Thread Subject: How do i get coefficients out of a function and assign it to a variable

For example, i have a function S^3+6S^2
i want a command that will automatically take out the coefficients and do
a=[1 6]
without having me physically typing in the coefficients every time
how could this be done?
thank you in advance

Your question is vague. Where did the function come from? How are you using it?

im sorry, for example, i have a polynomial that is something like
5S^6+3S^5
and in my function, this is my output.
under the same function in the same step file, i would like to use just the coefficients ([5 3] in this case) from the results as an input for the next step.

right now i have it say a= [5 3] taken from the results. However, matlab would use [5 3] as an input regardless of what the output is from the previous command.

is there a function that will pull out the coefficients from a polynomial?
thank you

"George " <iu23@drexel.edu> wrote in message <hemlb3$8t9$1@fred.mathworks.com>...
> im sorry, for example, i have a polynomial that is something like
> 5S^6+3S^5
> and in my function, this is my output.
> under the same function in the same step file, i would like to use just the coefficients ([5 3] in this case) from the results as an input for the next step.
>
> right now i have it say a= [5 3] taken from the results. However, matlab would use [5 3] as an input regardless of what the output is from the previous command.
>
> is there a function that will pull out the coefficients from a polynomial?
> thank you

one of the possible solutions
- this requires the symbolic math txb
- assume your function returns a CHAR string

     s='5*s^6+3*s^5';
     c=sym2poly(sym(s))
% c = 5 3 0 0 0 0 0
% also, see
     help coeffs;

us

this is exactly what im trying to do with a and b values


% a and b are top and bottom coefficients of transfer function
a=[0.0948 0.6302 1.271 1.271 0.6415 0.193 0.0287 0.001753 2,24e-5 0 0];
b=[1 8.835 29.68 54.28 62.13 47.62 25.21 9.256 2.309 0.3717 0.035 0.001606 2.24e-5 0 0];

[r,p,k] = residue(b,a)

"George " <iu23@drexel.edu> wrote in message <hemovb$hki$1@fred.mathworks.com>...
> this is exactly what im trying to do with a and b values
>
>
> % a and b are top and bottom coefficients of transfer function
> a=[0.0948 0.6302 1.271 1.271 0.6415 0.193 0.0287 0.001753 2,24e-5 0 0];
> b=[1 8.835 29.68 54.28 62.13 47.62 25.21 9.256 2.309 0.3717 0.035 0.001606 2.24e-5 0 0];
>
> [r,p,k] = residue(b,a)

well, since this obviously works
- why do you say ...what i'm trying to do...
- how does this relate to your OP

us

sorry i did not see you post before i posted mine. i made my post right after i posted the second time.
thanks for your function. yes it does work. but how could i pull out coefficients from both the numerator and denominator?
thank you in advance
i.e.
C=(2S^5+4S^3)/(3S^6)

"George " <iu23@drexel.edu> wrote in message
news:hep00a$kan$1@fred.mathworks.com...
> sorry i did not see you post before i posted mine. i made my post right
> after i posted the second time.
> thanks for your function. yes it does work. but how could i pull out
> coefficients from both the numerator and denominator?
> thank you in advance
> i.e.
> C=(2S^5+4S^3)/(3S^6)

If this is a symbolic expression:

syms S
C = (2*S^5+4*S^3)/(3*S^6);

use NUMDEN, then use SYM2POLY on the two outputs of that function.

--
Steve Lord
slord@mathworks.com
comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ

thank you again for the response. however, i still can not separate numerator and denominator.
I am trying to separate numerator and denominator of a transfer function but matlab does not accept Transfer function as an input...


rlocus(G_uncomp)
[d,n]=numden(G_uncomp);
d=sym2poly(sym(G_uncomp));
n=sym2poly(sym(G_uncomp));

((((((??? Undefined function or method 'numden' for input arguments of type 'tf'.

Error in ==> eval_water_sys at 90
[d,n]=numden(G_uncomp);))))))))


This is the error i get.


Transfer function:
 
0.474 s^8 + 3.143 s^7 + 6.302 s^6 + 5.977 s^5 + 3.106 s^4 + 0.9121 s^3 + 0.128 s^2
                                                                                    
                                                                        + 0.006587 s
                                                                                    
------------------------------------------------------------------------------------
s^12 + 8.835 s^11 + 29.68 s^10 + 54.28 s^9 + 62.13 s^8 + 47.62 s^7 + 25.21 s^6
                                                                                    
                     + 9.253 s^5 + 2.305 s^4 + 0.3687 s^3 + 0.03373 s^2 + 0.001317 s


and this is my G_uncomp

"George " <iu23@drexel.edu> wrote in message <hevr0j$m2k$1@fred.mathworks.com>...
> thank you again for the response. however, i still can not separate numerator and denominator.
> I am trying to separate numerator and denominator of a transfer function but matlab does not accept Transfer function as an input...
>
>
> rlocus(G_uncomp)
> [d,n]=numden(G_uncomp);
> ((((((??? Undefined function or method 'numden' for input arguments of type 'tf'.
> Error in ==> eval_water_sys at 90
> [d,n]=numden(G_uncomp);))))))))

well, yes - NUMDEN wants a symbolic var as input...

us

thanks for the response but im not really sure what that means. how can i transfer my transfer function into a symbolic form?

"George " <iu23@drexel.edu> wrote in message
news:hevr0j$m2k$1@fred.mathworks.com...
> thank you again for the response. however, i still can not separate
> numerator and denominator.
> I am trying to separate numerator and denominator of a transfer function
> but matlab does not accept Transfer function as an input...
>
>
> rlocus(G_uncomp)
> [d,n]=numden(G_uncomp);
> d=sym2poly(sym(G_uncomp));
> n=sym2poly(sym(G_uncomp));
>
> ((((((??? Undefined function or method 'numden' for input arguments of
> type 'tf'.

NUMDEN is only defined for symbolic objects. Since you don't have a
symbolic object but a tf object, you can't use this method.

What you should do instead is use the tf object's GET method to GET the
'num' and 'den' properties of the object as shown in the Example on this
documentation page.

http://www.mathworks.com/access/helpdesk/help/toolbox/control/ref/get.html

--
Steve Lord
slord@mathworks.com
comp.soft-sys.matlab (CSSM) FAQ: http://matlabwiki.mathworks.com/MATLAB_FAQ