Hi,
I had a piece of code which looped over a decimal vector like this:
for( i in where ){
thisdata <- subset(herde, herde$mlr >= i)
# do stuff with thisdata..
}
'where' is a vector like seq(-1, 1, by=0.1)
My problem was: 'nrow(thisdata)' in loop repetition 0.4 was different if
'where' was seq(-1, 1, by=0.1) than when 'where' was seq(-0.8,
1, by=0.1)
It went away after I changed the first line to:
thisdata <- subset(herde, herde$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
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, > AlexA 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$mlr >= i) # do stuff with thisdata.. } 'where' is a vector like 0.1*((-10):10) [ instead of seq(-1, 1, by=0.1) ] and then, when you want to change to seq(-0.8, 1, by=0.1), use instead 0.1*(-80,10). Hoping this helps, Ted. -------------------------------------------------------------------- E-Mail: (Ted Harding) <ted.harding at wlandres.net> Fax-to-email: +44 (0)870 094 0861 Date: 31-Mar-11 Time: 12:52:35 ------------------------------ XFMail ------------------------------
On 11-03-31 7:24 AM, Alexander Engelhardt wrote:> Hi, > I had a piece of code which looped over a decimal vector like this: > > > for( i in where ){ > thisdata<- subset(herde, herde$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 > > ______________________________________________ > 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.