Running Sum in R data.table [duplicate]

Question

I have a data.table in R, and I would like to apply rolling sum by group to it. But the problem is the group length is not the same and when the rollapply function reaches the shorter group, it will encounter an error. Is there a way to solve this, except for-loops?

The following is a simple example to illustrate the problem.

DT <- data.table(id = c(rep("A", 6), rep("B", 2), rep("C", 8)),
                 val = c(1:6, 1:2, 1:8))
> DT
    id val
 1:  A   1
 2:  A   2
 3:  A   3
 4:  A   4
 5:  A   5
 6:  A   6
 7:  B   1
 8:  B   2
 9:  B   1
10:  B   2
11:  B   3
12:  B   4
13:  B   5
14:  B   6
15:  C   7
16:  C   8

Rolling sum of 4 numbers, using rollapplyr()

DT[, cum.sum := rollapplyr(val, width = 4, FUN = sum, fill = NA), by = id]

But this will give me an error

Error in seq.default(start.at, NROW(data), by = by) : wrong sign in 'by' argument

And the output is

> DT
    id val cum.sum
 1:  A   1      NA
 2:  A   2      NA
 3:  A   3      NA
 4:  A   4      10
 5:  A   5      14
 6:  A   6      18
 7:  B   1      NA
 8:  B   2      NA
 9:  C   1      NA
10:  C   2      NA
11:  C   3      NA
12:  C   4      NA
13:  C   5      NA
14:  C   6      NA
15:  C   7      NA
16:  C   8      NA

Ideally, the output should be

> DT
    id val cum.sum
 1:  A   1      NA
 2:  A   2      NA
 3:  A   3      NA
 4:  A   4      10
 5:  A   5      14
 6:  A   6      18
 7:  B   1      NA
 8:  B   2      NA
 9:  C   1      NA
10:  C   2      NA
11:  C   3      NA
12:  C   4      10
13:  C   5      14
14:  C   6      18
15:  C   7      22
16:  C   8      26

Answer 1

We can do

DT[, cum.sum := Reduce(`+`, shift(val, 0:3)), by=id]

    id val cum.sum
 1:  A   1      NA
 2:  A   2      NA
 3:  A   3      NA
 4:  A   4      10
 5:  A   5      14
 6:  A   6      18
 7:  B   1      NA
 8:  B   2      NA
 9:  C   1      NA
10:  C   2      NA
11:  C   3      NA
12:  C   4      10
13:  C   5      14
14:  C   6      18
15:  C   7      22
16:  C   8      26

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I knew I'd seen this somewhere before - possibly a duplicate?