MATLAB: Suspicious value in vpa

precisoionvpa

How does vpa returns suspicious values like

vpa(1e50, 60) produces 100000000000000000000000000000000000000000000000000.0 which is fine,

while

vpa(1.1e50, 60) produces 110000000000000008392746825201075703624461468041216.0.

Where does "…8392746825201075703624461468041216" come from?

Best Answer

vpa('1.1e50', 60) 
ans =
110000000000000000000000000000000000000000000000000.0

When you say 1.1e50 not in single quotes, it converts it to a double (~16 digits) and then to vpa. If the number can't be evenly represented in double it shows up like above.

Related Solutions

MATLAB: Are symbolic expressions being rounded

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

MATLAB: Precise conversions from double to symbolic

Compare:

r = 2.1234
sym(1/r)
1/sym(r)
1/sym(r,'d')
1/sym(r,'e')
1/sym(r,'f')
1/sym(r,'r')
s = sym(sprintf('%.16g', r));
feval(symengine,'numeric::rationalize',1/s,'Exact')
1/feval(symengine,'numeric::rationalize',s,'Exact')

Best Answer

Related Solutions

MATLAB: Are symbolic expressions being rounded

MATLAB: Precise conversions from double to symbolic

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