[R] List indexing question

[R. Michael Weylandt]

It's a little funny, you actually need

x[[2]][1]

What's going on is the following:

lists can contain anything else in R, including more lists so
subsetting them takes a hair more work. x[2] returns the sublist of x
containing the second list element -- this is, however, not the same
as x[[2]] which truly returns the second element of x. The single
brackets allow more complex subsetting: x[1:2] would return the
sublist of first and second elements (still in a list) while x[[1:2]]
would be an error (because that doesn't really make any sense)

The way I heard this explained best is: if x is a train, x[2] is the
second car of the train, while x[[2]] is the contents of that car.

Hope this helps,
Michael

On Mon, May 21, 2012 at 8:07 PM, David Perlman <dperlman at wisc.edu> wrote:
> Consider the following:
>> x<-list(c(1,2,3),c(4,5,6))
>> x[1]
> [[1]]
> [1] 1 2 3
>
>> x[2]
> [[1]]
> [1] 4 5 6
>
> So far that all seems reasonable.  But now there's a problem.  I'm used to python, where I would say x[2][1] and get the value 4.  But I can't figure out how to do that in R.
>
>> x[2][1]
> [[1]]
> [1] 4 5 6
>
>> x[2,1]
> Error in x[2, 1] : incorrect number of dimensions
>
> I have no idea why x[2][1] returns the same thing as x[2]; that makes no sense to me at all.
>
> What is the proper syntax for what I'm trying to do?
>
> Thanks!
>
>
> -dave----------------------------------------------------------------------
> A neuroscientist is at the video arcade, when someone makes him a $1000 bet
> on Pac-Man. He smiles, gets out his screwdriver and takes apart the Pac-Man
> game. Everyone says "What are you doing?" The neuroscientist says "Well,
> since we all know that Pac-Man is based on electric signals traveling
> through these circuits, obviously I can understand it better than the other
> guy by going straight to the source!"
>
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