From tlumley at u.washington.edu  Fri Aug  8 18:41:37 2008
From: tlumley at u.washington.edu (Thomas Lumley)
Date: Fri, 8 Aug 2008 09:41:37 -0700 (PDT)
Subject: [Rd] Truncation error on simply mean & sum function (PR#12515)
In-Reply-To: <20080808113004.3B2FA282EFC2@mail.pubhealth.ku.dk>
References: <20080808113004.3B2FA282EFC2@mail.pubhealth.ku.dk>
Message-ID: <Pine.LNX.4.64.0808080940000.29914@homer24.u.washington.edu>


Not a bug. None of these numbers are exactly representable in binary, so 
you can't expect accuracy of better than machine epsilon.

Please don't report things that aren't bugs to r-bugs.

 	-thomas



On Fri, 8 Aug 2008, samaya6501 at yahoo.com.tw wrote:

> Full_Name: Tom Wang
> Version: 2.6.2
> OS: Linux, Windows
> Submission from: (NULL) (61.230.6.228)
>
>
> I've found an instance:
>
>> a <- c(0.187,-0.019,0.074,-0.06,0.221,-0.079,0.12,0.079,-0.281,-0.242)
>> sum(a)
> [1] -2.428613e-17
>
> The actual sum is 0 but it reports the wrong answer.
>
> Maybe it is due to the computer truncation error.
>
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>

Thomas Lumley			Assoc. Professor, Biostatistics
tlumley at u.washington.edu	University of Washington, Seattle



[Rd] Truncation error on simply mean & sum function (PR#12515)
Thomas Lumley