在R data.table中运行Sum

Question

我在R中有一个data.table,我想按组分别应用滚动总和.但问题是组长度不一样,当rollapply函数到达较短的组时,它会遇到错误.有没有办法解决这个问题,除了循环？

以下是一个简单的例子来说明问题.

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

滚动总和4个数字,使用 rollapplyr()

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

但这会给我一个错误

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

输出是

> 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

理想情况下,输出应该是

> 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

我们可以做的

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

我知道我以前见过这个 - 可能是重复的？