R help - Apr 2008 - problem with basic boolean selection in sequence

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Hi

Do not use computers, they are full of such traps and do not do what we 
think they do.

See FAQ 7.31 Why doesn't R think these numbers are equal
Regards

Petr
petr.pikal at precheza.cz

r-help-bounces at r-project.org napsal dne 09.04.2008 11:32:00:
> Dear R users,
> 
> I have a surprising problem while selecting values in a sequence created 
> with the seq() function...
> I've tried a lot of test before sending this here and hope I did not 
> disturb you for a foolish mistake from me
> 
> Please, have a look at the following lines:
> #_______________________________________________________________________
>  > test2<-seq(from=0,to=1,by=.1)
>  > test2
>  [1] 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
>  > test2==1
>  [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE
>  > test2==0.9
>  [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE
>  > test2==0.8
>  [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE
>  > test2==0.7
>  [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>  > test2==0.6
>  [1] FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>  > test2==0.5
>  [1] FALSE FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE
>  > test2==0.4
>  [1] FALSE FALSE FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE
>  > test2==0.3
>  [1] FALSE FALSE FALSE  FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>  > test2==0.2
>  [1] FALSE FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>  > test2==0.1
>  [1] FALSE  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>  > test2==0.0
>  [1]  TRUE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE FALSE
>  >
> # there is a problem with values 0.3, 0.6 and 0.7 impossible to select 
> in such a vector...
> # the same thing happens with other sequence including somme of these 
values
> 
> test3<-seq(from=0,to=1,by=0.01)
> test3
> test3==0.3   # not working
> test3==0.6   # working
> test3==0.7   # not working
> 
#________________________________________________________________________
> 
> Does anyones has an explanation and a solution ?
> 
> Kind regards
> 
> Ben
> 
> 
> 
> 
>    [[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.

> 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


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