[R] eliminating constant variables

[pdb]

Hi Jim, 

Thanks for your response, although I was probably not clear about exactly
what I want to achieve, please let me see if I can explain a little
better...

There are certain (unknown) columns in my data that contain either NULL in
every row, or the same value in every row (eg '1'). These columns are
useless for modelling as there is no variation in the data.

I need a way to automatically find and delete all these columns (it is not
rows I want to delete, but the whole column, as in 

train$Variablexxx = NULL

where Variablexxx needs to be automatically found.

Thanks in advance,

pdb
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