Try this:

aggregate(dt$value, list(long = dt$long, lat = dt$lat), FUN = sum)

On Mon, Jan 12, 2009 at 1:52 PM, Steve Murray <smurray444@hotmail.com>wrote:

>
> Dear all,
>
>
>
> I have a dataframe of 3 columns, consisting of 'longitude', 'latitude'
> and a corresponding 'value'. Where identical 'longitude' and 'latitude'
> pairs occur more than once, I want their corresponding 'value' to be
> summed and the 'pair' to only appear once.
>
>
>
> For example:
>
>
>
> long   lat   value
>
> 10     20      5
>
> 6      2       3
>
> 27    -3       9
>
> 10     20      10
>
> 4     -1       0
>
> 6      2       9
>
>
>
>
>
> would be converted to something like:
>
>
>
> long   lat   value
>
>
> 10     20      15
>
>
> 6      2       12
>
>
> 27    -3       9
>
>
> 4     -1       0
>
>
>
>
>
>
> ...as rows 1 and 4, 2 and 6 respectively are matched with respect to
> the 'long' and 'lat' columns. Their values in column 3 are then summed
> and reported as one row in the new dataframe.
>
>
>
> How would I go about coding this in R?
>
>
>
> Many thanks,
>
>
>
> Steve
>
>
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>



-- 
Henrique Dallazuanna
Curitiba-Paraná-Brasil
25° 25' 40" S 49° 16' 22" O

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