Say I have a data.frame:
df <- data.frame(A=c(10,20,30),B=c(11,22,33), C=c(111,222,333))
A B C
1 10 11 111
2 20 22 222
3 30 33 333
If I select two (or more) columns I get a data.frame:
x <- df[,1:2]
A B
1 10 11
2 20 22
3 30 33
This is what I want. However, if I select only one column I get a numeric vector:
x <- df[,1]
[1] 1 2 3
===================
Use drop=FALSE
> x <- df[,1, drop=FALSE]
> x
A
1 10
2 20
3 30
From the documentation (see ?"[") you can find:
If drop=TRUE the result is coerced to the lowest possible dimension.
===================
Omit the ,:
x <- df[1]
A
1 10
2 20
3 30
From the help page of ?"[":
Indexing by [ is similar to atomic vectors and selects a list of the specified element(s).
A data frame is a list. The columns are its elements.
===================
REF:
https://stackoverflow.com/questions/21025609/how-do-i-extract-a-single-column-from-a-data-frame-as-a-data-frame