[R] Odd behaviour of removing 'nothing' from an array or data frame

[Peter Dalgaard]

Richard.Cotton at hsl.gov.uk writes:

> I've just found some behaviour which strikes me as odd, but I'm not sure 
> whether it's a bug or a feature.  If you don't mind, I'd like to explain 
> via a couple of examples.
> 
> Let x = 1:10.
> Then intuitively, to me at least, the command x[-integer(0)] should leave 
> x untouched.  However the actual output under R2.4.0 is integer(0).
> 
> A slightly more involved example demonstrates why I think this behaviour 
> is back to front.
> First we define a data frame, in this case some people, with their 
> heights.
> peoples.heights = data.frame(names = c("Alice", "Bob", "Carol"), heights = 
> c(1.67, 1.85, 175))
> 
> To make sure the heights are sensible, we define a filter out impossibly 
> tall people.
> dubious.records = which(peoples.heights$heights > 2.5)          #3
> peoples.heights = peoples.heights[-dubious.records,]
> 
> This all works fine since dubious.records is not empty.  However, if all 
> the records had been entered properly, then we would get
> #dubious.records = integer(0)
> 
> Then the command peoples.heights = peoples.heights[-dubious.records,] 
> strips all the rows to give
> #[1] names   heights
> #<0 rows> (or 0-length row.names)
> 
> i.e. instead of removing the bad records, I've lost everything.
> I know that it's possible to recode this so problems don't occur, but the 
> point is that the answer is unexpected.
> 
> Can anybody explain if this behaviour is intentional or useful in some 
> way, or is it an oversight?

Consistency! It's not particularly useful, but it follows from general
principles, which it in the long run doesn't pay to depart from.

The issue is that the result of using an indexing operator ("[")
should depend only on the _value_ of its argument, not the expression
used to compute it. Just like you most likely expect log(2+2) not to be
different from log(4). And since

> dubious.records <- integer(0)
> identical(dubious.records, -dubious.records)
[1] TRUE

how can peoples.heights[-dubious.records,] be different from
peoples.heights[dubious.records,]? 

R could actually look at the expression and act on the minus sign, but
that way lies madness. Consider

keep <- -dubious.records
drop <- dubious.records

peoples.heights[keep,]
peoples.heights[-dubious.records,]
peoples.heights[-keep,]

etc... I think you'll get the picture.

-- 
   O__  ---- Peter Dalgaard             Øster Farimagsgade 5, Entr.B
  c/ /'_ --- Dept. of Biostatistics     PO Box 2099, 1014 Cph. K
 (*) \(*) -- University of Copenhagen   Denmark          Ph:  (+45) 35327918
~~~~~~~~~~ - (p.dalgaard at biostat.ku.dk)                  FAX: (+45) 35327907