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

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Mohammad Shojaei Arani 2022-12-15

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评论： Mohammad Shojaei Arani 2022-12-17

采纳的回答： Dyuman Joshi

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 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?

Mohammad Shojaei Arani 2022-12-15

Sorry, that was just a typo, which I correct now

syms x f(x) a

f(x) = a*x;

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采纳的回答

Dyuman Joshi 2022-12-15

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在 MATLAB Online 中打开

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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Dyuman Joshi 2022-12-15

What is the logic behind this?

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 2022-12-17

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1878742-can-we-distinguish-between-variables-and-parameters-in-a-symbolic-function#answer_1130012

在 MATLAB Online 中打开

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.