problem with subs function

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Robert
Robert 2016-11-30
评论： Walter Roberson 2021-2-3
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 个评论
Walter Roberson
Walter Roberson 2021-2-3
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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回答（4 个）

Tamir Suliman
Tamir Suliman 2016-12-1
try to add syms s and syms k then run the strings
or syms s k
bio lim
bio lim 2016-12-1
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 2016-12-1
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 个评论
Karan Gill
Karan Gill 2016-12-1
This is probably it. @Robert, did you read the error message?

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Jes Bel
Jes Bel 2018-7-13
After doing " syms s k " I copy pasted your code and worked fine for me.

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