How to fix the large and mad fractions when dealing with symbolic vars

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Mohd Aaquib Khan
Mohd Aaquib Khan 2022-9-18
评论： Mohd Aaquib Khan 2022-9-19
Ran in:
Hi guys,
I face this often, as you can see below that cos(phi) and cos(psi) are decent entities, but when they are used with a symbolic variable (kn here), it shows huge fractions. Is there an easy way to make the coefficients of k1 k2 and k3 in the short format or some decent fractions.
Even something as trivial as k1*cos(pi/2) is not evaluated to be 0 as shown below. I fear that this leads to truncation errors.
Thanks in advance.
cos(pi/2)
ans = 6.1232e-17
syms a
a*cos(pi/2)
ans = 
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Torsten
Torsten 2022-9-18
I could only see your screenshots after I opened your contribution using "Edit".

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采纳的回答

Walter Roberson
Walter Roberson 2022-9-19
The answer you are looking for is
digits(4)
sympref('FloatingPointOutput',true);
However, I do not recommend this.
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Mohd Aaquib Khan
Mohd Aaquib Khan 2022-9-19
Ran in:
Yes thank you, vpa() also works fine for the same result.
phi = [50, 25, 30 90]';
syms a
vpa(cosd(phi)*a,4)
ans = 
digits(4)
sympref('FloatingPointOutput',true);
phi = [50, 25, 30 90]';
cosd(phi)*a
ans = 

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更多回答（2 个）

Steven Lord
Steven Lord 2022-9-18
Ran in:
Symbolically, use vpa or double or use the sympref function to change the FloatingPointOutput preference.
Numerically, if you want to compute the sine or cosine of an angle expressed as a multiple of pi, use sinpi or cospi respectively.
format longg
[sin(0.5*pi), cos(0.5*pi); sinpi(0.5), cospi(0.5)]
ans = 2×2
1 6.12323399573677e-17 1 0
Paul
Paul 2022-9-18
Ran in:
In this expression
syms a
a * cos(pi/2)
ans = 
the cos() is evaluated numerically and the result converted to a sym to multiply with a. The numerical cos(pi/2) is not zero.
Force the cos to be evaluated symbolically
a * cos(pi/sym(2))
ans = 
0
or
a * cos(sym(pi)/2)
ans = 
0
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Torsten
Torsten 2022-9-19
For phi = 30 and phi = 90, cos(phi) is correctly simplified.
What would you expect for cos(50) and cos(25) ? I don't see a simple representation.
Walter Roberson
Walter Roberson 2022-9-19
You should never eval() a symbolic expression. eval() of a symbolic expression is not documented by Mathworks. In practice it is treated as eval(char()) of the expression. However, char() of a symbolic expression is written in a mix of MATLAB, English, and the internal MuPAD programming language, and is not generally something that can be computed.

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