On Sun, 15 Mar 2009, 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)?
I think some of these are because there are only NAs for character, logical,
and the numeric types. There isn't an NA of list type.
This one shouldn't be list(NA) - which NA would it use? It should be some sort
of list(_NA_list_) type, and list(NULL) is playing that role.
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?
Again, because there isn't an NA of generic vector type.
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?
And why is it OK to set an undefined ll[[3]], but not to get it?
Same reason for NULL vs NA. The fact that setting works may just be an
inconsistency -- as you can see from previous discussions, R often does not
effectively forbid code that shouldn't work -- or it may be bug-compatibility
with some version of S or S-PLUS.
-thomas
Thomas Lumley Assoc. Professor, Biostatistics
tlum...@u.washington.edu University of Washington, Seattle
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