[R] split / lapply over multiple columns

[David Winsemius]

On Aug 3, 2010, at 10:48 PM, Ralf B wrote:

> Hi all,
>
> I have a data frame with column over which I would like to run
> repeated functions for data analysis. Currently I am only running
> recursively over two columns where I column 1 has two states over
> which I split and column two has 3 states. The function therefore runs
> 2 x 3 = 6 times as shown when running the following code:
>
> mydata <- data.frame(userid = c(5, 6, 5, 6, 5, 6), taskid = c(1, 1,  
> 2, 2, 3, 3),
>      stuff = 11:16)
> mydata
> mydata <- mydata[with(mydata, order(userid, taskid)), ]
> mydata
>
> lapply(split(mydata, mydata[,1]), function(x){
> 	lapply(split(x, x[,2]), function(y){
> 		print(paste("result:",y))
> 	})
> })
>
> This traverses the tree like this:
>
> 5,1
> 5,2
> 5,3
> 6,1
> 6,2
> 6,3
>
> Is there an easier way of doing that? I would like to provide the two
> columns (index 1 and index 2) directly and have the ?lapply function
> perform its lambda function directly on each memebr of the tree
> automatically? How can I do that?

split(mydata, with(mydata, paste(userid, taskid, sep=".")))

Perhaps something like:

lapply( split(mydata, with(mydata, paste(userid, taskid, sep="."))),  
function(x) paste("result:", x))

>

David Winsemius, MD
West Hartford, CT