Sum of most recent values across groups

Question

For each row of my data I\'d like to compute the sum of most recent value for each group:

dt = data.table(group = c(\'a\',\'b\',\'a\',\         


        

Answer 1

I would create a column for each group showing the latest value for that group. Then just sum those columns:

library(zoo)
result <- rep(0, nrow(dt))
for(g in dt[, unique(group)]) {
  result <- result + dt[, na.fill(na.locf(ifelse(group==g, 1, NA)*value, na.rm=F), 0)]
}

all(dt[, desired] == result)