[R] Working with combinations

[David Winsemius]

On Mar 2, 2010, at 9:15 PM, Herm Walsh wrote:

> I am working with the combinations function (available in the gtools  
> package).  However, rather than store all of the possible  
> combinations I would like to check each combination to see if it  
> meets a certain criteria.  If it does, I will then store it.
>
> I have looked at the combinations code but am unsure where in the  
> algorithm I would be able to operate on each combination.

Logical indexing:

 > combinations(3,2,letters[1:3])[,1]=="a"
[1]  TRUE  TRUE FALSE

 > combinations(3,2,letters[1:3])[ combinations(3,2,letters[1:3])[, 
1]=="a", ]
      [,1] [,2]
[1,] "a"  "b"
[2,] "a"  "c"

-- 
David Winsemius.