[R] Why copying columns of a data.frame becomes numeric?

[Berend Hasselman]

On 12-04-2013, at 21:56, C W <tmrsg11 at 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 <bhh at xs4all.nl> wrote:
> 
> On 12-04-2013, at 21:32, C W <tmrsg11 at 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
> 
> 
>