On 11-03-31 7:24 AM, 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$mlr>= i)
# do stuff with thisdata..
}
'where' is a vector like seq(-1, 1, by=0.1)
The solution to this problem is to take steps by representable numbers,
not by numbers like 0.1 that can't be represented exactly. For example,
seq(-1, 1, by=0.25) has exact entries, because fractions with small
powers of 2 in the denominator are all exactly representable. ("Small"
depends on the numerator, but for fractions between 0 and 1 it's about
52, so not really so small.)
Duncan Murdoch
My problem was: 'nrow(thisdata)' in loop repetition 0.4 was different if
'where' was seq(-1, 1, by=0.1) than when 'where' was seq(-0.8, 1, by=0.1)
It went away after I changed the first line to:
thisdata<- subset(herde, herde$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
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