Can we distinguish between variables and parameters in a symbolic function?

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Mohammad Shojaei Arani
评论： Mohammad Shojaei Arani 2022-12-17
Hello,
I have a simple (perhaps naive, if so my appology) question. Consider the following
syms x f(x) x
f(x) = a*x;
Is there a way to distinguish between 'x' and 'a'? If I use symvar(f) it just gives the information about all vars and aparetly
cannot distinguish between x and a.
Any idea?
Thanks in advance,
Babak
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Dyuman Joshi
Dyuman Joshi 2022-12-15
symvar determines symbolic variables in the expression. Since you have not defined a as a symbolic variable in the above code, symvar won't classify a as an output.
What is the data type of a?

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Dyuman Joshi
Dyuman Joshi 2022-12-15
Ran in:
syms f(x) a m n z
f(x)=a*x
f(x) = 
y=symvar(f)
y = 
I understand what you mean by 'cannot distinguish between x and a'
But, this is how syms variable are expressed in arrays. For example -
z=[m n]
z = 
x and a are expressed similarly in the above expression obtained from symvar
However, you can convert the symbolic expression to string and obtain seperate variables -
z=symvar(char(f))
z = 2×1 cell array
{'a'} {'x'}
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Mohammad Shojaei Arani
Mohammad Shojaei Arani 2022-12-17
Hi Torsten and Dyuman,
After I thought deeper I came to believe that you are right. Thanks for your time.
Math is somehow different than programming :-)

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Walter Roberson
Walter Roberson 2022-12-17
https://www.mathworks.com/help/symbolic/sym.argnames.html
f_variables = argnames(f)
f_param = setdiff(symvar(f), f_variables)
This is not the same thing as "all variables mentioned that are not parameters". symvar does not report any "bound" variables or any variables being used as functions.
A bound variable is like x in
int(f(x), x, a, b)
provided that f does not itself contain x then you could substitute any other variable name without affecting the output, like
int(f(Dummy), Dummy, a, b)
int() and symsum() and symprod() can all use bound variables.
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Mohammad Shojaei Arani
Mohammad Shojaei Arani 2022-12-17
Hi Torsten,
So, it is actually possible to do it in matlab. Nice!
You also answered another question of me (somehow you read my mind!).
Thanks a lot for the very useful helps!
Babak

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