R help - Mar 2011 - Floating points and floor() ?

Perhaps somebody could clarify for me if the following is a floating
point matter or otherwise, and how am I to correct for it?
> floor(100*.1)
[1] 10
> 100*(1.0-.9)
[1] 10
> floor(100*(1-0.9))
[1] 9


Thanks!
Michael
_______________________________________________________
Michael Folkes
Salmon Stock Assessment
Canadian Dept. of Fisheries & Oceans     
Pacific Biological Station
3190 Hammond Bay Rd.
Nanaimo, B.C., Canada
V9T-6N7
Ph (250) 756-7264 Fax (250) 756-7053  Michael.Folkes@dfo-mpo.gc.ca


	[[alternative HTML version deleted]]

You are assuming that 0.1 is exactly represented. Since it is not, either 0.1 or
1-0.9 will be less than 0.1.

As to what you should do about it... that depends on what you are trying to
accomplish. Whatever it is, you need to use some other approach. Often the
alternative involves using integers as much as possible.
---------------------------------------------------------------------------
Jeff Newmiller The ..... ..... Go Live...
DCN:<jdnewmil@dcn.davis.ca.us> Basics: ##.#. ##.#. Live Go...
Live: OO#.. Dead: OO#.. Playing
Research Engineer (Solar/Batteries O.O#. #.O#. with
/Software/Embedded Controllers) .OO#. .OO#. rocks...1k
---------------------------------------------------------------------------
Sent from my phone. Please excuse my brevity.

"Folkes, Michael" <Michael.Folkes@dfo-mpo.gc.ca> wrote:

Perhaps somebody could clarify for me if the following is a floating point
matter or otherwise, and how am I to correct for it? > floor(100*.1) [1] 10
> 100*(1.0-.9) [1] 10 > floor(100*(1-0.9)) [1] 9 Thanks!
Michael_____________________________________________
Michael Folkes Salmon Stock Assessment Canadian Dept. of Fisheries & Oceans
Pacific Biological Station 3190 Hammond Bay Rd. Nanaimo, B.C., Canada V9T-6N7 Ph
(250) 756-7264 Fax (250) 756-7053 Michael.Folkes@dfo-mpo.gc.ca 	[[alternative
HTML version deleted]]_____________________________________________
R-help@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.


	[[alternative HTML version deleted]]

On Mar 3, 2011, at 8:23 PM, Folkes, Michael wrote:
> Perhaps somebody could clarify for me if the following is a floating
> point matter or otherwise, and how am I to correct for it?
>
>> floor(100*.1)
> [1] 10
>
>> 100*(1.0-.9)
> [1] 10
>
>> floor(100*(1-0.9))
> [1] 9
>
>
Yes. It's a "floating point matter". What do you mean by
"correct for
it"? What result would be "correct"?


David Winsemius, MD
Heritage Laboratories
West Hartford, CT

Hi Michael,

In floating point calculation, 1.0-.9 is not exactly 0.1.  This is easily seen
by subtracting.
> (1.0-.9)-0.1
[1] -2.775558e-17
> (1.0-.9)==0.1
[1] FALSE

David is right, you can't "correct" this.  You can only compensate
by taking care that you never, ever test whether 2 FP numbers are equal, because
they almost never are.  You must always ask whether the difference is small.
> round(1.0-.9-.1,15)==0
[1] TRUE

Unfortunately, most of us forget this rule once in a while and write a loop like
"while (x!=0)..." that won't terminate.
HTH
Rex



-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org]
On Behalf Of Folkes, Michael
Sent: Thursday, March 03, 2011 9:24 PM
To: r-help at r-project.org
Subject: [R] Floating points and floor() ?

Perhaps somebody could clarify for me if the following is a floating
point matter or otherwise, and how am I to correct for it?
> floor(100*.1)
[1] 10
> 100*(1.0-.9)
[1] 10
> floor(100*(1-0.9))
[1] 9


Thanks!
Michael
_______________________________________________________
Michael Folkes
Salmon Stock Assessment
Canadian Dept. of Fisheries & Oceans
Pacific Biological Station
3190 Hammond Bay Rd.
Nanaimo, B.C., Canada
V9T-6N7
Ph (250) 756-7264 Fax (250) 756-7053  Michael.Folkes at dfo-mpo.gc.ca


        [[alternative HTML version deleted]]

______________________________________________
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.




message may contain confidential information. If you are not the designated
recipient, please notify the sender immediately, and delete the original and any
copies. Any use of the message by you is prohibited.

On Thu, Mar 03, 2011 at 06:23:36PM -0800, Folkes, Michael
wrote:
> Perhaps somebody could clarify for me if the following is a floating
> point matter or otherwise, and how am I to correct for it?
> 
> > floor(100*.1)
> [1] 10
> 
> > 100*(1.0-.9)
> [1] 10
> 
> > floor(100*(1-0.9))
> [1] 9
As others pointed out, 0.1 is not exactly representable in base 2,
so we get

  formatC(0.1, digits=20)
  [1] "0.10000000000000000555"

  formatC(100*0.1, digits=20, width=-1)
  [1] "10"

  formatC(100*(1 - 0.9), digits=20)
  [1] "9.9999999999999982236"

A correct result may be obtained, if you reorganize your
calculation, so that all intermediate results are integers
and the inaccurate division is only the last operation. Then,
floor(n/10) will be correct and also n %/% 10 may be used.

Alternatively, if you work with decimal numbers with 1 or 2
decimal digits, then also floor(round(x, 1)) or floor(round(x, 2))
work correctly, if x is not too large.

See FAQ 7.31

 
http://cran.r-project.org/doc/FAQ/R-FAQ.html#Why-doesn_0027t-R-think-these-numbers-are-equal_003f

and

  http://rwiki.sciviews.org/doku.php?id=misc:r_accuracy:decimal_numbers

for further examples and some hints.

Hope this helps.

Petr Savicky.