A reshape puzzle in data.table

Question

Yet another reshape problem in data.table

set.seed(1234)
DT <- data.table(x=rep(c(1,2,3),each=4), y=c(\"A\",\"B\"), v=sample(1:100,12))
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Answer 1

Not sure this is the best solution, but you could do something like the following.

set.seed(1234)
DT <- data.table(x=rep(c(1,2,3),each=4), y=c("A","B"), v=sample(1:100,12))
DT[, id := seq_len(nrow(DT))]

setkey(DT, y)

uniqY <- unique(DT$y)

for(jj in uniqY){
  nc <- do.call(paste, c(expand.grid('Sum', c('x','v'),jj), sep ='.'))
  DT[.(jj), (nc) := list(cumsum(x), cumsum(v))]

}

setkey(DT, id)

DT[, 5:8 := lapply(.SD, function(x) { 
  xn <- is.na(x)
  x[xn] <- -Inf
  xx <- cummax(x)
  # deal with leading NA values
    if(xn[1]){
    xn1 <- which(xn)[1]
  xx[seq_len(xn1)] <- NA}   

  xx }), .SDcols = 5:8]