[R] That dreaded floating point trap

[(Ted Harding)]

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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