RE: [R] Getting subsets of a data frame

[Prof Brian Ripley]

Perhaps Fernando will also note that is documented in ?"[.data.frame",
a slightly more appropriate reference than Bill's.
It would be a good idea to read a good account of R's indexing: Bill 
Venables and I know of a couple you will find in the R FAQ.

On Sat, 16 Apr 2005, Liaw, Andy wrote:
Because a data frame can hold different data types (even matrices) in
different variables, one row of it can not be converted to a vector in
general (where all elements need to be of the same type).
Andy
From: Fernando Saldanha
Thanks, it's interesting reading.
I also noticed that
sw[, 1, drop = TRUE] is a vector (coerces to the lowest dimension)
but
sw[1, , drop = TRUE] is a one-row data frame (does not convert it into
a list or vector)
FS
On 4/16/05, [EMAIL PROTECTED] <[EMAIL PROTECTED]> wrote:
You should look at
?"["
and look very carefully at the "drop" argument.  For your example
sw[, 1]
is the first component of the data frame, but
sw[, 1, drop = FALSE]
is a data frame consisting of just the first component, as
mathematically fastidious people would expect.
This is a convention, and like most arbitrary conventions
it can be very
useful most of the time, but some of the time it can be a very nasty
trap.  Caveat emptor.
Bill Venables.
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On Behalf Of
Fernando Saldanha
Sent: Saturday, 16 April 2005 1:07 PM
To: Submissions to R help
Subject: [R] Getting subsets of a data frame
I was reading in the Reference Manual about Extract.data.frame.
There is a list of examples of expressions using [ and [[, with the
outcomes. I was puzzled by the fact that, if sw is a data
frame, then
sw[, 1:3]
is also a data frame,
but
sw[, 1]
is just a vector.
Since R has no scalars, it must be the case that 1 and 1:1
are the same:

1 == 1:1
[1] TRUE
Then why isn't sw[,1] = sw[, 1:1] a data frame?
FS
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--
Brian D. Ripley,                  [EMAIL PROTECTED]
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
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