[R] Re: coding factor replicates

[David Brahm]

Brad Buchsbaum <brad at aris.ss.uci.edu> has a factor:
  R> f <- factor(c("A","B","C","B","C","A","C","B","A","A","B"))
and wants to create a vector (same length as f) to indicate trial number:
  1 1 1 2 2 2 3 3 3 4 4

Here's a 2-line solution, done with matrices and cumsum():
  R> m <- outer(f, unique(f), "==")
  R> apply(m * apply(m, 2, cumsum), 1, sum)
     [1] 1 1 1 2 2 2 3 3 3 4 4

With package "colSums" loaded, the second line could use rowSums() instead.
Is that "swish"?
-- 
                              -- David Brahm (brahm at alum.mit.edu)
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