Deleting certain observations (and their imprint?)

-----Original Message-----
From: r-help-bounces at r-project.org [mailto:r-help-bounces at r-
project.org] On Behalf Of Stodola, Kirk
Sent: Thursday, November 29, 2012 5:32 PM
To: r-help at r-project.org
Subject: [R] Deleting certain observations (and their imprint?)

I'm manipulating a large dataset and need to eliminate some
observations based on specific identifiers.  This isn't a problem in
and of itself (using which.. or subset..) but an imprint of the deleted
observations seem to remain, even though they have 0 observations.
This is causing me problems later on.  I'll use the dataset warpbreaks
to illustrate, I apologize if this isn't in the best format

##Summary of warpbreaks suggests three tension levels (H, M, L)

summary(warpbreaks)

     breaks      wool   tension
 Min.   :10.00   A:27   L:18
 1st Qu.:18.25   B:27   M:18
 Median :26.00          H:18
 Mean   :28.15
 3rd Qu.:34.00
 Max.   :70.00

## Subset the dataset and keep only those observations with "L"

wb.subset <- warpbreaks[which(warpbreaks$tension=="L"),]

##Summary of the subsetted data shows: L=18, M=0, H=0, Why is M and H
still included?

summary(wb.subset)

     breaks      wool  tension
 Min.   :14.00   A:9   L:18
 1st Qu.:26.00   B:9   M: 0
 Median :29.50         H: 0
 Mean   :36.39
 3rd Qu.:49.25
 Max.   :70.00

##The subsetted dataset does not show M or H

wb.subset

Is there a way that M & H can be completely eliminated (i.e. they don't
show up in summary)? The only way I found was to export the dataset and
then reimport, which seems pretty cumbersome.  Thanks in advance for
any help.  -Kirk