problem with subs function

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Robert
Robert el 30 de Nov. de 2016
Comentada: Walter Roberson el 3 de Feb. de 2021
I use this command daily and sometimes it works and sometimes i get an error.
I am obviously not understanding the complexity of such a simple function
I have a simple symbolic function with two variables. I want to replace one of the variables with a numeric value. What is the issue here.
I have done this countless times with no problem now today its throwing a fit. of course s and k are already declared symbolic other wise open loop would not exists
openloop =
((10*s + 14)*(s + 10))/((10*k*s + s*(s + 1)*(s + 10))*(s + 5))
>> feedforward = subs(openloop,k,11.1)
Error using sym/subs>normalize (line 206) The sizes of the second and third arguments are inconsistent.
Error in sym/subs>mupadsubs (line 137) [X2,Y2,symX,symY] = normalize(X,Y); %#ok
Error in sym/subs (line 125) G = mupadsubs(F,X,Y);
  1 comentario
Walter Roberson
Walter Roberson el 3 de Feb. de 2021
of course s and k are already declared symbolic other wise open loop would not exists
Well, they were symbolic at the time that openloop was created, but they might not be symbolic now, or might have a different size.

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Respuestas (4)

Tamir Suliman
Tamir Suliman el 1 de Dic. de 2016
try to add syms s and syms k then run the strings
or syms s k

bio lim
bio lim el 1 de Dic. de 2016
I don't see any problems.
syms s k
openloop = ((10*s + 14)*(s + 10))/((10*k*s + s*(s + 1)*(s + 10))*(s + 5));
feedforward = subs(openloop,k,11.1);
The output is:
feedforward =
((10*s + 14)*(s + 10))/((111*s + s*(s + 1)*(s + 10))*(s + 5))

Walter Roberson
Walter Roberson el 1 de Dic. de 2016
Is your k possibly a vector of symbols? If so then
feedforward = subs(openloop,k,11.1)
would need one replacement value for each variable in k
  1 comentario
Karan Gill
Karan Gill el 1 de Dic. de 2016
This is probably it. @Robert, did you read the error message?

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Jes Bel
Jes Bel el 13 de Jul. de 2018
After doing " syms s k " I copy pasted your code and worked fine for me.

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