Does a multi-value purrr::pluck exist?

Question

Seems like a basic question and perhaps I'm just missing something obvious ... but is there any way to pluck a sublist (with purrr)? More specifically, here's an initial list:

l <- list(a = "foo", b = "bar", c = "baz")

And I want to return a new (sub-)list with only elements a and b. Normally, I'd just do 'base' R sub-listing:

l[c("a", "b")]

But this doesn't provide the nice .default handling of pluck. My understanding is that pluck 'replaces' [[, but is there a purrr equivalent for replacing [?

Revise

I don't think a direct 'replacement' for [ exists, at least not in purrr. I guess a workaround would be map(set_names(c("a", "b", "z")), partial(pluck, l), .default = "Not found!") but that's not very neat! – Holstein

Answer 1

Similar to jzadra's post, you could also do:

l %>% keep(names(.) %in% c("a","b"))

Keeleykeelhaul

This is a great solution but FYI it does not (currently) work when using the native pipe operator, |> (sadface). – Latishalatitude

This doesn't retain the order in the list of names unfortunately, which can be useful sometimes. – Devisor

Answer 2

Here's my take on it. Not based on purrr thus slightly off topic, but compatible with the tidyverse's general interface principle of playing nicely with pipes.

pluck_multiple <- function(x, ...) {
  `[`(x, ...)
}

x <- list(a = 1, b = 2, c = list(x = TRUE, y = FALSE))

pluck_multiple(x, c("a", "b"))
#> $a
#> [1] 1
#> 
#> $b
#> [1] 2
pluck_multiple(x, 2:3)
#> $b
#> [1] 2
#> 
#> $c
#> $c$x
#> [1] TRUE
#> 
#> $c$y
#> [1] FALSE

Honoria

Answer 3

I think that purrr::keep() does what you want.

l %>% keep(names(.) == "a" | names(.) == "b")

Rubidium

Or keep(names(.) %in% c("a", "b")) – Swanhilda

Answer 4

There's clearly a need for "muti-value pluck". It really comes down to extending flexibility of "pluck" to retrieving multiple values from the same level of list, as described in this github issue. For the example above, you can probably get away with:

> purrr::map(purrr::set_names(c("a", "b")), 
          ~purrr::pluck(l, .x, .default = NA_character_))
#> $a
#> [1] "foo"
#> 
#> $b
#> [1] "bar"

Karat

Answer 5

Not purrr, but magrittr has extract() as an alias for [:

library(magrittr)

l %>% 
  extract(c("a", "b"))
# $a
# [1] "foo"
# 
# $b
# [1] "bar"

magrittr implements a number of other pipeable operator aliases, too.

Superimposed

Answer 6

Riffing off Rappster, if you don't mind the ugliness, you could pipe to the functional form of the extraction operator as a one-off:

> list(a = 1, b = 2, c = list(x = TRUE, y = FALSE)) %>% `[`(c('a', 'b'))
$a
[1] 1

$b
[1] 2

> list(a = 1, b = 2, c = list(x = TRUE, y = FALSE)) %>% `[`(2:3)
$b
[1] 2

$c
$c$x
[1] TRUE

$c$y
[1] FALSE

This has the added benefit of keeping the order of your inputs vs the keep approach, or allowing repeated extractions:

> c(a = 1, b = 2, c = 3) %>% `[`(2:3)
b c 
2 3 
> c(a = 1, b = 2, c = 3) %>% `[`(3:2)
c b 
3 2 
> c(a = 1, b = 2, c = 3) %>% keep(names(.) %in% c('a', 'b'))
a b 
1 2 
> c(a = 1, b = 2, c = 3) %>% keep(names(.) %in% c('b', 'a'))
a b 
1 2 
> c(a = 1, b = 2, c = 3) %>% `[`(c('a', 'a'))
a a 
1 1 
> c(a = 1, b = 2, c = 3) %>% keep(names(.) %in% c('a', 'a'))
a 
1 

Devisor