How do i define a variable as a function?

Let us say there is a fucntion theta that is depedent on time: Theta(t); It is in a cos function so: cos(theta(t))

How do i differentiate this in matlab so it uses the chain rule?

clear all

syms t theta

theta=t;

f=cos(theta(t));

diff(f,t)

I have tried the above and got the errors:

Error using sym/subsref (line 49)

Error using maplemex

Error, (in MTM:-subsref) indices must be positive integers, got t

Error in test (line 5)

f=cos(theta(t));

Answer 1

[Star Strider]

I suspect this works the similiarly in Maple (that I do not currently have) —

syms t theta(t)
f=cos(theta(t));
dfdt(t) = diff(f,t)

dfdt(t) = 

.

[Hamza Yusuf]

I have tried to google after this error and which -all syms did not work

I get this error:

Error using assignin

Invalid variable name "theta(t)" in ASSIGNIN.

Error in syms (line 56)

assignin('caller',varargin{i},sym(varargin{i}));

Error in test (line 1)

syms t theta(t)

[Star Strider]

Maple may work differently than the MATLAB Symbolic Math Toolbox engine.

Check the Maple documentation, since my code obviously works in MATLAB R2021b.

I currently do not have Maple installed, so I cannot help with code using it.

.

[Hamza Yusuf]

I am using 2020b. I am not using maple. I dont know why it says "Error using maplemex" but i am using matlab

[Star Strider]

This error —

Error using maplemex

indicates that Maple is the symbolic engine in the posted original code.

.

[Hamza Yusuf]

How do i change it to matlab`?

I have found this:

https://www.mathworks.com/matlabcentral/answers/440286-error-using-sym-solve-and-maplemex

But that it did not work and this happened:

>> which -all syms
C:\Program Files\MATLAB\R2020b\toolbox\maple\syms.m
C:\Program Files\MATLAB\R2020b\toolbox\symbolic\symbolic\syms.m  % Shadowed
>> test
Error using sym/subsref (line 49)
Error using maplemex
Error, (in MTM:-subsref) indices must be positive integers, got t

Error in test (line 2)
f=cos(theta(t));

The other solution i did not get: Where do i have to look for the uninstaller?

[Hamza Yusuf]

The other solution where you remove the matlab directory worked. Thank you,

[Star Strider]

I hope that you do not have to uninstall Maple, and can simply disconnect it from MATLAB.

However if you need to uninstall it (and then reinstall it without connecting it to MATLAB next time), and since it appears that you are using Windows, open Control Panel, then click on Programs and Features, then follow the instructions. Then, reinstall Maple without connecting it to MATLAB.

.

Answer 2

[KSSV]

syms theta(t)

f=cos(theta(t));
diff(f,t)

[Hamza Yusuf]

I get this error:

Error using assignin

Invalid variable name "theta(t)" in ASSIGNIN.

Error in syms (line 47)

assignin('caller',varargin{1},sym(varargin{1}));

Error in test (line 1)

syms theta(t)