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* Round integer division
@ 2010-06-25 22:53 Lee Hachadoorian <lee.hachadoorian@gmail.com>
2010-06-25 23:00 ` Re: Round integer division Scott Marlowe <scott.marlowe@gmail.com>
2010-06-25 23:25 ` Re: Round integer division Adrian Klaver <adrian.klaver@gmail.com>
2010-06-26 13:09 ` Re: Round integer division Jasen Betts <jasen@xnet.co.nz>
0 siblings, 3 replies; 7+ messages in thread
From: Lee Hachadoorian @ 2010-06-25 22:53 UTC (permalink / raw)
To: pgsql-sql
Is it documented anywhere that floating-point numbers round
"scientifically", that is 0.5 rounds to the nearest even number? Compare:
SELECT round(2.5::real), round(2.5::numeric), round(3.5::real),
round(3.5::numeric);
generates
2 | 3 | 4 | 4
I stumbled across this when I was trying to use round(a::real/b::real)
to generate a rounded result to dividing integers, and noticed sometimes
0.5 was truncated and sometimes it was rounded up. Couldn't find
anything about this in the archives or the data type documentation. Is
there something obvious that I'm I missing?
Thanks,
--
Lee Hachadoorian
PhD Student, Geography
Program in Earth & Environmental Sciences
CUNY Graduate Center
^ permalink raw reply [nested|flat] 7+ messages in thread
* Re: Round integer division
2010-06-25 22:53 Round integer division Lee Hachadoorian <lee.hachadoorian@gmail.com>
@ 2010-06-25 23:00 ` Scott Marlowe <scott.marlowe@gmail.com>
2010-06-25 23:07 ` Re: Round integer division Lee Hachadoorian <lee.hachadoorian@gmail.com>
2 siblings, 1 reply; 7+ messages in thread
From: Scott Marlowe @ 2010-06-25 23:00 UTC (permalink / raw)
To: Lee Hachadoorian <lee.hachadoorian@gmail.com>; +Cc: pgsql-sql
On Fri, Jun 25, 2010 at 6:53 PM, Lee Hachadoorian
<lee.hachadoorian@gmail.com> wrote:
> Is it documented anywhere that floating-point numbers round
> "scientifically", that is 0.5 rounds to the nearest even number? Compare:
>
> SELECT round(2.5::real), round(2.5::numeric), round(3.5::real),
> round(3.5::numeric);
>
> generates
>
> 2 | 3 | 4 | 4
>
> I stumbled across this when I was trying to use round(a::real/b::real)
> to generate a rounded result to dividing integers, and noticed sometimes
> 0.5 was truncated and sometimes it was rounded up. Couldn't find
> anything about this in the archives or the data type documentation. Is
> there something obvious that I'm I missing?
That all floating point representations are approximate?
^ permalink raw reply [nested|flat] 7+ messages in thread
* Re: Round integer division
2010-06-25 22:53 Round integer division Lee Hachadoorian <lee.hachadoorian@gmail.com>
2010-06-25 23:00 ` Re: Round integer division Scott Marlowe <scott.marlowe@gmail.com>
@ 2010-06-25 23:07 ` Lee Hachadoorian <lee.hachadoorian@gmail.com>
2010-06-26 13:18 ` Re: Round integer division Jasen Betts <jasen@xnet.co.nz>
0 siblings, 1 reply; 7+ messages in thread
From: Lee Hachadoorian @ 2010-06-25 23:07 UTC (permalink / raw)
To: Scott Marlowe <scott.marlowe@gmail.com>; +Cc: pgsql-sql
On 06/25/2010 07:00 PM, Scott Marlowe wrote:
> That all floating point representations are approximate?
>
But if it's error due to approximation, shouldn't the result be random?
I tried this for a handful of larger numbers, and it appears to
consistently round to the even number. Wouldn't that have to be
intentionally programmed that way?
--Lee
--
Lee Hachadoorian
PhD Student, Geography
Program in Earth & Environmental Sciences
CUNY Graduate Center
^ permalink raw reply [nested|flat] 7+ messages in thread
* Re: Round integer division
2010-06-25 22:53 Round integer division Lee Hachadoorian <lee.hachadoorian@gmail.com>
2010-06-25 23:00 ` Re: Round integer division Scott Marlowe <scott.marlowe@gmail.com>
2010-06-25 23:07 ` Re: Round integer division Lee Hachadoorian <lee.hachadoorian@gmail.com>
@ 2010-06-26 13:18 ` Jasen Betts <jasen@xnet.co.nz>
0 siblings, 0 replies; 7+ messages in thread
From: Jasen Betts @ 2010-06-26 13:18 UTC (permalink / raw)
To: pgsql-sql
On 2010-06-25, Lee Hachadoorian <lee.hachadoorian@gmail.com> wrote:
>
>
> On 06/25/2010 07:00 PM, Scott Marlowe wrote:
>> That all floating point representations are approximate?
