Hi, The expected output is confusing. dat1 <- read.table(text="date, user, items_bought 2013-01-01, x, 2 2013-01-02, x, 1 2013-01-03, x, 0 2013-01-04, x, 0 2013-01-05, x, 3 2013-01-06, x, 1 2013-01-01, y, 1 2013-01-02, y, 1 2013-01-03, y, 0 2013-01-04, y, 5 2013-01-05, y, 6 2013-01-06, y, 1",sep=",",header=TRUE,stringsAsFactors=FALSE)
##Assuming that the data is ordered by date and no gaps in date res1 <- unsplit(lapply(split(dat1, dat1$user), function(x) { indx <- (seq(nrow(x)) - 1)%/%3 x$cum_items_bought_3_days <- ave(x$items_bought, indx, FUN = cumsum) x }), dat1$user) ##expected output res2 <- unsplit(lapply(split(dat1, dat1$user), function(x) { indx <- (seq(nrow(x)) - 1)%/%3 x$cum_items_bought_3_days <- ave(x$items_bought, indx, FUN = cumsum) indx2 <- seq(0, length(indx), by = 4) x[indx2, 4] <- x[indx2, 4] + indx[indx2] x }), dat1$user) A.K. Say I have data that looks like date, user, items_bought 2013-01-01, x, 2 2013-01-02, x, 1 2013-01-03, x, 0 2013-01-04, x, 0 2013-01-05, x, 3 2013-01-06, x, 1 2013-01-01, y, 1 2013-01-02, y, 1 2013-01-03, y, 0 2013-01-04, y, 5 2013-01-05, y, 6 2013-01-06, y, 1 to get the cumulative sum per user per data point I was doing data.frame(cum_items_bought=unlist(tapply(as.numeric(data$items_bought), data$user, FUN = cumsum))) output from this looks like date, user, items_bought 2013-01-01, x, 2 2013-01-02, x, 3 2013-01-03, x, 3 2013-01-04, x, 3 2013-01-05, x, 6 2013-01-06, x, 7 2013-01-01, y, 1 2013-01-02, y, 2 2013-01-03, y, 2 2013-01-04, y, 7 2013-01-05, y, 13 2013-01-06, y, 14 However I want to restrict my sum to only add up those that happened within 3 days of each row (relative to the user). i.e. the output needs to look like this: date, user, cum_items_bought_3_days 2013-01-01, x, 2 2013-01-02, x, 3 2013-01-03, x, 3 2013-01-04, x, 1 2013-01-05, x, 3 2013-01-06, x, 4 2013-01-01, y, 1 2013-01-02, y, 2 2013-01-03, y, 2 2013-01-04, y, 6 2013-01-05, y, 11 2013-01-06, y, 12 ______________________________________________ R-help@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.