[R] extracting index list when using tapply()

[hesicaia]

jholtman wrote:
> 
> Working code would help.  I would probably use 'lapply' since it
> appears that you want to return a variable number of items for each
> condition.
> 
> 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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>> Sent from the R help mailing list archive at Nabble.com.
>>
>> ______________________________________________
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>> PLEASE do read the posting guide
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>> and provide commented, minimal, self-contained, reproducible code.
>>
> 
> 
> 
> -- 
> Jim Holtman
> Cincinnati, OH
> +1 513 646 9390
> 
> What is the problem you are trying to solve?
> 
> ______________________________________________
> R-help at r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-help
> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
> 
> 

Thanks very much to all who replied. Post #2 (Charles Berry) did the trick.
I will be sure to give more relevant information and a small sample of data
in future posts.
Thanks again, your time is much appreciated.
Dan.
-- 
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