Re: Unexpected NANs in complex arithmetic

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From	Chris Angelico <rosuav@gmail.com>
Newsgroups	comp.lang.python
Subject	Re: Unexpected NANs in complex arithmetic
Date	Wed, 22 Jun 2016 16:30:49 +1000
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On Wed, Jun 22, 2016 at 4:17 PM, Steven D'Aprano
<steve+comp.lang.python@pearwood.info> wrote:
> On Wednesday 22 June 2016 13:54, Dan Sommers wrote:
>
>> By the time Python returns a result for inf+3j, you're already in
>> trouble (or perhaps Python is already in trouble).
>
> I don't see why. It is possible to do perfectly sensible arithmetic on INFs.

Technically, arithmetic on INF is a short-hand for a limit. What is
inf+1? Well, it's the limit of x+1 as x tends toward +∞, which is ∞.
So inf+1 evaluates to inf.

>>> inf = float("inf")
>>> inf+1
inf

What's 5-inf? Same again - the limit of 5-x as x tends toward +∞.
That's tending in the opposite direction, so we get infinity the other
way:

>>> 5-inf
-inf

So it's not really "doing arithmetic on infinity", as such. That said,
I'm not entirely sure why "inf+3j" means we're already in trouble -
Dan, can you elaborate? ISTM you should be able to do limit-based
arithmetic, just the same.

ChrisA

Thread

Unexpected NANs in complex arithmetic Steven D'Aprano <steve@pearwood.info> - 2016-06-22 12:48 +1000
  Re: Unexpected NANs in complex arithmetic Chris Angelico <rosuav@gmail.com> - 2016-06-22 12:57 +1000
    Re: Unexpected NANs in complex arithmetic Dan Sommers <dan@tombstonezero.net> - 2016-06-22 03:54 +0000
      Re: Unexpected NANs in complex arithmetic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-06-22 16:17 +1000
        Re: Unexpected NANs in complex arithmetic Paul Rubin <no.email@nospam.invalid> - 2016-06-21 23:25 -0700
        Re: Unexpected NANs in complex arithmetic Chris Angelico <rosuav@gmail.com> - 2016-06-22 16:30 +1000
          Re: Unexpected NANs in complex arithmetic Dan Sommers <dan@tombstonezero.net> - 2016-06-22 22:54 +0000
    Re: Unexpected NANs in complex arithmetic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-06-22 16:58 +1000
      Re: Unexpected NANs in complex arithmetic Steven D'Aprano <steve+comp.lang.python@pearwood.info> - 2016-06-22 17:05 +1000
  Re: Unexpected NANs in complex arithmetic Antoon Pardon <antoon.pardon@rece.vub.ac.be> - 2016-06-22 09:14 +0200
  Re: Unexpected NANs in complex arithmetic Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2016-06-22 10:18 +0100

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