[R] That dreaded floating point trap
(Ted Harding)
ted.harding at wlandres.net
Thu Mar 31 13:52:38 CEST 2011
On 31-Mar-11 11:24:01, Alexander Engelhardt wrote:
> Hi,
> I had a piece of code which looped over a decimal vector like this:
>
> for( i in where ){
> thisdata <- subset(herde, herde$mlr >= i)
> # do stuff with thisdata..
> }
>
> 'where' is a vector like seq(-1, 1, by=0.1)
>
> My problem was: 'nrow(thisdata)' in loop repetition 0.4 was different
> if 'where' was seq(-1, 1, by=0.1) than when 'where' was
> , when you wan
> It went away after I changed the first line to:
>
> thisdata <- subset(herde, herde$mlr >= round(i, digits=1))
>
> This is that "floating point trap" the R inferno pdf talked about,
> right? That file talked about the problem, but didn't offer a solution.
>
> Similar things happened when I created a table() from a vector with
> values in seq(-1, 1, by=0.1)
>
> Do I really have to round every float at every occurence from now on,
> or is there another solution? I only found all.equal() and identical(),
> but I want to subset for observations with a value /greater/ than
> something.
>
> Thanks in advance,
> Alex
A very straightforward way to avoid this problem is to construct the
sequence by multiplying a sequence of integers by an approriate
constant. E.g. for your first example:
for( i in where ){
thisdata <- subset(herde, herde$mlr >= i)
# do stuff with thisdata..
}
'where' is a vector like 0.1*((-10):10)
[ instead of seq(-1, 1, by=0.1) ]
and then, when you want to change to seq(-0.8, 1, by=0.1),
use instead 0.1*(-80,10).
Hoping this helps,
Ted.
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Date: 31-Mar-11 Time: 12:52:35
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