[Octave-bug-tracker] [bug #43313] Complex number relation operators (=, <, >) yield two logical trues

[Dan Sebald]

URL:
  <http://savannah.gnu.org/bugs/?43313>

                 Summary: Complex number relation operators (=, <, >) yield
two logical trues
                 Project: GNU Octave
            Submitted by: sebald
            Submitted on: Sun 28 Sep 2014 03:02:43 AM GMT
                Category: Interpreter
                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: dev
        Operating System: Any

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Details:

This was first discovered by R. in the sort() routine, in which complex
numbers are first sorted by modulus and then angle.

The relational operators (=, <, >) for complex numbers in which the real
portion is negative and the imaginary portion is -0 for one value and +0 for
the other value show two cases that evaluate to true:


octave:137> complex(-1,0) == complex(-1,-0)
ans =  1
octave:138> complex(-1,0) > complex(-1,-0)
ans =  1
octave:139> complex(-1,0) < complex(-1,-0)
ans = 0


I believe only equality should be true, because the similar test for the
floating point scenario behaves that way:


octave:143> -0 == 0
ans =  1
octave:144> -0 > 0
ans = 0
octave:145> -0 < 0
ans = 0


First modulus is compared and if the two numbers have equal modulus then angle
is compared.  Given the behavior of atan2(), i.e.,


octave:152> atan2(0,-1)
ans =  3.1416
octave:153> atan2(-0,-1)
ans = -3.1416


the non-unique angle for the same number makes one of the inequality tests
pass.  It's best to use the principle argument, which is created by mapping
-pi of atan2() to pi before performing the test.




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