[R] strange logical results

[Greg Snow]

This is FAQ 7.31 in the R FAQ.

Basically computers in general have problems exactly representing all
but a few floating point numbers, so while 2 numbers may look the same
when displayed (rounded to a couple of digits), they are different if
you look at them in full precision due to different roundings.
Functions like all.equal deal with this by checking to see if numbers
are close enough rather than equal.

The general rule is to not expect exact equality between floating point
numbers.  This is a problem for computers in general, not just R.

Hope this helps, 


-- 
Gregory (Greg) L. Snow Ph.D.
Statistical Data Center
Intermountain Healthcare
greg.snow at intermountainmail.org
(801) 408-8111
 

-----Original Message-----
From: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] On Behalf Of Erin Hodgess
Sent: Friday, December 29, 2006 3:36 AM
To: r-help at stat.math.ethz.ch
Subject: [R] strange logical results

Dear R People:  

I am getting some odd results when using logical operators:

> x <- seq(from=-1,to=1,by=0.1)

> > x
 [1] -1.0 -0.9 -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1  0.0  0.1  0.2
0.3  0.4 [16]  0.5  0.6  0.7  0.8  0.9  1.0
> x == -1
 [1]  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
FALSE [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> x == -0.9
 [1] FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
FALSE [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> x == -0.8
 [1] FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
FALSE [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> x == -0.7
 [1] FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
FALSE [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> x == -0.6
 [1] FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE
FALSE [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> x == -0.5
 [1] FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE
FALSE [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> x == -0.4
 [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
FALSE [13] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
> 

Should this show as true also, please?

I saw this in both Windows and LINUX Versions 2.4.0

Thanks in advance,
Sincerely,
Erin Hodgess
Associate Professor
Department of Computer and Mathematical Sciences University of Houston -
Downtown
mailto: hodgess at gator.uhd.edu

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