Path: csiph.com!x330-a1.tempe.blueboxinc.net!usenet.pasdenom.info!aioe.org!eternal-september.org!feeder.eternal-september.org!mx04.eternal-september.org!.POSTED!not-for-mail
From: Eric Sosman <esosman@ieee-dot-org.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Rounding values
Date: Tue, 10 Jan 2012 08:06:58 -0500
Organization: A noiseless patient Spider
Lines: 36
Message-ID: <jehd5k$1sd$1@dont-email.me>
References: <913cfe41-9fe7-4a3e-8ce3-52d468d92a30@r5g2000yqc.googlegroups.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit
Injection-Date: Tue, 10 Jan 2012 13:07:00 +0000 (UTC)
Injection-Info: mx04.eternal-september.org; posting-host="HSlJAUb3pGXi3i7ZL/HoAw"; logging-data="1933"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX18vxDfqdOfu1kaURFZjTldz"
User-Agent: Mozilla/5.0 (Windows NT 5.1; rv:9.0) Gecko/20111222 Thunderbird/9.0.1
In-Reply-To: <913cfe41-9fe7-4a3e-8ce3-52d468d92a30@r5g2000yqc.googlegroups.com>
Cancel-Lock: sha1:8IGXGwUFDITFM+SwKnJtPSwY3jQ=
Xref: x330-a1.tempe.blueboxinc.net comp.lang.java.programmer:11166

On 1/10/2012 7:43 AM, refreegrata wrote:
> Hi list. I'm a newbie with Java, just reading about BigDecimals.
> I have a question. I can't get a correct rounding operation.
>
> For example:
> ----------------------------------------------------------------------------------------------
> new BigDecimal(0.705).setScale(2,RoundingMode.HALF_UP);
> is 0.70
> ----------------------------------------------------------------------------------------------
> And
> ----------------------------------------------------------------------------------------------
> new BigDecimal(0.705).round(new MathContext(2, RoundingMode.HALF_UP));
> is also 0.70
> ----------------------------------------------------------------------------------------------
>
> However, with other values, like 1.705, the value is rounding to 1.71;
> I don't know if this happens only with values between zero to one. I
> still can't replicate the problem with greatest values. Maybe don't
> happens, or maybe happens with other values too. To me is a problem
> don't be sure about this.

     Try `new BigDecimal("0.705")' and similarly; note the "" marks.

     I suspect what you're seeing is not caused by BigDecimal itself,
but by small approximation errors in the representation of the `double'
values.  The system uses binary (base 2) for `double' numbers, and
just as there is no finite decimal representation of 3/7 there is no
finite binary representation of 705/1000 = 141/200.  Unable to provide
an exact 705/1000, the system does its best by giving you a nearby
value with as small an error as it can manage, but it will not be
exactly halfway between 700/1000 and 710/1000.  The damage has been
done before BigDecimal even arrives on the scene.

-- 
Eric Sosman
esosman@ieee-dot-org.invalid