[R] sampling problem - new to R

[Prof Brian Ripley]

On Tue, 5 Jun 2007, baldeck wrote:

> I have a data set of individual trees and the plots that they are in:
>
> Tree      Plot
> 56749    1
> 63494    1
> 87375    1
> 37494    2
> 92753    3
> 34847    3
> 38747    4 etc...

You haven't told us what form the 'data set' is, but I will presume a data 
frame called DF.

The obvious first step is to split by Plot.  Using 'resample' from ?sample

sapply(with(DF, split(Tree, Plot)), resample, size=1)

give a vector of trees ('individuals'?) with names the plots sampled from. 
That seems to be what you want, but if not please come back to us with a 
more extensive example including the desired output.


> So each plot is represented once for every individual that occurrs in it.
> Plots get different numbers of rows because there can be a different number
> of individuals in each plot.
>
> I want to make a data frame that consists of one individual from each plot.
> I would like to randomly choose one individual from each plot that is
> present in the data set.  I will have to do this to multiple data sets which
> may contain different plots, and may contain up to 1200 plots, so I can't
> choose the plots by hand.
>
> Please help me with this.  I'm an ecologist and I'm in Panama, with no one
> around who is educated in R.  Whoever solves this problem for me will be
> acknowledged in any resulting publications.
>
> Thanks!
> -Claire
>

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595