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

[William Dunlap]

> class(mtcars[1:4,]) # select some rows
[1] "data.frame"
> class(mtcars[,1:4]) # select some columns
[1] "data.frame"
> class(mtcars[,3]) # select one column
[1] "numeric"
> class(mtcars[, 3, drop=FALSE]) # select one column
[1] "data.frame"

I cannot give a definitive reason why it is done this way, but you do need
some way to get from a data.frame to a column it contains and some way
to get from a data.frame to a single-column data.frame.  The above methods
do give you that choice.

Also note that rows and columns of data.frame are intrinsically different.
A column is generally a vector of one type while a row is a list of 1-long
vectors.

Bill Dunlap
Spotfire, TIBCO Software
wdunlap tibco.com


> -----Original Message-----
> From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-project.org] On Behalf
> Of C W
> Sent: Friday, April 12, 2013 12:33 PM
> To: r-help
> Subject: [R] Why copying columns of a data.frame becomes numeric?
> 
> 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
> 
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> 
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