[R] REmove level with zero observations

[GL]

If I have a column with 2 levels, but one level has no remaining
observations. Can I remove the level? 

Had intended to do it as listed below, but soon realized that even though
there are no observations, the level is still there. 

For instance

summary(dbs3.train.sans.influential.obs$HAC) 

yields

0 ,1 
4685,0 

nlevels(dbs3.train.sans.influential.obs$HAC)

yields
[1] 2

drop.list <- NULL
for (i in 1:ncol(dbs3.train.sans.influential.obs)) {       
     if (nlevels(dbs3.train.sans.influential.obs[,i]) < 2) {drop.list <-
cbind(drop.list,i)}}

yields 
nothing because HAC still has two levels, even though there aren't any
observations in on of the levels.

What I want to do is loop through all columns that are factors and create a
list of items to drop because there will subsequently be < 2 levels when I
try to run a linear model.


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