found coding a bit annoying but will improve over time!

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