[R] An index of all possible combinations of variables in a d ata frame

[Gabor Grothendieck]

And here is a variation on your idea of using the factors 
attribute of terms with some minor reductions, this one
returning a list of numeric vectors:

   fac <- attributes(terms(~ a * b * c))$factors
   apply(fac == 1, 2, which)

Liaw, Andy <andy_liaw <at> merck.com> writes:

: 
: This might be a bit of an abuse of the functions, but it seems to work:
: 
: > dat <- data.frame(A=1, B=2, C=3)
: > f <- ~ .^3
: > colnames(attr(terms(f, data=dat), "factors"))
: [1] "A"     "B"     "C"     "A:B"   "A:C"   "B:C"   "A:B:C"
: 
: Cheers,
: Andy
: 
: > From: Alan Simpson
: > 
: > Hello list
: >  
: > Does anybody know of any way to create an index of all the possible
: > combinations of variables (factors) in a data frame? ie for 3 
: > factors A, B &
: > C we have 
: >  
: > A
: > B
: > C
: > AB
: > AC
: > BC
: > ABC
: >  
: > which equates to columns 1, 2, 3, 1:2, (1,3), 2:3 and 1:3.
: >  
: > I realise that a function like model.matrix does this, but 
: > how to get the
: > seqence of the index?
: >  
: > Any help would be greatly appreciated.
: >  
: > Regards
: >  
: > Alan Simpson
: > Roberts Research Group
: >  
: > 
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: > 
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