R help - Jul 2011 - Max within groups

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

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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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.
>
> ThomasThis message and any attachment are intended solely for the  
> addressee and may contain confidential information. If you have  
> received this message in error, please send it back to me, and  
> immediately delete it.   Please do not use, copy or disclose the  
> information contained in this message or in any attachment.  Any  
> views or opinions expressed by the author of this email do not  
> necessarily reflect the views of the University of Nottingham.
>
> This message has been checked for viruses but the contents of an  
> attachment
> may still contain software viruses which could damage your computer  
> system:
> you are advised to perform your own checks. Email communications  
> with the
> University of Nottingham may be monitored as permitted by UK  
> legislation.
> ______________________________________________
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David Winsemius, MD
West Hartford, CT