RE: [R] Getting subsets of a data frame

From: Liaw, Andy <andy_liaw_at_merck.com>
Date: Sat 16 Apr 2005 - 22:46:55 EST


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@csiro.au <Bill.Venables@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@stat.math.ethz.ch
> > [mailto:r-help-bounces@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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> > https://stat.ethz.ch/mailman/listinfo/r-help
> > PLEASE do read the posting guide!
> > http://www.R-project.org/posting-guide.html
> >
>
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>



R-help@stat.math.ethz.ch mailing list
https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide! http://www.R-project.org/posting-guide.html Received on Sat Apr 16 22:58:42 2005

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