how to remove columns from data frame in R

Question

I am trying to remove columns from data.frame. The following expression works

a1$"STR_COL_4_" <- NULL

but

xx <- "STR_COL_4_"   
a1$xx <- NULL

does not work...what is the problem ???

By using `$`, the name, in this case `xx` would need to be in `a1`. It is not evaluated but treated as literal so is not found — user20650, Nov 22 '14 at 00:14
typing 'a$xx' R will search for a columns named explicitely 'xx', and it does not exist. — agenis, Nov 22 '14 at 00:15
@RogerFilmyer - The basics are explained in `help("$")` in the *Recursive objects* section — Rich Scriven, Nov 22 '14 at 00:17

score 1 · Answer 1 · answered Nov 22 '14 at 00:21

As Richard Scriven, user20650, and agenis have pointed out, when you use $ to refer to a column in a data frame, it merely looks for that column in the data frame. In your second example, R looks for a column named xx and fails, even though there is already a variable named xx. Using [[ ]] lets R evaluate an expression.

As an example using the builtin cars dataset:

> data(mtcars)
> 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   
> var <- "wt"
> mtcars$var
NULL
> mtcars[[var]]
 [1] 2.620 2.875 2.320 3.215 3.440 3.460 3.570 3.190 3.150 3.440 3.440 4.070
[13] 3.730 3.780 5.250 5.424 5.345 2.200 1.615 1.835 2.465 3.520 3.435 3.840
[25] 3.845 1.935 2.140 1.513 3.170 2.770 3.570 2.780

how to remove columns from data frame in R

1 Answers1