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))
#             


        

Answer 1

Try this:

cumsum0 <- function(x) { x <- cumsum(x); ifelse(x == 0, NA, x) }
DT2 <- DT[, {SUM.<-y; lapply(data.table(model.matrix(~ SUM.:x + SUM.:v + 0)), cumsum0)}]
setnames(DT2, sub("(.):(.)", "\\2.\\1", names(DT2)))

Simplifications:

1) If using 0 in place of NA is ok then it can be simplified by omitting the first line which defines cumsum0 and replacing cumsum0 in the next line with cumsum.

2) The result of the second line has these names:

> names(DT2)
[1] "SUM.A:x" "SUM.B:x" "SUM.A:v" "SUM.B:v"

so if that is sufficient the last line can be dropped since its only purpose is to make the names exactly the same as in the question.

The result (without the simplifications) is:

> DT2
    SUM.x.A SUM.x.B SUM.v.A SUM.v.B
 1:       1      NA      12      NA
 2:       1       1      12      62
 3:       2       1      72      62
 4:       2       2      72     123
 5:       4       2     155     123
 6:       4       4     155     220
 7:       6       4     156     220
 8:       6       6     156     242
 9:       9       6     255     242
10:       9       9     255     289
11:      12       9     318     289
12:      12      12     318     338