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 [[alternative HTML version deleted]]
Sarah Goslee
2013-Apr-12 19:42 UTC
[R] Why copying columns of a data.frame becomes numeric?
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$mpg >> class(a) > [1] "numeric" >> b <- mtcars[1:5, ] >> class(b) > [1] "data.frame" > > > Thanks a lot, > Mike >-- Sarah Goslee http://www.functionaldiversity.org
Berend Hasselman
2013-Apr-12 19:44 UTC
[R] Why copying columns of a data.frame becomes numeric?
On 12-04-2013, at 21:32, C W <tmrsg11 at gmail.com> wrote:> Dear list, > > I want the 1st, 2nd, 5th, and 6th columns of mtcars. After copying them, > the columns become numeric class rather than data frame. > > But, when I copy rows, they data frame retains its class. Why is this? I > don't see why copying rows vs columns is so different. > >> class(mtcars) > [1] "data.frame" >> head(mtcars) > mpg cyl disp hp drat wt qsec vs am gear carb > Mazda RX4 21.0 6 160 110 3.90 2.620 16.46 0 1 4 4 > Mazda RX4 Wag 21.0 6 160 110 3.90 2.875 17.02 0 1 4 4 > Datsun 710 22.8 4 108 93 3.85 2.320 18.61 1 1 4 1 > Hornet 4 Drive 21.4 6 258 110 3.08 3.215 19.44 1 0 3 1 > Hornet Sportabout 18.7 8 360 175 3.15 3.440 17.02 0 0 3 2 > Valiant 18.1 6 225 105 2.76 3.460 20.22 1 0 3 1 >> a <- mtcars$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
William Dunlap
2013-Apr-12 19:44 UTC
[R] Why copying columns of a data.frame becomes numeric?
> 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 > > [[alternative HTML version deleted]] > > ______________________________________________ > R-help at r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code.