Difference between a[[i]] and a[i]

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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