[R] Problem about zero

Yen Lee b88207001 at ntu.edu.tw
Wed Jun 16 00:43:15 CEST 2010

Hello, everyone,

Thank you for all your kindness.

I've solved the problem through your help with the function all.equal.

Thank you very much!


-----Original Message-----
From: Gabor Grothendieck [mailto:ggrothendieck at gmail.com] 
Sent: Wednesday, June 16, 2010 6:40 AM
To: Yen Lee
Cc: r-help at r-project.org
Subject: Re: [R] Problem about zero

On Tue, Jun 15, 2010 at 6:00 PM, Yen Lee <b88207001 at ntu.edu.tw> wrote:
> Hello, everyone,
> There's a problem about zero in R and I really need your help.
> I have a vector shown as x=c(0.1819711,0.4811463,0.1935151,0.1433675),
> The sum of this vector is shown as 1 in R, but when I type 1-sum(x), the
> value is not zero, but -2.220446e-16.
> I can accept that this value is quite small and could be seen as zero, but
> there would be a problem when it's not really zero but a negative value in
> my algorithm.
> Therefore I would like to know that how could it be avoid.
> One way I think is to define the value 1-sum(x) as zero when it is smaller
> than a particular value, but the particular value is not be set yet.
> I would like to know more about the definition of the shown zero in R.

As others have pointed out please read the R FAQ.

Actually on my machine: Windows Vista running C2D BLAS, "R version
2.11.1 Patched (2010-05-31 r52167)"
I do get zero:

> x <- c(0.1819711,0.4811463,0.1935151,0.1433675)
> 1-sum(x)
[1] 0

Also check out sum.exact in the caTools package.

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