R help - Jul 2008 - Fwd: Re: extracting index list when using tapply()

The following message is provided by Erik

Please provide the reproducible code to do this.  Generate a sample data
set using the random data generating functions and show us what you'd
like, we can then more easily help.



ctu at bigred.unl.edu wrote:
> Hi,
> How about using "subset"?
> x1<-tapply(subset(years, length(area)>20), function(x)
length(unique(x)))
>
> I hope this works
> Chunhao
>
>
> Quoting hesicaia <dboyce at dal.ca>:
>
>>
>> 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.
>> -- 
>> View this message in context:   
>>
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>>
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>>
>
> ______________________________________________
> R-help at r-project.org mailing list
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> PLEASE do read the posting guide
http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.

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