r - Populate NAs in a vector using prior non-NA values?

Question

What's a compact/efficient way to populate NAs using the prior non-NA value? For example:

test = c( 1 , 2 , NA , NA , 5 , NA , 9 , NA , NA )
expected = c( 1 , 2 , 2 , 2 , 5 , 5 , 9 , 9 , 9 )

Here, all of the NA values 'look back' to the first non-NA value. I'm trying to avoid a for loop

Answer 1

library(zoo)
na.locf(test)
[1] 1 2 2 2 5 5 9 9 9