R help - Oct 2007 - Bug or just a problem with the computer precision?

Hi

I have the following problem: I have a lot of numbers that identify
import goods according to the Harmonised System (8 numbers in two groups
with a dot in between). I want to move to the 
6 numbers (4 numbers, dot, two numbers). My trick to do this works for
all the numbers in the Harmonised Sytem but not for this one:

A<-4709.9000
(A<-floor(100 * A ) / 100) = 4709.89

But this has to be 4709.90

If I do this in Excel it works fine...Any idea how to make sure that I
get 4709.90 and not 4709.89?

Renger



_________________________________________________

ECOPLAN 
Forschung und Beratung in Wirtschaft und Politik
Economic Research and Policy Consultancy

Thunstrasse 22 / CH-3005 Berne (Switzerland)
Phone: +41 31 356 61 61 / Fax: +41 31 356 61 60
mailto:renger at ecoplan.ch  / http://www.ecoplan.ch>

You are getting round-off problems (I think it is FAQ 7.33).  Use
characters since thats what the numbers really are. Convert the
characters and then use substring.  Also sprintf works:
> sprintf("%.2f", A*100)
[1] "470990.00"
> sprintf("%.2f", A)
[1] "4709.90"
>

On 10/4/07, Renger van Nieuwkoop <renger at ecoplan.ch>
wrote:
> Hi
>
> I have the following problem: I have a lot of numbers that identify
> import goods according to the Harmonised System (8 numbers in two groups
> with a dot in between). I want to move to the
> 6 numbers (4 numbers, dot, two numbers). My trick to do this works for
> all the numbers in the Harmonised Sytem but not for this one:
>
> A<-4709.9000
> (A<-floor(100 * A ) / 100) = 4709.89
>
> But this has to be 4709.90
>
> If I do this in Excel it works fine...Any idea how to make sure that I
> get 4709.90 and not 4709.89?
>
> Renger
>
>
>
> _________________________________________________
>
> ECOPLAN
> Forschung und Beratung in Wirtschaft und Politik
> Economic Research and Policy Consultancy
>
> Thunstrasse 22 / CH-3005 Berne (Switzerland)
> Phone: +41 31 356 61 61 / Fax: +41 31 356 61 60
> mailto:renger at ecoplan.ch  / http://www.ecoplan.ch>
>
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>

-- 
Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem you are trying to solve?

First, I think you are misusing numbers for what are probably character 
strings.  But with numbers,
> A <- 4709.9000
> sprintf("%7.2f", A)
[1] "4709.90"

R will never print a single number as 4709.90: it will drop the trailing 
zero.

With strings:
> A <- "4709.9000"
> substr(A, 1, 7)
[1] "4709.90"


On Thu, 4 Oct 2007, Renger van Nieuwkoop wrote:
> Hi
>
> I have the following problem: I have a lot of numbers that identify
> import goods according to the Harmonised System (8 numbers in two groups
> with a dot in between). I want to move to the
> 6 numbers (4 numbers, dot, two numbers). My trick to do this works for
> all the numbers in the Harmonised Sytem but not for this one:
>
> A<-4709.9000
> (A<-floor(100 * A ) / 100) = 4709.89
>
> But this has to be 4709.90
No, as 4709.9000 is not the exact number represented in R. Consider
> A <- 4709.9000
> round(A, 2)
[1] 4709.9
> 100*A - 470990
[1] -5.820766e-11

so A is represented a number slightly less than 4709.9000

> If I do this in Excel it works fine...Any idea how to make sure that I
> get 4709.90 and not 4709.89?
Then Excel would appear to have a different internal representation (or a 
bug).

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595