For a package application, I want to generate all 1-way, or 2-way, ...
combinations of factors, symbolically, as a matrix.
E.g., all two-way terms among 4 factors.

> factors <- LETTERS[1:4]
> combn(4,2)
  [,1] [,2] [,3] [,4] [,5] [,6]
[1,]   1   1   1   2   2   3
[2,]   2   3   4   3   4   4

But I want to replace the numbers I in the above by factors[I]. I know
I can do this with gtools::combinations:

> t(combinations(4,2,factors))
  [,1] [,2] [,3] [,4] [,5] [,6]
[1,] "A" "A" "A" "B" "B" "C"
[2,] "B" "C" "D" "C" "D" "D"

But I'd prefer not to have to require an extra package if there is a
simpler way using base R, e.g., by indexing
the result of combn() using factors. I tried, among other non-working
things:

> res <- combn(4,2)
> res[] <- LETTERS[1:4]
> res
  [,1] [,2] [,3] [,4] [,5] [,6]
[1,] "A" "C" "A" "C" "A" "C"
[2,] "B" "D" "B" "D" "B" "D"

right shape, but wrong result. What is the magic incantation here?




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Michael Friendly   Email: friendly AT yorku DOT ca
Professor, Psychology Dept.
York University    Voice: 416 736-5115 x66249 Fax: 416 736-5814
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