[R] Numbers that look equal, should be equal, but if() doesn't see as equal (repost with code included)

[Paul Lemmens]

Hoi Thomas,

--On woensdag 28 mei 2003 7:16 -0700 Thomas Lumley 
<tlumley at u.washington.edu> wrote:

> On Wed, 28 May 2003, Paul Lemmens wrote:
>
>> Hi!
>>
>> Apologies for sending the mail without any code. Apparently somewhere
>> along the way the .R attachments got filtered out. I have included the
>> code below as clean as possible. My original mail is below the code.
>
> I still think you need not to be using ==.  You want something like
>
> if ( abs(mean.b-mean.orig)/(epsilon+abs(mean.orig) < epsilon){
>
> You are effectively using epsilon=0, but epsilon=10e-10 should be
> adequate.
>
Based on all the hints and explanations I've changed the test to 
'identical(all.equal(mean.b, mean.orig, tolerance=.Machine$double.eps), 
FALSE)'.

I still need to look into the concept of finite precision, because I still 
don't grasp how sometimes (as an extreme example, probably) 0.25 != 1/4. 
That this will happen for a number with a lot of different decimals I can 
understand (by an accumulation of rounding errors).


thnx all 4 your help!



-- 
Paul Lemmens
NICI, University of Nijmegen              ASCII Ribbon Campaign /"\
Montessorilaan 3 (B.01.03)                    Against HTML Mail \ /
NL-6525 HR Nijmegen                                              X
The Netherlands                                                 / \
Phonenumber    +31-24-3612648
Fax            +31-24-3616066