[R] extracting index list when using tapply()

From: hesicaia <dboyce_at_dal.ca>
Date: Tue, 08 Jul 2008 11:23:16 -0700 (PDT)

  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.

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Received on Tue 08 Jul 2008 - 18:32:31 GMT

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