Remove rows from a single-column data frame

When I try to remove the last row from a single column data frame, I get a vector back instead of a data frame:

> df = data.frame(a=1:10)
> df
    a
1   1
2   2
3   3
4   4
5   5
6   6
7   7
8   8
9   9
10 10

> df[-(length(df[,1])),]
[1] 1 2 3 4 5 6 7 8 9

The behavior I'm looking for is what happens when I use this command on a two-column data frame:

> df = data.frame(a=1:10,b=11:20)
> df
    a  b
1   1 11
2   2 12
3   3 13
4   4 14
5   5 15
6   6 16
7   7 17
8   8 18
9   9 19
10 10 20

> df[-(length(df[,1])),]
  a  b
1 1 11
2 2 12
3 3 13
4 4 14
5 5 15
6 6 16
7 7 17
8 8 18
9 9 19

My code is general, and I don't know a priori whether the data frame will contain one or many columns. Is there a开发者_开发知识库n easy workaround for this problem that will let me remove the last row no matter how many columns exist?

Try adding the drop = FALSE option:

R> df[-(length(df[,1])), , drop = FALSE]
  a
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8 8
9 9