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

[Sarah Goslee]

It's another reason not to use $ for extraction.

By default, R reduces dimensionality when subsetting, so mtcars$mpg
actually returns a numeric vector. With $, there's no way to override
the default behavior.

> data(mtcars)
> a <- mtcars$mpg
> class(a)
[1] "numeric"
> dim(a)
NULL
>
> a <- mtcars[, "mpg", drop=FALSE]
> class(a)
[1] "data.frame"
> dim(a)
[1] 32  1

Sarah

On Fri, Apr 12, 2013 at 3:32 PM, 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"
>> b <- mtcars[1:5, ]
>> class(b)
> [1] "data.frame"
>
>
> Thanks a lot,
> Mike
>

-- 
Sarah Goslee
http://www.functionaldiversity.org