[Octave-bug-tracker] [bug #59890] inf+1i*inf gives nan+1i*inf

[Rik]

Update of bug #59890 (project octave):

                Priority:              5 - Normal => 3 - Low                
              Item Group:        Incorrect Result => Matlab Compatibility   
                  Status:                    None => Confirmed              

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Follow-up Comment #9:

The decision comes down to whether it is worth the programmer time to
implement an exception for scalar multiplication of complex numbers.  I
believe (and jwe wrote this in comment #1) that we just forward scalar
multiplication to the C++ math library.  There, the rules for scalar
multiplication say that 0 x Inf = NaN so a complex scalar value with a 0 real
becomes NaN.  Matlab, by contrast, seems to not perform multiplications when
one of the values is zero.

For vectors and matrices, Matlab is equivalent to Octave and just forwards to
a BLAS library.  Taking jwe's example,


x = [i,2,3], y = Inf(3,1), x*y
Matlab: NaN + NaNi
Octave: NaN - NaNi


Aside from a difference in the sign of the complex value, this is the same
result.  However, now change to using scalar multiplication which because of
the orientation of the vectors also results in broadcasting.


x = [i,2,3], y = Inf(3,1), x.*y
Matlab:
[ 0 + Infi, Inf + 0i, Inf + 0i
  0 + Infi, Inf + 0i, Inf + 0i
  0 + Infi, Inf + 0i, Inf + 0i ]

Octave:
[ NaN + Infi, Inf - NaNi, Inf - NaNi
[ NaN + Infi, Inf - NaNi, Inf - NaNi
[ NaN + Infi, Inf - NaNi, Inf - NaNi ]


At the core of the difference are these four multiplications which in Matlab
result in


complex (0,0) * Inf
ans = NaN
complex (0,1) * Inf
ans = 0 + Infi
complex (1,0) * Inf
ans = Inf
complex (1,1) * Inf
ans = Inf + Infi


In Octave the results are


complex (0,0) * Inf
ans = NaN - NaNi
complex (0,1) * Inf
ans = NaN + Infi
complex (1,0) * Inf
ans = Inf - NaNi
complex (1,1) * Inf
ans = Inf + Infi


I'm changing the Item Group to Matlab Compatibility since we are different in
this regard, and marking as Confirmed.  But I'm lowering the priority since I
think there are more interesting bugs to fix ahead of this one.

 

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