Getting subsets of a data frame

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