Question

Following code subsets a dataframe, but includes the row names in the subset:

mpg = subset(mtcars, cyl=='6', select=mpg)

How can I convert the resulting mpg object to a vector containing the values only? In other words, I need to get rid of the row names from the resulting mpg object

Answer 1

You have a number of options.

Firstly, since you are using subset, you can specify the argument drop=TRUE, but see the warning later in this answer):

subset(mtcars, cyl=='6', select=mpg, drop=TRUE)
[1] 21.0 21.0 21.4 18.1 19.2 17.8 19.7

But in the longer run you should get familiar with the R subset operators [, [[, and $:

mtcars[mtcars$cyl==6, "mpg"]
[1] 21.0 21.0 21.4 18.1 19.2 17.8 19.7

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And you can simplify that last line of code by using the function with (but, again, see the warning):

with(mtcars, mtcars[cyl==6, "mpg"])
[1] 21.0 21.0 21.4 18.1 19.2 17.8 19.7

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Warning: Some aspects of subset and with may be considered harmful. All objects are first evaluated in the context of the data frame. So, if you happen to use a variable with the same name as a column in your data frame, your expression will evaluate to the column name, not your variable:

with(mtcars,mtcars[cyl==6,"mpg"])
[1] 21.0 21.0 21.4 18.1 19.2 17.8 19.7

gear <- 6
with(mtcars,mtcars[cyl==gear,"mpg"])
[1] 22.8 24.4 22.8 32.4 30.4 33.9 27.3 21.4

subset(mtcars, cyl==gear, select=mpg, drop=TRUE)
[1] 22.8 24.4 22.8 32.4 30.4 33.9 27.3 21.4

The with and subset call has got gear from the data frame, and not from where you set it to 6.