[R] Removing and restoring factor levels

[Duncan Murdoch]

I'm doing a big slow computation, and profiling shows that it is 
spending a lot of time in match(), apparently because I have code like

x %in% listofxvals

Both x and listofxvals are factors with the same levels, so I could 
probably speed this up by stripping off the levels and just treating 
them as integer vectors, then restoring the levels at the end.

What is the safest way to do this?  I am worried that at some point x 
and listofxvals will *not* have the same levels, and the optimization 
will give the wrong answer.  So I need code that guarantees they have 
the same coding.

I think this works, where "master" is a factor with the master list of 
levels (guaranteed to be a superset of the levels of x and listofxvals), 
but can anyone spot anything that might go wrong?

# Strip the levels
x <- as.integer( factor(x), levels = levels(master) )

# Restore the levels
x <- structure( x, levels = levels(master), class = "factor" )

Thanks for any advice...

Duncan Murdoch