list all factor levels of a data.frame

If your problem is specifically to output a list of all levels for a factor, then I have found a simple solution using :

unique(df$x)

For instance, for the infamous iris dataset:

unique(iris$Species)

Here are some options. We loop through the 'data' with sapply and get the levels of each column (assuming that all the columns are factor class)

sapply(data, levels)

Or if we need to pipe (%>%) it, this can be done as

library(dplyr)
data %>% 
     sapply(levels)

Or another option is summarise_each from dplyr where we specify the levels within the funs.

 data %>%
      summarise_each(funs(list(levels(.))))

Or using purrr:

data %>% purrr::map(levels)

Or to first factorize everything:

data %>% dplyr::mutate_all(as.factor) %>% purrr::map(levels)

And answering the question about how to get the lengths:

data %>% map(levels) %>% map(length)