[SOLVED] purrr::pluck - how to get last element of list

purrr::pluck - how to get last element of list

I am trying to extract the last element of nested lists. How can I do this with purrr? In the example below the results should be all the elements with the value "c". I have seen this, but I am particularly interested in a solution with purrr.

Many thanks

x <- list(list(1,2,"c"), list(3,"c"), list("c"))
x
#> [[1]]
#> [[1]][[1]]
#> [1] 1
#> 
#> [[1]][[2]]
#> [1] 2
#> 
#> [[1]][[3]]
#> [1] "c"
#> 
#> 
#> [[2]]
#> [[2]][[1]]
#> [1] 3
#> 
#> [[2]][[2]]
#> [1] "c"
#> 
#> 
#> [[3]]
#> [[3]][[1]]
#> [1] "c"

map(x, purrr::pluck, tail)
#> Error in map(x, purrr::pluck, tail): could not find function "map"
map(x, purrr::pluck, length(x))
#> Error in map(x, purrr::pluck, length(x)): could not find function "map"

^{Created on 2021-05-21 by the reprex package (v2.0.0)}

Solution

Using purrr::pluck simply:

library(purrr)

map(x, ~ pluck(.x, length(.x)))

Or even easier use dplyr::last:

library(purrr)
library(dplyr)

map(x, last)

In just base R, reverse each list element and take the first item:

lapply(x, function(y) rev(y)[[1]])

lapply(x, \(y) rev(y)[[1]]) # R >= 4.1.0

mapply(`[[`, x, sapply(x, length), SIMPLIFY = F)

Which would return a single vector if remove the argument SIMPLIFY = F because the default is TRUE.

This works by iterating through your list x and the output of sapply(x, length) in parallel and applying it to the function `[[` which is an extraction operator.