On 12-04-2013, at 21:56, C W <tmrs...@gmail.com> wrote:
> What if my data is much larger, and I don't know what column number but know
> its name? Do I have to grep its by name?
> How about subset()? Is that what people commonly use?
>
Continuing with mtcars and the desired columns
b <- mtcars[,c("mpg","cyl","drat","wt")]
Berend
> Mike
>
> On Fri, Apr 12, 2013 at 3:44 PM, Berend Hasselman <b...@xs4all.nl> wrote:
>
> On 12-04-2013, at 21:32, C W <tmrs...@gmail.com> wrote:
>
> > Dear list,
> >
> > I want the 1st, 2nd, 5th, and 6th columns of mtcars. After copying them,
> > the columns become numeric class rather than data frame.
> >
> > But, when I copy rows, they data frame retains its class. Why is this? I
> > don't see why copying rows vs columns is so different.
> >
> >> class(mtcars)
> > [1] "data.frame"
> >> head(mtcars)
> > mpg cyl disp hp drat wt qsec vs am gear carb
> > Mazda RX4 21.0 6 160 110 3.90 2.620 16.46 0 1 4 4
> > Mazda RX4 Wag 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4
> > Datsun 710 22.8 4 108 93 3.85 2.320 18.61 1 1 4 1
> > Hornet 4 Drive 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1
> > Hornet Sportabout 18.7 8 360 175 3.15 3.440 17.02 0 0 3 2
> > Valiant 18.1 6 225 105 2.76 3.460 20.22 1 0 3 1
> >> a <- mtcars$mpg
> >> class(a)
> > [1] "numeric"
>
> Here you are assigning a single column of mtcars, which is a numeric vector,
> to another object. So that is a numeric vector.
>
> >> b <- mtcars[1:5, ]
> >> class(b)
> > [1] "data.frame"
> >
>
> Here you are assigning a couple of rows of the complete dataframe and the
> result is a dataframe.
>
> If you want the 1st, 2nd, 5th, and 6th columns of mtcars in a new datafrmae
> why don't you do this:
>
> a <- mtcars[,c(1,2,5,6)]
>
> then
>
> > class(a)
> [1] "data.frame"
>
>
> Berend
>
>
>
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