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R: Deprecate logical subsetting?

Since I discovered it, I have found logical subsetting in R a very elegant idiom. Recently I’ve had a change of heart due to two observations. Here is why I propose staying away from it.

First of all, there is this question on stackoverflow which proves that subsetting numerically using which is faster than logical subsetting.

I know this may not be a good enough reason for some of you. It wasn’t for me either till I found this bug.1 It’s a subtle, and therefore, the sinister kind. See the code snippet below to reproduce the bug.

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## Suppose you get an empty data.frame from somewhere. Probably from
## logical subsetting itself. For example:
R> xx <- data.frame("u"=runif(10), "n"=rnorm(10))

## Now let's get an empty data.frame:
R> xx[FALSE]
data frame with 0 columns and 10 rows

# But wait! That's got 10 rows? Anyway, let's continue.
R> xx <- xx[FALSE]

## Now, you are happily coding away when you find your code
## splutters and throws some garbage error message.
## What happened? May be this:

R> yy3 <- xx[c(TRUE, FALSE, TRUE, FALSE, TRUE), ]

R> yy3
'data.frame':   3 obs. of  0 variables

R> nrow(yy3) # Btw, NROW, dim all have this bug.
[1] 3

R> attributes(yy3)
$names
character(0)

$row.names
[1] "NA"   "NA.1" "NA.2"

$class
[1] "data.frame"

## You, the oblivious one, have checked for nrow and go to do
## something innocent such as this:

R> sum(yy[1, ])
Error in FUN(X[[1L]], ...) :
  only defined on a data frame with all numeric variables
Calls: Summary.data.frame -> lapply -> FUN

## What the #@$*@ !!!

And you are left wondering where you called summary in your code.

Even if one discounts efficiency, it may be helpful to give up on the idiom of logical subsetting for reasons of correctness. At least for data.frames.

  1. Or at least I’ll consider this a bug. ↩