Getting subsets of a data frame

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.
BTW,

sw <- swiss
sw[1,,drop=TRUE] *is* a list (not as claimed, but as documented)
sw[1, ]          is a data frame
sw[, 1]          is a numeric vector.

I should have pointed out that "[.data.frame" is in the See Also of Bill's 
reference.

BTW to Andy: a list is a vector, and Kurt and I recently have been trying 
to correct documentation that means `atomic vector' when it says `vector'.
(Long ago lists in R were pairlists and not vectors.)
is.vector(list(a=1))
[1] TRUE
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, Bill.Venables at csiro.au <Bill.Venables at csiro.au> 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: r-help-bounces at stat.math.ethz.ch
[mailto:r-help-bounces at stat.math.ethz.ch] 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,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

Getting subsets of a data frame

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