Some code to test purrr::walk

purrr_walk_test.R

# Sample df with missing values
df <- data.frame(col1=sample(c('','Y','N'),100,replace = TRUE),
                 col2=sample(c('', as.character(0:9)),100,replace = TRUE),
                 stringsAsFactors = FALSE)

cols <- colnames(df) # store colnames

lapply(df,table) # check levels

df.forFor <- df # to be fixed with a for loop
df.forPurrr <- df # to be fixed using purrr::walk

# Correct levels
levels <- list(
  col1 = factor(c('N','Y')),
  col2 = ordered(as.character(0:9))
)

# impute df.forFor and df.forPurr by
# changing '' to the first level of each column.
# i.e. in col1 '' becomes 'N' and in col2 '' becomes '0'

# impute df.forFor using for loop
for(col in cols) {
  df.forFor[,col][which(df.forFor[,col]=='')] <- levels(levels[[col]])[1]
}

# impute df.forPurrr using purrr::walk
purrr::walk(cols,function(col){
  df.forPurrr[,col][which(df.forPurrr[,col]=='')] <- levels(levels[[col]])[1]
})

lapply(df.forFor,table) # imputation succeeds
lapply(df.forPurrr,table) # imputation fails