On 2024-09-06 12:44 a.m., Richard O'Keefe wrote:
I expect that atan(1i) = (0 + infinity i) and that atan(1i)/5 = (0 +
infinity i)/5 = (0 + infinity i).
Here's what I get in C:
(0,1) = (0, 1)
atan((0,1)) = (0, inf)
atan((0,1))/5 = (0, inf)
Note the difference between I*infinity = (0,1)*infinity =
(0*infinity,1*infinity) = (NaN,infinity)
and (0,infinity)/5 = (0/5,infinity/5) = (0,infinity).
The former involves multiplying 0 by infinity, which yields NaN.
The latter does not.
complex(1,0,Inf)*2
[1] NaN+Infi
There is no good reason for this. 0*2 is 0, not NaN.
In IEEE arithmetic, multiplying or dividing a complex number by a real number is
NOT the same as multiplying or dividing by the complex version of that
real number.
(0,Inf) * 2 = (0*2, Inf*2) = (0, Inf).
(0,Inf) * (2,0) = (0*2 - Inf*0, 0*0 + Inf*2) = (NaN, Inf).
There really truly is a bug here, and it is treating R*Z, Z*R, and Z/R
as if they were
the same as W*Z, Z*W, and Z/W where W = complex(1,R,0).
I would only disagree with the statement above by distinguishing between
a "bug" (where R is not behaving as documented) and a "design flaw"
(where it is behaving as documented, but the behaviour is undesirable).
I think this is a design flaw rather than a bug.
The distinction is important: if it is a design flaw, then a change is
harder, because users who rely on the behaviour deserve more help in
adapting than those who rely on a bug. Bugs should be fixed. Design
flaws need thinking about, and sometimes shouldn't be fixed.
On the other hand, I was unable to find documentation saying that the
current behaviour is intended, so I could be wrong.
Duncan Murdoch
On Fri, 6 Sept 2024 at 10:12, Bert Gunter <bgunter.4...@gmail.com> wrote:
Perhaps
Inf*1i
[1] NaN+Infi
clarifies why it is *not* a bug.
(Boy, did that jog some long dusty math memories :-) )
-- Bert
On Thu, Sep 5, 2024 at 2:48 PM Duncan Murdoch <murdoch.dun...@gmail.com>
wrote:
On 2024-09-05 4:23 p.m., Leo Mada via R-help wrote:
Dear R Users,
Is this desired behaviour?
I presume it's a bug.
atan(1i)
# 0+Infi
tan(atan(1i))
# 0+1i
atan(1i) / 5
# NaN+Infi
There's no need to involve atan() and tan() in this:
> (0+Inf*1i)/5
[1] NaN+Infi
Why do you think this is a bug?
Duncan Murdoch
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