calculating sum of previous 3 rows in R data.table (by grid-square)

Question

I would like to calculate the rainfall that has fallen over the last three days for each grid square, and add this as a new column in my data.table. To be clear, I want to s

Answer 1

Here's a quick and efficient solution using the latest data.table version (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

The basic idea here is to shift the rain column twice and then sum up the rows.