[datatable-help] "ungrouping" a data.table

[statquant3]

I am trying to replicate the kdb ungroup function
Say you have a table with several list columns, each list have same number
of elements on the same row

R) t = Sys.time()
R)
DT=data.table(a=c(1,2,3),b=c('q','w','e'),c=list(rep(t,2),rep(t+1,3),rep(t,0)),d=list(rep(1,2),rep(20,3),rep(1,0)))
R) DT
    a b  c                                                                                       
d
1: 1 q  2016-01-08 13:45:04.16544,2016-01-08 13:45:04.16544      1,1
2: 2 w 2016-01-08 13:45:05.16544,2016-01-08 13:45:05.16544,2016-01-08
13:45:05.16544 20,20,20
3: 3 e

The idea is to unlist all list columns keeping the non-list unchanged

I have the following:

dtUngroup <- function(DT){
	colClasses <- lapply(DT,FUN=class)
	listCols <- which(colClasses=='list')
	if(length(listCols)>0){
		nonListCols <- setdiff(colnames(DT),listCols)
		DT[,nbListElem:=lapply(.SD,FUN=lengths),.SDcols=(listCols[1L])]
		DT1 <- DT[,lapply(.SD,FUN=rep,times=DT$nbListElem),.SDcols=(nonListCols)]
		DT1[,(listCols):=DT[,lapply(.SD,FUN=unlist),.SDcols=(listCols)]]
		DT1[,nbListElem:=NULL]
		return(DT1)
	}
	return(DT)
}
R) dtUngroup(DT)[]
    a b c                   d
1: 1 q 1452260946  1
2: 1 q 1452260946  1
3: 2 w 1452260947 20
4: 2 w 1452260947 20
5: 2 w 1452260947 20

Buy as you can see 
1. it is verbose
2. empty lists are unsupported
3. POSIXct type is downcasted to numeric

Any idea how to fix those ?



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