[R] Re: coding factor replicates

[Douglas Bates]

Douglas Bates <bates at cs.wisc.edu> writes:

> 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.

Before others point out the obvious simplification (I did it in stages
and assembled the "swish" result, as Bill would term it - apparently
swish has a different connotation in Australia than it does in North
America), the second line could be

> sp <- split(result, f)   # split the index vector on factor levels
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