On 3/02/2009, at 12:45 PM, David Epstein wrote:
I'm sure I've read about the difference between a[[i]] and a[i] in
R, but I
cannot recall what I read. Even more disturbing is the fact that I
don't
know how to search the newsgroup for this. All the different
combinations I
tried were declared not to be valid search syntax.
Essentially: a is a list; a[i] is a list of length 1 whose sole entry is
the i-th entry of a; a[[i]]] is the i-th entry of a. If you are of a
mathematical bent it may be illuminating to think of this as being
analogous
to, say, {3} being a subset of the set {1,2,3,4,5} and 3 being an
element
of this set.
1. What sort of object can the operators [] and [[]] be applied to?
How do
they differ? I mean objects in standard R, not in packages that
provide
conceivable overloading of these operators (if that's possible).
Essentially lists. Note that any vector can be considered to be a list.
Note that data frames are lists.
2. Meta-question: how could one search for an answer to this
question in the
newsgroup?
RSiteSearch("[") or RSiteSearch("[[")
3. Meta-question: how could one search for an answer to this in the
FAQ?
Doesn't seem to be there, AFAICS.
4. Meta-question: how could one search for an answer to this in any
of the
links one is shown after typing help.start()?
Click on ``Search Engine and Keywords''; then search on ``[[''. Then
click on ``Extract''. This last is less than totally perspicuous. ;-)
Instead of trying to follow these links, just do
?"[" or ?"[["
HTH.
cheers,
Rolf Turner
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