[R] Subsetting a data.frame degenerates at one column?

[Allen S. Rout]

Greetings.

At the moment, I'm applying R to some AIX 'nmon' output, trying to get
a handle on some disk performance metrics.  In case anyone's
interested:

http://docs.osg.ufl.edu/tsm/pdf/

some of them are more edifying than others. (ahem)

I'm trying to develop a somewhat general framework for plotting these
measures, in the hopes that it's of some use to people other than me.
One obstacle I encounter is that, when I select one column out of a
data.frame, the result is no longer a data.frame.  So, say I've got,
in data frame 'input'

      disk1 disk2 disk3 disk4
T0000     0     1     0     4
T0001     0     1     0     5
T0002     0     1     0     5
T0003     0     2     0     4
T0004     0     2     0     3
T0005     0     1     0     3
T0006     0     0     0     3

and somewhere I've noted a list

targets <- c('disk2','disk3')

I can say 

input[,targets]
      disk2 disk3
T0000     1     0
T0001     1     0
T0002     1     0
T0003     2     0
T0004     2     0
T0005     1     0
T0006     0     0

but if 

targets <- c('disk2')
input[,targets]
[1] 1 1 1 2 2 1 0

Ick.

I've been reading through the indexing and data.frame docs, and remain
unsatisfied so far.  Where is my thinking going wrong? 



- Allen S. Rout