[R] 2D look up (within interval)

[David Winsemius]

On Apr 29, 2010, at 1:58 PM, YasirKaheil wrote:
> Hi,
> I have a matrix of X and Y coordinates, and one pair of coordinates  
> that I
> need to know it corresponds (or is closest) to which row in the  
> matrix. For
> example:
> MC<- cbind(rep(1:1000,each=1000),rep(1:1000,1000));
> p<-c(543.1,440.05);
>
> I know that if you do which.min you'll get the answer, which will be  
> row #
> 542440, but which.min is slow and this process needs to be repeated  
> a lot.

It didn't seem to be particularly slow when I tried it:

 > which.min( abs(MC[,1]-p[1])+ abs(MC[,2]-p[2]) )   # L1 metric
[1] 542440

system.time( which.min(abs(MC[,1]-p[1])+ abs(MC[,2]-p[2])) )
    user  system elapsed
   0.068   0.034   0.103

What was your code and experience with it?


> I tried using match and findInterval and %in% but I don't know how I  
> can use
> them to find in 2 columns simultanuously.
-- 

David Winsemius, MD
West Hartford, CT