[Octave-bug-tracker] [bug #45507] acos returns different results on big input values

[H. G.]

URL:
  <http://savannah.gnu.org/bugs/?45507>

                 Summary: acos returns different results on big input values
                 Project: GNU Octave
            Submitted by: hardy
            Submitted on: Do 09 Jul 2015 18:53:00 GMT
                Category: Octave Function
                Severity: 3 - Normal
                Priority: 5 - Normal
              Item Group: Regression
                  Status: None
             Assigned to: None
         Originator Name: 
        Originator Email: 
             Open/Closed: Open
         Discussion Lock: Any
                 Release: 4.0.0
        Operating System: Any

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Details:

Here is a tiny "script" to show the behavior:


acos(1e7)
acos(1e8)


This gives you different results, depending where you run it (Octave 3.8.2,
Octave 4.0.0, or Matlab):

* The first calculation returns acos(1e7) = 0 + 16.8 i in all three programs.

* The second calculation returnes acos(1e8) = 0 + 19.1 i in Matlab. It
returnes the same value in Octave 3.8.2.

* But in Octave 4.0.0 the result of the second line now is acos(1e8) = NaN +
Inf i .

So Octave WAS behaving Matlab compatible here, but version 4.0.0 is NOT Matlab
compatible any more in this case!

I don't know if acos is well defined for input values >1 at all. But Matlab
decided to always returns something with a real part of 0, for big input
numbers. Octave did the same, but now (version 4.0.0) it suprisingly "jumps"
from a real part of 0 up to a real part of NaN somewhere between 1e7 and 1e8.

I have observed this change in behavior on the Linux release versions as well
as on the Windows release versions (3.8.2-3 mxe inofficial built, 4.0.0
official release).





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