r - 计算 R data.table 中前 3 行的总和(通过 grid-square)

Question

我想计算每个网格方块在过去三天内下降的降雨量，并将其添加为我的 data.table 中的新列。明确地说，我想总结当前和以前两 (2) 天的降雨量，对于每个气象网格方块

library ( zoo )
library (data.table)


# making the data.table
rain           <- c(NA, NA, NA, 0, 0, 5, 1, 0, 3, 10)  # rainfall values to work with
square         <- c(1,1,1,1,1,1,1,1,1,2)               # the geographic grid square for the rainfall measurement
desired_result <- c(NA, NA, NA, NA, NA, 5, 6, 6, 4, NA )  # this is the result I'm looking for (the last NA as we are now on to the first day of the second grid square)
weather <- data.table(rain, square, desired_result)  # making the data.table

我试图回答:这条线曾经有效，但不再有效

weather[, rain_3 := filter(rain, rep(1, 2), sides = 1), by = list(square)]  

所以在这里我尝试另一种方法:

# this next line gets the numbers right, but sums the following values, not the preceeding ones. 
weather$rain_3 <- rollapply(zoo(weather$rain), list(seq(-2,0)), sum)

# here I add in the by weather$ square, but still no success
weather$rain_3 <- rollapply(zoo(weather$rain), list(seq(-2,0)), sum, by= list(weather$square))

如果您有任何见解或建议，我将不胜感激。

非常感谢!

Answer 1

这是使用最新 data.table 的快速有效的解决方案版本 (v 1.9.6+)

weather[, rain_3 := Reduce(`+`, shift(rain, 0:2)), by = square]
weather
#     rain square desired_result rain_3
#  1:   NA      1             NA     NA
#  2:   NA      1             NA     NA
#  3:   NA      1             NA     NA
#  4:    0      1             NA     NA
#  5:    0      1             NA     NA
#  6:    5      1              5      5
#  7:    1      1              6      6
#  8:    0      1              6      6
#  9:    3      1              4      4
# 10:   10      2             NA     NA

这里的基本思想是 shift rain列两次，然后总结行。