Applying the same factor levels to multiple variables in an R data frame

Question

I am working with a dataset that includes 16 questions where the response set is identical (Yes, No, Unknown or Missing). I am processing the data using R and I want to turn

Answer 1

An R base solution using apply

 data.frame(apply(df, 2, factor, 
                 levels=c(-9, 0, 1), 
                 labels = c("Unknown or Missing", "No", "Yes")))

Using sapply

data.frame(sapply(df, factor, levels=c(-9, 0, 1), 
         labels = c("Unknown or Missing", "No", "Yes")))