[R] Re: coding factor replicates

[Douglas Bates]

Uwe Ligges <ligges at statistik.uni-dortmund.de> writes:

> Brad Buchsbaum wrote:
> > 
> > Hi All,
> > 
> > If I have a factor f:
> > 
> > A B C B C A C B A A B ....
> > 
> > and I would like to generate a factor to indicate the trial number
> > as a function of condition: e.g.
> > 
> > 1 1 1 2 2 2 3 3 3 4 4 ...
> > 
> > how might I attack this in R?
> 
> What about something like
>   as.factor(outer(rep(1, 3), 1:4))

I think the point is that the 1's are at the first occurence of the
level, the 2's at the second occurence, etc.  This seems like the sort
of problem that Bill Venables would come up with a devilishly clever
way of solving.

I would do it as

> result <- seq(along = f)         # create an vector to hold the response
> sp <- split(seq(along = f), f)   # split the factor on levels
> result[unlist(sp)] <- unlist(lapply(sp, function(x) seq(along = x)))
> result
 [1] 1 1 1 2 2 2 3 3 3 4 4

but I'm sure Bill would do it much more elegantly than that.

-- 
Douglas Bates                            bates at stat.wisc.edu
Statistics Department                    608/262-2598
University of Wisconsin - Madison        http://www.stat.wisc.edu/~bates/
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