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Creating Symbolic Functions

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Itamar
Itamar 2012-3-15
Hey all. I'm new to Matlab and a first time poster here. I'm trying to create a multivariate symbolic polynomial and access its variables using the argnames function, which I found here:
http://www.mathworks.com/help/toolbox/symbolic/argnames.html
Running the example code at the linked page, I get the following:
>> syms f(x, y);
??? Error using ==> syms at 61
Not a valid variable name.
Then I looked at the documentation for sym here:
http://www.mathworks.com/help/toolbox/symbolic/sym.html
I tried running the example code for a symbolic function found there, which produced:
>> x = sym('x');
>> y = sym('y');
>> f(x, y) = x + y;
??? Error using ==> mupadmex
Error in MuPAD command: DOUBLE cannot convert the input expression into a double
array.
If the input expression contains a symbolic variable, use the VPA function instead.
Error in ==> sym.sym>sym.double at 936
Xstr = mupadmex('symobj::double', S.s, 0);
Error in ==> sym.sym>privformatscalar at 2678
x = double(x);
Error in ==> sym.sym>privformat at 2663
s = privformatscalar(x);
Error in ==> sym.sym>sym.subsasgn at 1433
[inds{k},refs{k}] = privformat(inds{k});
Then I tried the following, just to see if it would work (it didn't):
>> x = sym('x');
>> y = sym('y');
>> f = x+y;
>> argnames(f);
??? Undefined function or method 'argnames' for input arguments of type 'sym'.
What am I doing wrong?
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G A
G A 2012-3-15
Thanks, Walter.
Walter Roberson
Walter Roberson 2012-3-16
Dang, they introduced new functionality that used to be illegal.

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采纳的回答

Alexander
Alexander 2012-3-16
The concept of 'symbolic functions' including the function argnames have been introduced in MATLAB 2012a. You cannot use this feature in earlier versions.
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G A
G A 2012-3-16
That was my feeling.
Itamar
Itamar 2012-3-16
I'm running MATLAB 7.12.0.635 (R2011a), so that explains it. I really should have checked what version of the software I was using. Thanks.

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更多回答（2 个）

Walter Roberson
Walter Roberson 2012-3-15
f = evalin(symengine, '(x,y) -> x+y;');
For something more complicated,
f = evalin(symengine, 'proc(x) local S, N; S := 0; for N to x do S := S + cos(Pi/N) end_do; S end_proc;');
Only parts of MuPAD have direct MATLAB interfaces, and creating new functions (procedures) does not have a direct interface, unfortunately. Though it could be that I have missed a trick.
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Itamar
Itamar 2012-3-16
I only started using MATLAB relatively recently, and I don't have a very clear picture of how things are structured; in particular I didn't realize there was a separate system responsible for performing symbolic computations. I'll probably go take a look at some MuPAD documentation now. Thanks for the help.

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G A
G A 2012-3-15
Could it be an alternative solution to use symvar() instead of argnames()?
syms x y
f = x+y;
symvar(f)
ans =
[ x, y]
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Itamar
Itamar 2012-3-16
I think that symvar is probably fine for me. And if I'm reading the documentation correctly, then symvar should return the variable names in lexicographic order:
http://www.mathworks.com/help/toolbox/symbolic/symvar.html
Also, it doesn't seem as though function evaluation notation works for 'sym' objects; it looks like I would be using the 'subs' function (or something similar), i.e.:
>> x = sym('x');
>> y = sym('y');
>> f = x+y^2;
>> subs(f,[x,y],[1,2])
ans =
5
>> subs(f,[y,x],[2,1])
ans =
5
So it seems like I could distinguish between asymmetric functions in this context.

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