[R] Interaction versus combinations

[Phil Spector]

Perhaps not the best or easiest way, but does:

> apply(t(combn(idx,2)),1,paste,collapse='.')
[1] "1.3" "1.6" "1.9" "3.6" "3.9" "6.9"

get you in the right direction?
 					- Phil Spector
 					 Statistical Computing Facility
 					 Department of Statistics
 					 UC Berkeley
 					 spector at stat.berkeley.edu


On Thu, 3 Jun 2010, Alex van der Spek wrote:

> I can get the interactions between factors like this:
>
>> idx=c(1,3,6,9)
>> jdx=idx
>> levels(interaction(idx,jdx,lex.order=TRUE))
> [1] "1.1" "1.3" "1.6" "1.9" "3.1" "3.3" "3.6" "3.9" "6.1" "6.3" "6.6" "6.9"
> [13] "9.1" "9.3" "9.6" "9.9"
>
> This list contains all possible interactions. Whereas I need only the 
> combinations, e.g 4 over 2 = 6 total being
>
> "1.3 " "1.6" "1.9" "3.6" "3.9" "6.9"
>
> I have been unable to find how to get the combinations. Of course with some 
> string manipulations this can be done for this toy example. There should be 
> something in R to make this easy?
>
> Thanks in advance
> Alex van der Spek
>
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