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 sum up the current and PREVIOUS two (2) days of rainfall, for each meterological grid square
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
My attempt to answer: this line used to work, but no longer does
weather[, rain_3 := filter(rain, rep(1, 2), sides = 1), by = list(square)]
So here I am trying another method:
# 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))
I would greatly appreciate any insights or suggestions you may have.
Many thanks!
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