> 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-boun...@r-project.org [mailto:r-help-boun...@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@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. ______________________________________________ R-help@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.
