Getting subsets of a data frame

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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