[Rd] split() is slow on data.frame (PR#14123)

[pengyu.ut at gmail.com]

Please see the following code for the runtime comparison between
split() and mysplit.data.frame() (they do the same thing
semantically). mysplit.data.frame() is a fix of split() in term of
performance. Could somebody include this fix (with possible checking
for corner cases) in future version of R and let me know the inclusion
of the fix?

m=300000
n=6
k=30000

set.seed(0)
x=replicate(n,rnorm(m))
f=sample(1:k, size=m, replace=T)

mysplit.data.frame<-function(x,f) {
  print('processing data.frame')
  v=lapply(
      1:dim(x)[[2]]
      , function(i) {
        split(x[,i],f)
      }
      )

  w=lapply(
      seq(along=v[[1]])
      , function(i) {
        result=do.call(
            cbind
            , lapply(v,
                function(vj) {
                  vj[[i]]
                }
                )
            )
        colnames(result)=colnames(x)
        return(result)
      }
      )
  names(w)=names(v[[1]])
  return(w)
}

system.time(split(as.data.frame(x),f))
system.time(mysplit.data.frame(as.data.frame(x),f))