如何使用 purrr::map 到 base::assign？

Question

How to get from meow to purr?

Let's say I have a list with 2 elements "a" and "b". I want to copy these elements into my Global environment. If I wanted to just add these elements to the search path, I'd use assign. However, I want to create a copy. And I'd like to use purrr::map to do this. How does it work?

mylist <- list(a=1:2, b=3:4)

#nope
map(mylist, assign, x=names(.), value=.)

#no
map(mylist, assign(x=names(.), value=.))

#no
map2(mylist, names(mylist), assign(x=.y, value=.x) )

#no
map2(mylist, names(mylist), assign, x=.y, value=.x )

Answer 1

Several responses to this :-)

1. Don't. My first visceral and emphatic response: don't do that. Very rarely do I find the use of get / assign justified, most commonly they are indications of inefficient (if not poor) data management. Having similar-structured things in a list means that you can efficiently operate on them (using map or lapply or similar).

2. Base R does this very efficiently with list2env .

    ls() # character(0) list2env(list(a=1:2, b=3:4), envir = .GlobalEnv) # <environment: R_GlobalEnv> ls() # [1] "a" "b"

3. If you must use purrr , then what you need is both the name and the value, so use purrr::imap :

    ls() # character(0) imap(list(a=1:2, b=3:4), ~ assign(..2, ..1, envir = .GlobalEnv)) # $a # [1] 1 2 # $b # [1] 3 4 ls() # [1] "a" "b"

   The reason your assign calls failed was that it was assigning it to the then-current environment, which is neither global nor something you get to use after your call to map / map2 .

Answer 2

Easier is list2env from base R applied directly on the named list to create the objects from the names in the global environment

list2env(mylist, .GlobalEnv)

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Regarding the use of map , it returns only the value and not the names

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A slightly related to this topic would be destructuring assignment. In R , this can be done with %<-% operator from zeallot

library(zeallot)
c(a, b) %<-% mylist