[R] Access only part of last dimension of table/matrix

[David Winsemius]

On Jul 1, 2011, at 7:44 PM, Marc Schwartz wrote:

> On Jul 1, 2011, at 4:50 PM, David Winsemius wrote:
>
>>
>> I would like to do some operations inside a function using only one  
>> value for the last dimension of a table/matrix:
>>
>> tabfn <- function (dfrm, facvec, YN ="event"){
>>            return( Etbl <- do.call(table, dfrm[ , c(facvec,  
>> "event") ]) )
>>             # just want Etbl[,,,"TRUE"] or Etbl[,, "TRUE"] or  
>> Etbl[,"TRUE"]
>>             }
>> tbl <- tabfn(testdf, c("x", "y") )
>> tbl   # all value of event returned
>>
>> At the console it is easy for me to count the number of factors and  
>> use the right number of commas
>>
>> tbl[ , , "TRUE"] if I only want the slice with that value. How can  
>> I do this programmatically?
>>
>> Thnks.
>
>
> David,
>
> I had a vague recollection of something like this coming up at some  
> point in the past and it took me a bit to get the right keywords to  
> find it.
>
> I did not realize how far back it was (2001), but here are two  
> possible solutions by Peter Dalgaard and Thomas Lumley from the same  
> thread:
>
> https://stat.ethz.ch/pipermail/r-help/2001-October/016110.html
> https://stat.ethz.ch/pipermail/r-help/2001-October/016122.html
>
> It looks like Peter's solution is along the lines of the one that  
> you just posted.


Yeah. Thanks, Mark. Pretty much the same. Guess I'm in good company.  
(Surprised this isn't asked more frequently.)

testdf <- data.frame(x=sample(letters[1:5], 25, replace=TRUE),
                     y=sample(letters[1:5], 25, replace=TRUE),
                     z=sample(letters[1:5], 25, replace=TRUE),
                     event=sample(c(TRUE, FALSE), 25, replace=TRUE) )

etbl <- table(testdf[ , c(c("x", "y"), "event")])
apply(etbl, seq(length=length(dim(etbl))-1),"[", 2)  # Dalgaard 2001
apply(etbl, 1:(length(dim(etbl))-1), "[", 2)   # Winsemius 2011

Any idea how to make it a "selection"? By that I mean how to select  
values of the last dimension whose "event" values are "TRUE". I get  
"Error: object 'event' not found" with any expression involving event,  
since it is only an attribute in dimnames.


>
> Hope that this helps.

Yes. At least there is nothing (yet) that I would call "truly  
elegant". Maybe it's just that I stumbled on my solution, rather than  
seeing it as a glaringly obvious application of apply(..., "[" , index)

>
> Regards,

Sorry for the duplicate private message. Meant to hit reply to all.
>
> Marc Schwartz
>

David Winsemius, MD
West Hartford, CT