[R] Max within groups

[David Winsemius]

On Jul 16, 2011, at 9:55 AM, Thomas Chesney wrote:

> I know there's a really easy way to do this but I just can't track  
> it down. I experimented with various apply functions but couldn't  
> get it quite right.
>
> I have a matrix like this:
>
> 1, 16
> 1, 23
> 1, 21
> 1, 6
> 1, 25
> 2, 4
> 2, 17
> 2, 45
> 2, 11
> 2, 20

tapply(mtx[,2], mtx[,1], max)
>
> and I'd like to find the max value in Column 2 (or the index to the  
> max value) grouped by the first column. So the max for Group 1, the  
> max for Group 2 etc.
>
> Any help would be appreciated.
>
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David Winsemius, MD
West Hartford, CT