How to find degree of polynomial in Matlab?

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Sohail 2013-12-30

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评论： Sohail 2013-12-30

Dear All, Good day! I am trying to find the degree of polynomial in MATLAB. I know it is possible in MuPAD by using degree(p). But couldn't find a way by using editor. Kindly help me on this. Thanks

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Walter Roberson 2013-12-30

How is the polynomial represented ?

Sohail 2013-12-30

Actually I want to compare the nominator and denominator degree and depending on that I have to take some decision. Here is the small code that shows the z(p) Rational polynomial (RP). I put limit on that to get new RP. Now I want to compare the degree of num and den of znew(p). How I can extract the degree?

clc;

close all;

clear all;

syms p

z(p)=(2*p^4+5*p^2+1)/(p^3+p);

a=limit(z(p)/p, p, inf);

z1(p)=z(p)-a*p;

Znew=simplify(z1(p));

[num,den] = numden(Znew);

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Answer 1

Walter Roberson 2013-12-30

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在 MATLAB Online 中打开

In the case you use, where you are working with ratios of symbolic polynomials, then

degree_num = length(coeffs(num,p)) - 1;
degree_den = length(coeffs(den,p)) - 1;

You can also simplify (at least logically) the taking of the limit:

[n, d] = numden(z(p));
coeffn = coeffs(n,p);
coeffd = coeffs(d,p);
lenn = length(coeffn);
lend = length(coeffd);
if lenn > lend + 1
  error('limit would have been infinite')
elseif lenn <= lend
  error('limit would have been zero');
else
  a = coeffn(end);
end

If you are sure that neither of the two exception cases can occur, then

t = coeff(numden(z(p)));
a = t(end);

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Walter Roberson 2013-12-30

You could use sym2poly() . Again length() - 1 for degree, but the first entry of the matrix instead of the last entry for the highest coefficient.

Sohail 2013-12-30

Thanks alot mate. Have a good day. :)

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Answer 2

The Matlab Spot 2013-12-30

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编辑：The Matlab Spot 2013-12-30

See if polyval helps

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Sohail 2013-12-30

We cannot find degree of poly. using polyval().

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Answer 3

Roger Stafford 2013-12-30

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Suppose you evaluate your polynomial, P(x), at a large number of equally-spaced values of x. Then if diff(P,n) exhibits a non-zero constant value (except of course for small round-off errors,) you can conclude that P has degree n. Moreover, that constant value is indicative of the coefficient of the highest degree term.

I would think however, that an inspection of whatever process is creating your polynomial should more easily tell you its degree.