[R] problem with basic boolean selection in sequence

[Richard.Cotton at hsl.gov.uk]

> I have a surprising problem while selecting values in a sequence created 

> with the seq() function...
>...
>  > test2<-seq(from=0,to=1,by=.1)
>...
>  > test2==0.3
>  [1] FALSE FALSE FALSE  FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>...
> Does anyones has an explanation and a solution ?

I suspect that this is a problem with floating point rounding errors.  The 
values in test2 are not exactly what you requested:
format(test2, nsmall=20)
#[1] "0.00000000000000000000" "0.10000000000000000555" 
"0.20000000000000001110"
#[4] "0.30000000000000004441" "0.40000000000000002220" 
"0.50000000000000000000"
#[7] "0.60000000000000008882" "0.70000000000000006661" 
"0.80000000000000004441"
#[10] "0.90000000000000002220" "1.00000000000000000000"

A workaround is to check that the difference between test2 and the target 
value is very small:
abs(test2 - .3) < .Machine$double.eps
# [1] FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE

Regards,
Richie.

Mathematical Sciences Unit
HSL


------------------------------------------------------------------------
ATTENTION:

This message contains privileged and confidential inform...{{dropped:20}}