>>
> But if it's error due to approximation, shouldn't the result be random?
> I tried this for a handful of larger numbers, and it appears to
> consistently round to the even number. Wouldn't that have to be
> intentionally programmed that way?
Yes it is, an enginneer at intel made that decision in the 70s when
the 8087 coprocessor was developed.
If you're not running on x86-like hardware the round operation may
do something different.
^ permalink raw reply [nested|flat] 7+ messages in thread
* Re: Round integer division
2010-06-25 22:53 Round integer division Lee Hachadoorian <lee.hachadoorian@gmail.com>
@ 2010-06-25 23:25 ` Adrian Klaver <adrian.klaver@gmail.com>
2010-06-25 23:32 ` Re: Round integer division Lee Hachadoorian <lee.hachadoorian@gmail.com>
2 siblings, 1 reply; 7+ messages in thread
From: Adrian Klaver @ 2010-06-25 23:25 UTC (permalink / raw)
To: pgsql-sql; +Cc: Lee Hachadoorian <lee.hachadoorian@gmail.com>
On Friday 25 June 2010 3:53:01 pm Lee Hachadoorian wrote:
> Is it documented anywhere that floating-point numbers round
> "scientifically", that is 0.5 rounds to the nearest even number? Compare:
>
> SELECT round(2.5::real), round(2.5::numeric), round(3.5::real),
> round(3.5::numeric);
>
> generates
>
> 2 | 3 | 4 | 4
>
> I stumbled across this when I was trying to use round(a::real/b::real)
> to generate a rounded result to dividing integers, and noticed sometimes
> 0.5 was truncated and sometimes it was rounded up. Couldn't find
> anything about this in the archives or the data type documentation. Is
> there something obvious that I'm I missing?
>
> Thanks,
>
> --
>
> Lee Hachadoorian
> PhD Student, Geography
> Program in Earth & Environmental Sciences
> CUNY Graduate Center
See here for explanation:
http://archives.postgresql.org/pgsql-general/2010-03/msg00969.php
--
Adrian Klaver
adrian.klaver@gmail.com
^ permalink raw reply [nested|flat] 7+ messages in thread
* Re: Round integer division
2010-06-25 22:53 Round integer division Lee Hachadoorian <lee.hachadoorian@gmail.com>
2010-06-25 23:25 ` Re: Round integer division Adrian Klaver <adrian.klaver@gmail.com>
@ 2010-06-25 23:32 ` Lee Hachadoorian <lee.hachadoorian@gmail.com>
0 siblings, 0 replies; 7+ messages in thread
From: Lee Hachadoorian @ 2010-06-25 23:32 UTC (permalink / raw)
To: pgsql-sql
Thanks. That was a fairly recent post, too, but I couldn't come up with
the right keyword search to find it.
On 06/25/2010 07:25 PM, Adrian Klaver wrote:
> On Friday 25 June 2010 3:53:01 pm Lee Hachadoorian wrote:
>
>> Is it documented anywhere that floating-point numbers round
>> "scientifically", that is 0.5 rounds to the nearest even number?
>>
> See here for explanation:
> http://archives.postgresql.org/pgsql-general/2010-03/msg00969.php
>
>
--
Lee Hachadoorian
PhD Student, Geography
Program in Earth & Environmental Sciences
CUNY Graduate Center
^ permalink raw reply [nested|flat] 7+ messages in thread
* Re: Round integer division
2010-06-25 22:53 Round integer division Lee Hachadoorian <lee.hachadoorian@gmail.com>
@ 2010-06-26 13:09 ` Jasen Betts <jasen@xnet.co.nz>
2 siblings, 0 replies; 7+ messages in thread
From: Jasen Betts @ 2010-06-26 13:09 UTC (permalink / raw)
To: pgsql-sql
On 2010-06-25, Lee Hachadoorian <lee.hachadoorian@gmail.com> wrote:
> Is it documented anywhere that floating-point numbers round
> "scientifically", that is 0.5 rounds to the nearest even number?
That's swiss rounding. And no, as I understand it documented that
most arithmetic) is platform specific.
Postgres is written in C and the relevant portions of the
C standards douments (and discussions thereof) give a good
picture of the functioning of postgres arithmetic.
^ permalink raw reply [nested|flat] 7+ messages in thread
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2010-06-25 22:53 Round integer division Lee Hachadoorian <lee.hachadoorian@gmail.com>
2010-06-25 23:00 ` Scott Marlowe <scott.marlowe@gmail.com>
2010-06-25 23:07 ` Lee Hachadoorian <lee.hachadoorian@gmail.com>
2010-06-26 13:18 ` Jasen Betts <jasen@xnet.co.nz>
2010-06-25 23:25 ` Adrian Klaver <adrian.klaver@gmail.com>
2010-06-25 23:32 ` Lee Hachadoorian <lee.hachadoorian@gmail.com>
2010-06-26 13:09 ` Jasen Betts <jasen@xnet.co.nz>
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