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

Late to the party, but a more recent version of data.table package (1.12.8 for me) has frollsum function that will accomplish this a bit more cleanly than earlier (but very much valid) answers:

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

# using `frollsum`
weather[, rain3 := frollsum(rain, n = 3), by = square][]
#>     rain square desired_result rain3
#>  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

^{Created on 2020-07-09 by the reprex package (v0.3.0)}