Here is the revised version of your code with comments and explanations: # Install required packages install.packages("data.table") library(data.table) # Create a sample dataset set.seed(0L) nr <- 1e7 nid <- 1e5 DT <- data.table(id = sample(nid, nr, TRUE), value = sample(c("A", NA_character_), nr, TRUE)) # Define four functions to fill missing values mtd1 <- function(test) { # Use zoo's na.locf() function to fill missing values test[, value := zoo::na.locf(value, FALSE), id] } mtd2 <- function(test) { # Find the index of non-missing values test[!

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