Creating a new r data.table column based on values in another column and grouping

Question

I have a data.table with date, zipcode and purchase amounts.

library(data.table)
set.seed(88)
DT <- data.table(date = Sys.Date()-365 + sort(s         


        

Answer 1

I didn't find any data.table solutions, this is how I got it though:

library(dplyr)
earlierPurchases <- vector()

for(i in 1:nrow(DT)) {
  temp <- dplyr::filter(DT, zip == zip[i] & date < date[i])
  earlierPurchases[i] <- sum(temp$purchaseAmount)
}

DT <- cbind(DT, earlierPurchases)

It worked quite fast.