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Groups > comp.lang.java.programmer > #35752
| From | Michael Jung <miju@golem.phantasia.org> |
|---|---|
| Newsgroups | comp.lang.java.programmer |
| Subject | Re: double == double |
| Date | 2016-10-13 21:10 +0200 |
| Organization | albasani.net |
| Message-ID | <8737k0qd38.fsf@golem.phantasia.org> (permalink) |
| References | <double-20161012130409@ram.dialup.fu-berlin.de> <ntledh$lfi$1@dont-email.me> |
Eric Sosman <esosman@comcast-dot-net.invalid> writes:
> On 10/12/2016 8:15 AM, Stefan Ram wrote:
>> Sometimes it is taught that it makes no sense to compare
>> two double values with »==«.
>> But what about this function?
>> public static double max( final double a, final double b)
>> { return
>> a == 0.0 && b == 0.0 && java.lang.Double.doubleToRawLongBits( a )
>> == Double.doubleToRawLongBits( -0.0d )? b : a >= b ? a : b; }
>>
>> which tries to return positive zero if one argument is
>> positive zero and the other one negative zero?
The situations, where you'd want max(+0,-0) = +0 and not -0 are
extremely limited, considering that +0 = -0, even for (Java) doubles.
>> It seems that there are special cases when a comparison
>> of double values with »==« is appropriate. I have observed
>> that this is usually the case when comparing with »0.0« -
>> another example would be avoiding to divide by 0.0.
In fact, one of the extremely rare applications of +0 and -0 is division
by them: 1/+0 = +Infinity , 1/-0 = -Infinity.
And, of course, "a == b iff a - b == 0.0" intuitively; allowing equality
on 0.0 opens it for all numbers.
Put in other words, using +0 and -0 as different turns doubles into
something they were not meant to be.
>
> The problem with comparing double or float values for equality
> (or inequality) isn't the values themselves, but their "provenance."
> Calculations that yield identical results when performed with real
> numbers and their infinite precision may yield different outcomes
> when finite-precision floating-point values are used instead. We
> know, for example, that `ln xy = ln x + ln y' for all positive x,y,
> yet we cannot rely on `Math.log(x*y) == Math.log(x) + Math.log(y)'.
> A rearrangement that appears innocuous can produce a result that is
> slightly different (sometimes more than "slightly!") from that of
> the original, and it's no simple matter to say which, if either,
> is "right."
Maybe it is easiest to see how fast you get there by evaluating the
following code:
public static void main(String[] args) {
System.out.println(0.1 + 0.2 == 0.3);
System.out.println(0.1 + 0.2 - 0.3 == 0);
}
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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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