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

[Liaw, Andy]

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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