Extract num and den in symbolic expression.

Question

Bruce Taylor 2019-7-24

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https://ww2.mathworks.cn/matlabcentral/answers/473318-extract-num-and-den-in-symbolic-expression

编辑： Walter Roberson 2019-7-26

The 'tf' function in Control Systems needs numerator and denometer in a vector of decreasing powers of s.

For example a numerator of a*s^2 + b*s + c needs to be num= [a b c];. I have been cutting and pasting the necessary coefficients

but to do this I have to pause my program. Is there a way to extract the appropriate vector(s) programatically?

Thanks,

Bruce Taylor

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Answer 1

Star Strider 2019-7-24

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https://ww2.mathworks.cn/matlabcentral/answers/473318-extract-num-and-den-in-symbolic-expression#answer_384653

在 MATLAB Online 中打开

Use the sym2poly funciton:

syms a b c s 
num = 1*s^2 + 2*s + 3;
n = sym2poly(num)
n = 
     1     2     3

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Star Strider 2019-7-24

在 MATLAB Online 中打开

I thought you had already isolated the numerator and denominator.

Try this:

syms a b c d e f g s 
sys=(a*s^2+b*s+c)/(d*s^3+e*s^2+f*s+g)
sys = subs(sys,{a,b,c,d,e,f,g},{3,6,8,1,2,9,5})
[np,dp] = numden(sys)
n = sym2poly(np)
d = sym2poly(dp)

producing:

sys =
(a*s^2 + b*s + c)/(d*s^3 + e*s^2 + f*s + g)
 
sys =
(3*s^2 + 6*s + 8)/(s^3 + 2*s^2 + 9*s + 5)
 
np =
3*s^2 + 6*s + 8
 
dp =
s^3 + 2*s^2 + 9*s + 5
 
n =
     3     6     8
d =
     1     2     9     5