In R ,how can i replac the NA by the previous character [duplicate]

Question

there is a dataframe as blow(with NA values)

md <- data.frame(cat=c('a','b','d',NA,'E',NA),
                subcat=c('A','C',NA,NA,NA,'D')) 

 cat subcat
1    a      A
2    b      C
3    d   <NA>
4 <NA>   <NA>
5    E   <NA>
6 <NA>      D

i want to replace the NA by the previous character ,the result as below.

Using loop statement like 'for ...' can do it, but it's not that efficient . is there any formula or package can do it ? thanks!

  cat subcat
1   a      A
2   b      C
3   d      C
4   d      C
5   E      C
6   E      D

Answer 1

You can use the na.locf function from the zoo package.

zoo::na.locf(md)
  cat subcat
1   a      A
2   b      C
3   d      C
4   d      C
5   E      C
6   E      D

Or use fill and everything from the tidyr and dplyr, respectively.

library(dplyr)
library(tidyr)

md %>% fill(everything())
#   cat subcat
# 1   a      A
# 2   b      C
# 3   d      C
# 4   d      C
# 5   E      C
# 6   E      D

Answer 2

One approach is to use run length encoding rle(). Because it does not encode NAs, I replaced them with a string "NA".

roll_na <- function(.) {
  .[is.na(.)] <- "NA"
  var <- rle(.)
  na_ind <- which(var$values == "NA")
  var_lag <- c(NA, var$values[-length(var$values)])
  var$values[na_ind] <- var_lag[na_ind]
  
  rep(var$values, times = var$lengths)
}

library(dplyr)

md %>% 
  mutate(across(everything(), roll_na))

#   cat subcat
# 1   a      A
# 2   b      C
# 3   d      C
# 4   d      C
# 5   E      C
# 6   E      D