当列表中有一个向量时，如何将列表列变异为一个普通的列，只留下最后一个值？

Question

I am trying to use purrr::map_chr to get the last element of the vector in a list-column as the actual value in case that it exists.

THE reproducible example:

library(data.table)
library(purrr)

x <- data.table(one = c("a", "b", "c"), two = list("d", c("e","f","g"), NULL))

I want data as it is but changing my list-column to a common one with "g" as the value for x[2,2] . What I've tryed:

x %>% mutate(two = ifelse(is.null(.$two), map_chr(~NA_character_), map_chr(~last(.))))

The result should be the next one.

# one  two
#  a    d 
#  b    g
#  c    NA

Thaks in advance!

Answer 1

Here is an option. We can use if/else instead of ifelse here

library(dplyr)
library(tidyr)
x %>% 
   mutate(two = map_chr(two, ~ if(is.null(.x)) NA_character_ else last(.x)))
#   one two
#1   a   d
#2   b   g
#3   c  NA

---

Or replace the NULL elements with NA and extract the last

x %>% 
   mutate(two = map_chr(two, ~ last(replace(.x, is.null(.), NA))))

Answer 2

I would propose this solution which is a bit cleaner.

library(tidyverse)

df <- tibble(one = c("a", "b", "c"), two = list("d", c("e","f","g"), NULL))

df %>% 
  mutate_at("two", replace_na, NA_character_) %>% 
  mutate_at("two", map_chr, last)