[R] combinations/ indexing a matrix

[Michael Friendly]

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?




-- 
Michael Friendly     Email: friendly AT yorku DOT ca 
Professor, Psychology Dept.
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