[R] extracting index list when using tapply()

jim holtman jholtman at gmail.com
Wed Jul 9 01:55:59 CEST 2008

Working code would help.  I would probably use 'lapply' since it
appears that you want to return a variable number of items for each

On Tue, Jul 8, 2008 at 2:23 PM, hesicaia <dboyce at dal.ca> wrote:
> Hello,
>  The quick version of my question is how can I extract a matrix instead of
> a vector using tapply()? I would like to be able to access both the results
> of tapply() and also the index variables.
> In case further explanation would help:  I am analyzing a large (3million
> rows x 9 columns) spatial/temporal dataset and am attempting to calculate
> the number of unique years containing any data within each geographic area
> (10 degree cells in this case). I can do this, but I also want to extract a
> subset vector of the index variable (area).
> My script to calculate the number of unique years containing any data for
> each area is:
> x<-tapply(years, area, function(x) length(unique(x)))
> Now, I want to extract the vector of areas where the number of unique years
> containing any data is >20, but tapply() only returns a vector of unique
> years and I was a matrix.
> I could use a looping function to do this, but tapply() is much faster with
> large datasets and so I would like to use it if possible.
> Any help is appreciated.
> Thanks.
> --
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Jim Holtman
Cincinnati, OH
+1 513 646 9390

What is the problem you are trying to solve?

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