I cannot remove 0 elements of a symbolic array

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

Answer 1

[Walter Roberson]

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.

[Mohammad Shojaei Arani]

Thanks Walter a lot!

I spent 2 hours and I believe I could not fix it by myself. But, why matlab does not also consider symbolic functions. Is this a bug in matlab?

[Walter Roberson]

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]

Thanks Walter for your kins explanation!

Although I am a mathematician I found coding a bit annoying but will improve over time!