R help - Jul 2006 - seq unexpected behavior

seq(0.1, 0.9 - 0.8, by = 0.1) gives the following error message:

Error in seq.default(0.1, 0.9 - 0.8, by = 0.1) : 
        wrong sign in 'by' argument

but seq(0.1, 0.8 - 0.7, by = 0.1) gives
[1] 0.1
(no error message)

Why do I get an error message in the first case?
Han


> sessionInfo()
R version 2.2.1, 2005-12-20, i386-pc-mingw32

attached base packages:
[1] "methods"   "stats"     "graphics" 
"grDevices" "utils"
"datasets" 
[7] "base"   

(NB I also tried version 2.3.1 and got the same result - both versions
are precompiled)
  
> Sys.getlocale()
[1] "LC_COLLATE=English_United States.1252;LC_CTYPE=English_United
States.1252;LC_MONETARY=English_United
States.1252;LC_NUMERIC=C;LC_TIME=English_United States.1252"


--------------------

This email message is for the sole use of the intended recip...{{dropped}}

On Wed, 2006-07-26 at 18:35 -0700, Vries, Han de wrote:
> seq(0.1, 0.9 - 0.8, by = 0.1) gives the following error message:
> 
> Error in seq.default(0.1, 0.9 - 0.8, by = 0.1) : 
>         wrong sign in 'by' argument
> 
> but seq(0.1, 0.8 - 0.7, by = 0.1) gives
> [1] 0.1
> (no error message)
> 
> Why do I get an error message in the first case?
> Han

See R FAQ 7.31 Why doesn't R think these numbers are equal?
> print(0.9 - 0.8, 20)
[1] 0.09999999999999997780
> print(0.8 - 0.7, 20)
[1] 0.10000000000000008882


In the first case, the result of the subtraction is slightly less than
0.1, resulting in a negative interval. In the second case, it is
slightly greater than 0.1, which is OK.

HTH,

Marc Schwartz