problem with subs function

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]

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.

Answer 1

[Tamir Suliman]

try to add syms s and syms k then run the strings

or syms s k

Answer 2

[bio lim]

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))

Answer 3

[Walter Roberson]

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]

This is probably it. @Robert, did you read the error message?

Answer 4

[Jes Bel]

After doing " syms s k " I copy pasted your code and worked fine for me.