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
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