Abs and angle of complex number with symbolic object

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john
john 2013-3-4
Hi,
If r=sym('a')*exp(j*sym('b')) is a symbolic object, and
1. if I write abs(r), I get abs(a).....I need get only "a" without abs
2. if I write angle(r), I get error
??? Undefined function or method 'atan2' for input arguments of type 'sym'.
Error in ==> angle at 14 p = atan2(imag(h), real(h));
Can you help me please? How can I solve this?

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Walter Roberson
Walter Roberson 2013-3-5
There is no "complex argument" function for symbolic variables exposed at the MATLAB level. You can, though, define
arg = @(V) feval(symengine, 'arg', V);
http://www.mathworks.com/help/symbolic/mupad_ref/arg.html
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john
john 2013-3-7
Did you mean this code
r=sym('a')*exp(j*sym('b'));
arg = @(r) feval(symengine, 'arg', r);
number=arg(r);
????
But I got again error
??? Undefined function or method 'atan2' for input arguments of type 'sym'.
Error in ==> angle at 14 p = atan2(imag(h), real(h));
Btw, what is better to use, atan or atan2 in form "P = 2 * atan( (sqrt(X.^2 + Y.^2) - X) ./ Y )";
Thank you

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Brian B
Brian B 2013-3-4
编辑：Brian B 2013-3-4
Well, for part 1 you have to assume a is a non-negative real:
syms a
assume(a>0)
Then
>> abs(a)
ans =
a
I have not tried the second part, but you can imagine dividing by the magnitude and taking a log....
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Brian B
Brian B 2013-3-4
编辑：Brian B 2013-3-4
Try
a = sym('a','positive')
b = sym('b','real')
That works for me in R2011b, including computing the magnitude of r. I don't see a good way to get the argument, though.

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