[Rd] Definition of [[

[Duncan Murdoch]

On 15/03/2009 2:31 PM, Stavros Macrakis wrote:
> The semantics of [ and [[ don't seem to be fully specified in the
> Reference manual.  In particular, I can't find where the following
> cases are covered:
> 
>> cc <- c(1); ll <- list(1)
> 
>> cc[3]
> [1] NA
> OK, RefMan says: If i is positive and exceeds length(x) then the
> corresponding selection is NA.
> 
>> dput(ll[3])
> list(NULL)
> ? i is positive and exceeds length(x); why isn't this list(NA)?

Because the sentence you read was talking about "simple vectors", and ll 
is presumably not a simple vector.  So what is a simple vector?  That is 
not explicitly defined, and it probably should be.  I think it is 
"atomic vectors, except those with a class that has a method for [".

> 
>> ll[[3]]
> Error in list(1)[[3]] : subscript out of bounds
> ? Why does this return NA for an atomic vector, but give an error for
> a generic vector?
> 
>> cc[[3]] <- 34; dput(cc)
> c(1, NA, 34)
> OK
> 
> ll[[3]] <- 34; dput(ll)
> list(1, NULL, 34)
> Why is second element NULL, not NA?

NA is a length 1 atomic vector with a specific type matching the type of 
c.  It makes more sense in this context to put in a NULL, and return a 
list(NULL) for ll[3].

> And why is it OK to set an undefined ll[[3]], but not to get it?

Lots of code grows vectors by setting elements beyond the end of them, 
so whether or not that's a good idea, it's not likely to change.

I think an argument could be made that ll[[toobig]] should return NULL 
rather than trigger an error, but on the other hand, the current 
behaviour allows the programmer to choose:  if you are assuming that a 
particular element exists, use ll[[element]], and R will tell you when 
your assumption is wrong.  If you aren't sure, use ll[element] and 
you'll get NA or list(NULL) if the element isn't there.

> I assume that these are features, not bugs, but I can't find
> documentation for them.

There is more documentation in the man page for Extract, but I think it 
is incomplete.  The most complete documentation is of course the source 
code, but it may not answer the question of what's intentional and 
what's accidental.

Duncan Murdoch