R help - Mar 2009 - numeric equality

Dear all,

 

I am totally confused by the following R output, but don't have a clue
for it.

 
> a <- 1 - 0.2
> a == 0.8
[1] TRUE
> a <- 1 - 0.8
> a == 0.2
[1] FALSE
> a <- 1 - 0.5
> a == 0.5
[1] TRUE
> a <- 1 - 0.6
> a == 0.4
[1] TRUE
> a <- 1 - 0.9
> a == 0.1
[1] FALSE

 

 

My R version is Windows XP R version 2.8.1 (2008-12-22).

 

Thanks very much.

 

 

Changhong


==================================
P Please consider the environment before printing this e-mail

Cleveland Clinic is ranked one of the top hospitals
in America by U.S. News & World Report (2008).  
Visit us online at http://www.clevelandclinic.org for
a complete listing of our services, staff and
locations.


Confidentiality Note:  This message is intended for use\...{{dropped:16}}

This is a FAQ:
http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f
and also covered nicely in Burns' R Inferno:
www.burns-stat.com/pages/Tutor/R_inferno.pdf

Sarah

On Wed, Mar 18, 2009 at 11:58 AM, Yu, Changhong <YuC at ccf.org>
wrote:
> Dear all,
>
>
>
> I am totally confused by the following R output, but don't have a clue
> for it.
>
>
>
>> a <- 1 - 0.2
>
>> a == 0.8
>
> [1] TRUE
>
>> a <- 1 - 0.8
>
>> a == 0.2
>
> [1] FALSE
>
>> a <- 1 - 0.5
>
>> a == 0.5
>
> [1] TRUE
>
>> a <- 1 - 0.6
>
>> a == 0.4
>
> [1] TRUE
>
>> a <- 1 - 0.9
>
>> a == 0.1
>
> [1] FALSE
>
>
>
>
>
> My R version is Windows XP R version 2.8.1 (2008-12-22).
>
>
>
> Thanks very much.
>
>
>
>
>
> Changhong
>
>
-- 
Sarah Goslee
http://www.functionaldiversity.org

On Wed, Mar 18, 2009 at 8:58 AM, Yu, Changhong <YuC at ccf.org>
wrote:
> Dear all,
> I am totally confused by the following R output, but don't have a clue
> for it.
>
>> a <- 1 - 0.2
>
>> a == 0.8
>
> [1] TRUE
>
>> a <- 1 - 0.8
>
>> a == 0.2
>
> [1] FALSE
This is expected behaviour.  The bottom line is that you shouldn't
rely on the equality operator when dealing with floating point values
in any numerical software, including R.

I write about this in detail at:

http://blog.revolution-computing.com/2009/03/when-is-a-zero-not-a-zero.html

which will hopefully clear up your questions.

# David Smith

-- 
David M Smith <david at revolution-computing.com>
Director of Community, REvolution Computing www.revolution-computing.com
Tel: +1 (206) 577-4778 x3203 (Seattle, USA)

Check out our upcoming events schedule at www.revolution-computing.com/events

On Wed, 2009-03-18 at 11:58 -0400, Yu, Changhong wrote:
> Dear all,
> 
>  
> 
> I am totally confused by the following R output, but don't have a clue
> for it.
> 
>  
> 
> > a <- 1 - 0.2
> 
> > a == 0.8
> 
> [1] TRUE
> 
> > a <- 1 - 0.8
> 
> > a == 0.2
> 
> [1] FALSE
Hi Yu,

First of all read FAQ 7.31 (Why doesn't R think these numbers are
equal?)

Second, in this case, use all.equal

a <- 1 - 0.8

a == 0.2
[1] FALSE

all.equal(a,0.2)
[1] TRUE

-- 
Bernardo Rangel Tura, M.D,MPH,Ph.D
National Institute of Cardiology
Brazil