how to get each column as data.frame (instead of a vector) from a data.frame?

Question

Normally when you get a column, it is a vector. How can I keep it as the data.frame with the same row names and corresponding column name?

Answer 1

Instead of calling the desired column with a comma ie data.frame[,i] use data.frame[i] to preserve the class as data.frame and also retain row names.

data.frame[,i] #As a vector
data.frame[i] #As a data.frame

Answer 2

use the argument drop = FALSE as in:

mtcars[, 1, drop = FALSE]

Answer 3

If you specify a single number when subsetting a data.frame, you get a one-column data.frame. This is different than matrix subsetting, which requires a "missing" i argument to return the entire column (which it then converts to a vector).

# mtcars is a data.frame
mtcars[1]       # first column
str(mtcars[1])  # is still a data.frame
# 'data.frame':   32 obs. of  1 variable:
#  $ mpg: num  21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ...
# MTCARS is a matrix
MTCARS <- as.matrix(mtcars)
as.matrix(MTCARS)[1]        # only the first element
# [1] 21
str(as.matrix(MTCARS)[,1])  # the first column, as a vector
 Named num [1:32] 21 21 22.8 21.4 18.7 18.1 14.3 24.4 22.8 19.2 ...
 - attr(*, "names")= chr [1:32] "Mazda RX4" "Mazda RX4 Wag" "Datsun 710" ...