CODEWITHSUNDEEP
rpurrr
mnist
mnist

Reputation: 6956

How to replace a modifying for loop by purrr

I would like to replace a simple for loop by a purrr alternative. How can I code this and keep the original structure of my object

Here is my example:

my_list <- list(
      a = list(
        list(
          aaa = c(1:3),
          aab = c(4:6), 
          aac = c(7:9)),
        list(
          aaa = c(10:12),
          aab = c(13:15), 
          aac = c(16:18)),
        list(
          aaa = c(19:21),
          aab = c(22:24), 
          aac = c(25:27))
      ))

and my original solution

x <- purrr::map_lgl(my_list$a, .f = ~ !is.null(.x$aaa))

my_list1 <- my_list
for (i in which(x)) {
      my_list1$a[[i]]$aaa <- 99
}
str(my_list1)

However, I would like to replace my for loop by a purrr-solution, as well. I tried something like this, but map always returns a new list instead of just updating it

my_list2 <- map(c(1, 3),
        function(x){
          my_list[["a"]][[x]][["aab"]] <- 99
          return(my_list)
        }
)
str(my_list2)

Modify should be more applicable in this context but I could'nt make it work either.

my_list3 <- purrr::map(
      .x = which(x), 
      .f = function(i = .x) {
        purrr::modify_in(.x = my_list, 
                         .where = list("a", i, "aaa"), 
                         .f = ~99)
      }
)
str(my_list3)

The structure of the new list should look the same like the initial list:

# str(my_list)
List of 1
 $ a:List of 3
  ..$ :List of 3
  .. ..$ aaa: int [1:3] 1 2 3
  .. ..$ aab: int [1:3] 4 5 6
  .. ..$ aac: int [1:3] 7 8 9
  ..$ :List of 2
  .. ..$ aab: int [1:3] 13 14 15
  .. ..$ aac: int [1:3] 16 17 18
  ..$ :List of 3
  .. ..$ aaa: int [1:3] 19 20 21
  .. ..$ aab: int [1:3] 22 23 24
  .. ..$ aac: int [1:3] 25 26 27

# str(my_list1) is correct
List of 1
 $ a:List of 3
  ..$ :List of 3
  .. ..$ aaa: num 99
  .. ..$ aab: int [1:3] 4 5 6
  .. ..$ aac: int [1:3] 7 8 9
  ..$ :List of 2
  .. ..$ aab: int [1:3] 13 14 15
  .. ..$ aac: int [1:3] 16 17 18
  ..$ :List of 3
  .. ..$ aaa: num 99
  .. ..$ aab: int [1:3] 22 23 24
  .. ..$ aac: int [1:3] 25 26 27

# str(my_list2) or str (my_list3) is not correct
List of 2
 $ :List of 1
  ..$ a:List of 3
  .. ..$ :List of 3
  .. .. ..$ aaa: num 99
  .. .. ..$ aab: int [1:3] 4 5 6
  .. .. ..$ aac: int [1:3] 7 8 9
  .. ..$ :List of 2
  .. .. ..$ aab: int [1:3] 13 14 15
  .. .. ..$ aac: int [1:3] 16 17 18
  .. ..$ :List of 3
  .. .. ..$ aaa: int [1:3] 19 20 21
  .. .. ..$ aab: int [1:3] 22 23 24
  .. .. ..$ aac: int [1:3] 25 26 27
 $ :List of 1
  ..$ a:List of 3
  .. ..$ :List of 3
  .. .. ..$ aaa: int [1:3] 1 2 3
  .. .. ..$ aab: int [1:3] 4 5 6
  .. .. ..$ aac: int [1:3] 7 8 9
  .. ..$ :List of 2
  .. .. ..$ aab: int [1:3] 13 14 15
  .. .. ..$ aac: int [1:3] 16 17 18
  .. ..$ :List of 3
  .. .. ..$ aaa: num 99
  .. .. ..$ aab: int [1:3] 22 23 24
  .. .. ..$ aac: int [1:3] 25 26 27

Any hints where I go wrong with purrr?

Upvotes: 3

Views: 600

Answers (2)

Joris C.
Joris C.

Reputation: 6234

Given that the list components to modify are located at the second nested level, you can also directly access them with modify_depth:

library(purrr)

my_list <- modify_depth(my_list, 2, ~list_modify(.x, aaa = 99))

str(my_list)
#> List of 1
#>  $ a:List of 3
#>   ..$ :List of 3
#>   .. ..$ aaa: num 99
#>   .. ..$ aab: int [1:3] 4 5 6
#>   .. ..$ aac: int [1:3] 7 8 9
#>   ..$ :List of 3
#>   .. ..$ aaa: num 99
#>   .. ..$ aab: int [1:3] 13 14 15
#>   .. ..$ aac: int [1:3] 16 17 18
#>   ..$ :List of 3
#>   .. ..$ aaa: num 99
#>   .. ..$ aab: int [1:3] 22 23 24
#>   .. ..$ aac: int [1:3] 25 26 27

NB: This also works for multiple list components at the first list level, (e.g. $a, $b, ...), without having to add an additional iteration step.

Edit: as listed in the documentation of modify_depth:

modify_depth(x, 2, fun) is equivalent to x <- modify(x, ~ modify(., fun))

In order to replace only a selection of list components on the second nested level, we could rewrite the call to modify_depth in a more verbose way as:

library(purrr)

## replace aaa only for components 1 and 3
modify_loc <- c(TRUE, FALSE, TRUE)

my_list <- modify(my_list, ~imodify(., ~if(modify_loc[.y]) list_modify(.x, aaa = 99) else .x))

str(my_list)
#> List of 1
#>  $ a:List of 3
#>   ..$ :List of 3
#>   .. ..$ aaa: num 99
#>   .. ..$ aab: int [1:3] 4 5 6
#>   .. ..$ aac: int [1:3] 7 8 9
#>   ..$ :List of 3
#>   .. ..$ aaa: int [1:3] 10 11 12
#>   .. ..$ aab: int [1:3] 13 14 15
#>   .. ..$ aac: int [1:3] 16 17 18
#>   ..$ :List of 3
#>   .. ..$ aaa: num 99
#>   .. ..$ aab: int [1:3] 22 23 24
#>   .. ..$ aac: int [1:3] 25 26 27

Upvotes: 3

Ronak Shah
Ronak Shah

Reputation: 388982

One way would be using map and change a single element in each list.

my_list[["a"]][x] <- purrr::map(my_list[["a"]][x], ~{.[["aaa"]] <- 99;.})


my_list
#$a
#$a[[1]]
#$a[[1]]$aaa
#[1] 99

#$a[[1]]$aab
#[1] 4 5 6

#$a[[1]]$aac
#[1] 7 8 9


#$a[[2]]
#$a[[2]]$aaa
#[1] 99

#$a[[2]]$aab
#[1] 13 14 15

#$a[[2]]$aac
#[1] 16 17 18
#......

which is same as using lapply in base R

my_list[["a"]][x] <- lapply(my_list[["a"]][x], function(x) {x[["aaa"]] <- 99;x})

Upvotes: 2

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