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

[Richard.Cotton at hsl.gov.uk]

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?

Regards,
Richie.

Mathematical Sciences Unit
HSL
Buxton
SK17 9JN
01298 21(x8672)


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