How can we compute the derivative of polynomials?

How can we compute the derivative of the (m-1) polynomials at a point?

I got the following error:

The following error occurred converting from sym to double:

Unable to convert expression into double array.

Error in untitled (line 42)

p(1)=((x-alpha(1))*p0)/beta(1);

syms x 
 p0=1;
x0=2;
p(1)=((x-.2)*p0)/.3;
p(2)=((x-.5)*p(1)-.3*p0)/1.3;
dp(1)=eval((subs(diff(p(1),x,1),x,x0)) )
dp(2)=eval((subs(diff(p(2),x,1),x,x0)) )
for j=3:m-1
    p(j)=((x-.2)*p(j-1) - .3*p(j-2))/1.3;
    dp(j)=eval((subs(diff(p(j),x,1),x,x0)));
end

[James Tursa]

What is m? When I define m, I don't get any errors when I run your code in R2020a. What version are you running?

[Omar B.]

m=6 any number.

[James Tursa]

I don't get any errors in R2017a either.

[Omar B.]

It dose not work.

[Walter Roberson]

If I set m=6 then I do not get any error in R2020a.

[Walter Roberson]

You should never eval() a symbolic expression.

eval() of a symbolic expression is equivalent to eval(char()) of the expression. However, char() of a symbolic expression is not generally a MATLAB expression.

>> char(repmat(sym('x'),[2 2 2]))
ans =
    'array(1..2, 1..2, 1..2, (1, 1, 1) = x, (1, 1, 2) = x, (1, 2, 1) = x, (1, 2, 2) = x, (2, 1, 1) = x, (2, 1, 2) = x, (2, 2, 1) = x, (2, 2, 2) = x)'

char() of a symbolic expression is also not generally a MuPAD expression.

In context, the subs() are returning symbolic rational numbers, and those only need to be double() not eval() .

[Omar B.]

what should I use insted of eval() ?

[James Tursa]

double( )

That being said, I don't know what is going on in your actual case since your example code doesn't produce an error at my end.

[Omar B.]

do you mean that we need to use double() insted of eval() with a symbolic expression?

[James Tursa]

Both eval( ) and double( ) work for me with your example code. What is your actual code?

[Omar B.]

yes, I got the same results. Thank you so much

Answer 1

[David Hill]

Why not just use polynomial representation

p = [3 0 -2 0 1 5];
pd=polyder(p);
x=1:10;
val=polyval(pd,x);

[Omar B.]

I just here wrote an example , but I need to refer the coefficients to another code. Is there another way?

[Walter Roberson]

The coedfficients would be in pd