[Octave-bug-tracker] [bug #56414] sin function incorrect results when not slicing input

[Kevin Anderson]

URL:
  <https://savannah.gnu.org/bugs/?56414>

                 Summary: sin function incorrect results when not slicing
input
                 Project: GNU Octave
            Submitted by: kandersondax
            Submitted on: Wed 29 May 2019 02:30:42 PM UTC
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Incorrect Result
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 5.1.0
        Operating System: Microsoft Windows

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Details:

The "sin" function can be made to have different output in very specific
circumstances depending on if an input vector is sliced or not.  Slicing to
copy actually gives the correct result, whereas the original (or a simple
copy) does not.

Here is a minimal example from Octave 5.1.0 on Windows 10 x64:


>> x = -30:-20
x =

  -30  -29  -28  -27  -26  -25  -24  -23  -22  -21  -20

>> sin(pi*x)
ans =

 Columns 1 through 9:

   1.0779e-14  -1.4209e-14   1.7640e-14  -6.8592e-15   1.0289e-14  -1.3720e-14
  2.9391e-15  -6.3693e-15   9.7996e-15

 Columns 10 and 11:

  -1.3230e-14   2.4492e-15

>> sin(pi*x(1:end))
ans =

 Columns 1 through 9:

   1.0779e-14   1.3553e-18   3.4289e-15  -6.8592e-15  -3.9215e-15   4.9120e-16
  2.9391e-15  -6.3693e-15   9.7996e-15

 Columns 10 and 11:

   9.8104e-16   2.4492e-15




As can be seen above, the results from columns 5, 6, and 10 differ.  The
results from the x(1:end) set are correct if you type in the numbers
individually.  The results from the first form where x is used directly are
incorrect.  It is also only certain values this happens with, though the
example above is not all of them.

The multiplication by "pi" also seems necessary for this bug to occur, but I
cannot guarantee that.  I only know I've only seen it with "pi" as a factor. 
This bug was also observed when multiplying by a number which was a multiple
of pi as well.




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