How to fill NAs with LOCF by factors in data frame, split by country

Question

I have the following data frame (simplified) with the country variable as a factor and the value variable has missing values:

country value
AUT     NA
AUT            


        

Answer 1

If speed is a consideration then this unstack/stack solution is about 4 to 6 times faster than the others on my system although it does entail a slightly longer line of code:

stack(lapply(unstack(data, value ~ country), na.locf, na.rm = FALSE))

Another approach is:

transform(data, value = ave(value, country, FUN = na.locf0))