The previous exercise carried the last observation of the prior day forward into the first observation of the following day. This exercise will show you how to fill missing values by trading day, without using the prior day's final value.

You will use the same split-lapply-rbind paradigm from the Introduction to xts and zoo course. For reference, the pattern is below.

x_split <- split(x, f = "months")
x_list <- lapply(x_split, cummax)
x_list_rbind <- do.call(rbind, x_list)

Recall that the do.call(rbind, ...) syntax allows you to pass a list of objects to rbind() instead of having to type all their names.

Your workspace has a trade_day object that contains the regular series from the previous exercise, but without any NA filled in.