[R] Match numeric vector against rows in a matrix?

[David Winsemius]

On Jan 5, 2011, at 2:16 PM, Kevin Ummel wrote:

> Two posts in one day is not a good day...and this question seems  
> like it should have an obvious answer:
>
> I have a matrix where rows are unique combinations of 1's and 0's:
>
>> combs=as.matrix(expand.grid(c(0,1),c(0,1)))
>> combs
>     Var1 Var2
> [1,]    0    0
> [2,]    1    0
> [3,]    0    1
> [4,]    1    1
>
> I want a single function that will give the row index containing an  
> exact match with vector x:
>
>> x=c(0,1)

 > intersect( which(combs[,1]==x[1]), which(combs[,2]==x[2]) )
[1] 3

Or maybe even faster:

 >  which( combs[,1]==x[1] & combs[,2]==x[2])
[1] 3

>
> The solution needs to be applied many times, so I need something  
> quick -- I was hoping a base function would do it, but I'm drawing a  
> blank.
>

-- 

David Winsemius, MD
West Hartford, CT