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Date: Thu, 30 Jun 2005 09:47:40 -0700
From: Yitzchak Scott-Thoennes <sthoenna AT efn DOT org>
To: cygwin AT cygwin DOT com
Subject: Re: Bug in printf ?
Message-ID: <20050630164740.GA3104@efn.org>
References: <20050630 DOT 202057 DOT 83613389 DOT haro AT kgt DOT co DOT jp> <20050630 DOT 202554 DOT 129758646 DOT haro AT kgt DOT co DOT jp> <8864897 DOT 1120133151080 DOT JavaMail DOT adm-moff AT moffice2 DOT nsc DOT no> <20050630 DOT 214457 DOT 02636322 DOT haro AT kgt DOT co DOT jp>
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On Thu, Jun 30, 2005 at 09:44:57PM +0900, haro AT kgt DOT co DOT jp wrote:
> ::Dealing with integers illustrates the matter more clearly. When
> ::the decimal value is exactly 0.5, then printf should round to the
> ::nearest *even* integer, as far as I know, so you should get
> 
> Hi Peter,
> 
> Thank you for your explanation. I didn't know, until now, that
> rounding should be done to the "nearest *even* integer".
> Need to learn every day. ;-)

This is done to prevent systemic bias.

However, note that
http://www.opengroup.org/onlinepubs/009695399/functions/printf.html says:
"The low-order digit shall be rounded in an implementation-defined manner."
so while people have come to expect unbiased rounding, it isn't actually
mandatory AIUI under vanilla SUSv3.

Also see "What Every Computer Scientist Should Know About
Floating-Point Arithmetic" at (among other places)
http://docs.sun.com/source/806-3568/ncg_goldberg.html.

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