Message-ID: <29F81207-4E03-4AA5-A976-33237AB1D2FD@comcast.net>
Date: 2011-07-16T14:16:23Z
From: David Winsemius
Subject: Max within groups
In-Reply-To: <5EAA21940C65214F9C11DA5FBBC14F0B2561165A2C@EXCHANGE2.ad.nottingham.ac.uk>
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