Groups > comp.lang.java.programmer > #35769

Re: double == double

From	4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid>
Newsgroups	comp.lang.java.programmer
Subject	Re: double == double
Date	2016-10-15 17:36 +0100
Organization	A noiseless patient Spider
Message-ID	<nttlum$u6r$2@dont-email.me> (permalink)
References	(2 earlier) <8737k0qd38.fsf@golem.phantasia.org> <87zim5es30.fsf@golem.phantasia.org> <ntta21$osp$1@dont-email.me> <nttemi$mt3$1@gioia.aioe.org> <nttfev$bgj$1@dont-email.me>

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On 15/10/2016 15:46, Eric Sosman wrote:

[...]
>     That is, I contend there's no substitute for understanding.
> My own understanding is imperfect -- the field called Numerical
> Analysis is large, intricate, and subtle -- but nonetheless I feel
> that dicta like "Never" are, er, "Always" out of place.  (Almost.)

My opinion is that suggesting that == should *never* be used to test 
equality for doubles is totally sensible. I only object to "it doesn't 
make sense", from a formal point of view. If you choose - as the golden 
rule prescribes - a small enough delta, you can still test equality 
between two identical values. If a developer understands how fp numbers 
are represented internally (e.g IEEE 754), they also understand why two 
different representations of the same number are going to fail the 
comparison, while using == for two identical representations (e.g. two 
literals) makes perfect sense.

The problem, in some cases, is how to choose that delta. I was 
developing a 3D geometry library in C++ a while ago. The problem I had 
was testing the library. I had to test for equality with 0.0 when 
checking whether a point belonged to a plane or a rect was the result of 
the intersection between two planes. Determining the delta to use turned 
out to be extremely difficult, if not impossible. I ended up tranforming 
my classes into templates with a generic type for the coordinate, and 
write tests using integers instead of doubles. I'd be curious to know if 
there can be a better solution.

-- 
4ndre4

Thread

Re: double == double Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-10-12 09:39 -0400
  Re: double == double Michael Jung <miju@golem.phantasia.org> - 2016-10-13 21:10 +0200
    Re: double == double Michael Jung <miju@golem.phantasia.org> - 2016-10-15 14:04 +0200
      Re: double == double Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-10-15 09:13 -0400
        Re: double == double Arne Vajhøj <arne@vajhoej.dk> - 2016-10-15 10:32 -0400
          Re: double == double Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-10-15 10:46 -0400
            Re: double == double Michael Jung <miju@golem.phantasia.org> - 2016-10-15 17:44 +0200
            Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-15 17:36 +0100
              Re: double == double Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-10-15 14:15 -0400
                Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-15 19:41 +0100
            Re: double == double Arne Vajhøj <arne@vajhoej.dk> - 2016-10-15 20:39 -0400
      Re: double == double Arne Vajhøj <arne@vajhoej.dk> - 2016-10-15 10:41 -0400
        Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-15 23:09 +0100
          Re: double == double Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-10-15 18:28 -0400
            Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-16 03:13 +0100
              Re: double == double Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-10-16 08:37 -0400
                Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-16 14:06 +0100
          Re: double == double Arne Vajhøj <arne@vajhoej.dk> - 2016-10-15 20:30 -0400
            Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-16 03:16 +0100
              Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-16 03:18 +0100
              Re: double == double Arne Vajhøj <arne@vajhoej.dk> - 2016-10-16 09:45 -0400
                Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-16 14:51 +0100
                Re: double == double Arne Vajhøj <arne@vajhoej.dk> - 2016-10-16 11:24 -0400
                Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-16 16:43 +0100
                Re: double == double Arne Vajhøj <arne@vajhoej.dk> - 2016-10-16 12:54 -0400
                Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-16 18:37 +0100
                Re: double == double Arne Vajhøj <arne@vajhoej.dk> - 2016-10-16 14:14 -0400
                Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-16 22:19 +0100
                Re: double == double Arne Vajhøj <arne@vajhoej.dk> - 2016-10-16 22:49 -0400
                Re: double == double a.laforgia@gmail.com - 2016-10-17 01:53 -0700
                Re: double == double Patrick Roemer <sangamon@netcologne.de> - 2016-10-17 19:04 +0200
                Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-17 20:17 +0100
                Re: double == double Patrick Roemer <sangamon@netcologne.de> - 2016-10-17 23:11 +0200
                Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-17 22:46 +0100
                Re: double == double Patrick Roemer <sangamon@netcologne.de> - 2016-10-18 18:55 +0200
                Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-18 18:52 +0100
                Re: double == double Patrick Roemer <sangamon@netcologne.de> - 2016-10-20 15:03 +0200
                True O-O [Was: double == double] Eric Sosman <esosman@comcast-dot-net.invalid> - 2016-10-17 18:06 -0400
                Re: True O-O Patrick Roemer <sangamon@netcologne.de> - 2016-10-18 18:32 +0200
                Re: True O-O [Was: double == double] Arne Vajhøj <arne@vajhoej.dk> - 2016-10-19 22:09 -0400
                Re: double == double Arne Vajhøj <arne@vajhoej.dk> - 2016-10-19 21:58 -0400
                Re: double == double Patrick Roemer <sangamon@netcologne.de> - 2016-10-17 18:58 +0200
                Re: double == double 4ndre4 <626401fd-4bc7-4f8f-b641-563ae0f72920@626401fd-4bc7-4f8f-b641-563ae0f72920.invalid> - 2016-10-17 20:32 +0100
                Re: double == double Patrick Roemer <sangamon@netcologne.de> - 2016-10-17 23:21 +0200
                Re: double == double Arne Vajhøj <arne@vajhoej.dk> - 2016-10-19 21:38 -0400
                Re: double == double Arne Vajhøj <arne@vajhoej.dk> - 2016-10-19 21:47 -0400
                Re: double == double Patrick Roemer <sangamon@netcologne.de> - 2016-10-20 15:01 +0200

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