Symbolic dependency apparently sometimes gives wrong answers

Question

Mohammad Shojaei Arani 2022-10-28

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1838163-symbolic-dependency-apparently-sometimes-gives-wrong-answers

评论： Mohammad Shojaei Arani 2022-10-29

采纳的回答： Walter Roberson

Hello,

I have a simple question but could not figour out how to tackle it by myself. Consider the following:

syms p1 y

f=[p1*y;p1];

hasSymType(f,'symfunDependingOn',y)

0

I am not happy with this since clearly I expected to get the answer [1,0]. I do not know why it behaves like this!!!

Please not that my actual problem are much more complicated and this is just to explain to you my question. So, please do not give me any answer which

just works for this simple example, rather I would like a generic sort of answer.

Any idea?

thanks in advance!

Babak

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Answer 1

Walter Roberson 2022-10-28

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在 MATLAB Online 中打开

Not a bug. Your first element is a symbolic expression involving the desired variable, but symfundependingon is for unresolved symbolic functions that have the variable as a parameter.

syms f(x) g(x, y) h(y)

expressions involving f or g would be symfun depending on x, but h would not be.

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Mohammad Shojaei Arani 2022-10-28

Well, I am sure there has been a good reason matlab developers decided to do so, as you mentioned chan rule is one example. But, I think I do not like this and will stick to my own code to tackle this problem. However, I have a suggestion and if you find it useful please add a new command in MATLAB.

I a (simple) mathematician. John D'Errico seems also to be a mathematician whose level is much higher than me. Both of us had inconveniences with this. People need to know the functional dependecies of ANYTHING in an easy and intuitive manner and when they google it they like to find the proper matlab command as fast as possible without having a great knowledge of mathematics. ANYTHING can be a 'symbolic expression', a 'symbolic function', 'a symbolic vector', a 'symbolic matrix', a 'symbolic cell', whatever. For instance, imagine we want to check dependency in variable 'x'. if E=[x y f(x,y)] then I expect to get the answer [1 0 1]. If E=[1 2 x^2;sin(x*y) x z] then I expect to get the answer [0 0 1;1 1 0].

Well, you are the boss!

Thanks for your time Walter!

Babal

Steven Lord 2022-10-28

If you click on the three dots at the right side of the question, an answer, or a comment one of the options is Link. This will give you a hyperlink you can send to Support that the Support staff can use to see the discussion that's taken place here.

For example, if you click on this link you should be directed to the comment you made on John D'Errico's answer.

https://www.mathworks.com/matlabcentral/answers/1838163-symbolic-dependency-apparently-sometimes-gives-wrong-answers#comment_2436153

Mohammad Shojaei Arani 2022-10-28

I coontacted them!

Thanks Steven!

And, have a great weekend!

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Answer 2

John D'Errico 2022-10-28

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编辑：John D'Errico 2022-10-28

To be honest, this appears to be more of a bug report than anything else, since that option in hasSymType does not apprear to be working as per the documentation. So I would report it as such, but Answers is not the correct place to report bugs.

Could you find some other solution that would work? Even if someone could find something that works here, it does not get past the fact that hasSymType is the correct tool to solve your specidfic problem, and it should work as per the help.

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Mohammad Shojaei Arani 2022-10-28

Hi John,

Thanks!

Of course, I have my alternative way to tackle this issue (I had to write a rather complicated function which

finds the functional dependencies. I first convert my symbolic expression to a character or string and then detect functional dependencies using character manipulations). But, I would be happier when MATLAB fixes this bug.

Thanks again,

Babak

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