[Octave-bug-tracker] [bug #51748] vpa producing strange results

[Luis Mendo]

URL:
  <http://savannah.gnu.org/bugs/?51748>

                 Summary: vpa producing strange results
                 Project: GNU Octave
            Submitted by: lmendo
            Submitted on: Mon 14 Aug 2017 09:59:26 AM UTC
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Inaccurate Result
                  Status: None
             Assigned to: None
         Originator Name: Luis Mendo
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 4.2.1
        Operating System: Any

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Details:

Running these statements in Octave


vpa('5^2',30)
vpa('sqrt(5)^4',30)
vpa('.5',30)
vpa('(5^.5)^4',30)


produces the following results, the fourth of which is not correct:


ans = (sym) 25.0000000000000000000000000000
ans = (sym) 25.0000000000000000000000000000
ans = (sym) 0.500000000000000000000000000000
ans = (sym) 25.0000000000000035527136788005


This is strange, specially considering that .5 is represented exactly as a
float (see third statement).

For reference, SymPy produces the correct result:


from sympy import *
print(N(sqrt(5)**4, 30))
print(N(5**2, 30))


gives


25.0000000000000000000000000000
25.0000000000000000000000000000





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