# problem with subs function

9 views (last 30 days)
Robert on 30 Nov 2016
Commented: Walter Roberson on 3 Feb 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);
Walter Roberson on 3 Feb 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.

Tamir Suliman on 1 Dec 2016
try to add syms s and syms k then run the strings
or syms s k

bio lim on 1 Dec 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 on 1 Dec 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
Karan Gill on 1 Dec 2016
This is probably it. @Robert, did you read the error message?

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