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Why are symbolic expressions being rounded?

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Bill Tubbs
Bill Tubbs on 2 Sep 2020
Answered: Bill Tubbs on 3 Sep 2020
I'm deriving some expressions symbolically and then substituting values and computing the results and I only recently noticed that the results are being rounded.
>> format long
>> syms x; double(subs(10/x,x,3))
ans =
3.333300000000000
or simply,
>> x = sym(3.33333333)
x =
3.3333
>> double(x)
ans =
3.333300000000000
I can't even reproduce results from the documentation it seems:
>> syms x
>> p = sym(pi);
>> piVpa = vpa(p)
piVpa =
3.1416
According to the documentation, this is supposed to produce:
piVpa =
3.1415926535897932384626433832795
  1 Comment
Bill Tubbs
Bill Tubbs on 2 Sep 2020
Actually, I think it is double that is doing the rounding:
>> syms x; 10/3 - subs(10/x,x,3)
ans =
0
>> 10/3 - double(subs(10/x,x,3))
ans =
3.333333333355171e-05

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Accepted Answer

Bill Tubbs
Bill Tubbs on 3 Sep 2020
Mathworks technical support have diagnosed the problem. Don't know what caused it originally but the 'FloatingPointOutput' setting in the symbolic preferences was set to 1.
Here is the solution to fix it:
>> sympref
ans =
struct with fields:
FourierParameters: [1×2 sym]
HeavisideAtOrigin: [1×1 sym]
AbbreviateOutput: 1
TypesetOutput: 1
FloatingPointOutput: 1
PolynomialDisplayStyle: 'default'
MatrixWithSquareBrackets: 0
>> sympref('FloatingPointOutput','default');
>> double(sym('pi'))
ans =
3.141592653589793
>> p = sym(pi);
>> piVpa = vpa(p)
piVpa =
3.1415926535897932384626433832795

More Answers (1)

Steven Lord
Steven Lord on 2 Sep 2020
Have you changed your digits setting?
help digits
Are you calling the correct double function? What does this show?
which double(x)
  1 Comment
Bill Tubbs
Bill Tubbs on 2 Sep 2020
Digits = 32
which double(x)
returns
/Applications/MATLAB_R2019b.app/toolbox/symbolic/symbolic/@sym/sym.m % sym method

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