Purrr's Modify-In Function - r

Question

I'm trying to use purrr's modify_in to modify elements of a list. An example of the list:
tib_list <- map(1:3, ~ tibble(col_one = runif(5),
col_two = runif(5), col_three = runif(5)))
Let's say I want to change elements 2 and 3 of the list to unselect col_one. I imagined doing this:
modify_in(tib_list, 2:length(tib_list), ~ select(.x, -col_one)
But this yields an error. I then thought of doing something like this, but this ends up duplicating the list
map(1:3, ~ modify_in(tib_list, .x, ~ select(.x, -col_one))

Answer 1

I think you wanted to use modify_at which lets you specify either element names or positions. modify_in allows us to use only one position like purrr::pluck.
library(tidyverse)
tib_list <- map(1:3, ~ tibble(col_one = runif(5), col_two = runif(5), col_three = runif(5)))
modify_at(tib_list, c(2,3), ~ select(.x, -col_one))
#> [[1]]
#> # A tibble: 5 x 3
#> col_one col_two col_three
#> <dbl> <dbl> <dbl>
#> 1 0.190 0.599 0.824
#> 2 0.214 0.172 0.106
#> 3 0.236 0.666 0.584
#> 4 0.373 0.903 0.252
#> 5 0.875 0.196 0.643
#>
#> [[2]]
#> # A tibble: 5 x 2
#> col_two col_three
#> <dbl> <dbl>
#> 1 0.513 0.113
#> 2 0.893 0.377
#> 3 0.275 0.675
#> 4 0.529 0.612
#> 5 0.745 0.405
#>
#> [[3]]
#> # A tibble: 5 x 2
#> col_two col_three
#> <dbl> <dbl>
#> 1 0.470 0.789
#> 2 0.181 0.289
#> 3 0.680 0.213
#> 4 0.772 0.114
#> 5 0.314 0.895
Created on 2021-08-27 by the reprex package (v0.3.0)
We can use modify_in with one position, but supplying a vector such as c(2,3) would mean that we want to access the third element of the second parent element in a nested list. This is why we see the error below.
# works
modify_in(tib_list, 2, ~ select(.x, -col_one))
#> [[1]]
#> # A tibble: 5 x 3
#> col_one col_two col_three
#> <dbl> <dbl> <dbl>
#> 1 0.109 0.697 0.0343
#> 2 0.304 0.645 0.851
#> 3 0.530 0.786 0.600
#> 4 0.708 0.0324 0.605
#> 5 0.898 0.232 0.567
#>
#> [[2]]
#> # A tibble: 5 x 2
#> col_two col_three
#> <dbl> <dbl>
#> 1 0.766 0.157
#> 2 0.0569 0.0422
#> 3 0.943 0.0850
#> 4 0.947 0.0806
#> 5 0.761 0.297
#>
#> [[3]]
#> # A tibble: 5 x 3
#> col_one col_two col_three
#> <dbl> <dbl> <dbl>
#> 1 0.878 0.864 0.540
#> 2 0.168 0.745 0.120
#> 3 0.943 0.338 0.535
#> 4 0.353 0.478 0.204
#> 5 0.267 0.669 0.478
# doesn't work
modify_in(tib_list, c(2,3), ~ select(.x, -col_one))
#> Error in UseMethod("select"): no applicable method for 'select' applied to an object of class "c('double', 'numeric')"

Answer 2

I never used modify_in, but you could use
library(purrr)
library(dplyr)
tib_list %>%
imap(~ if (.y > 1) { select(.x, -col_one) } else { .x })
to get
[[1]]
# A tibble: 5 x 3
col_one col_two col_three
<dbl> <dbl> <dbl>
1 0.710 0.189 0.644
2 0.217 0.946 0.955
3 0.590 0.770 0.0180
4 0.135 0.101 0.888
5 0.640 0.645 0.346
[[2]]
# A tibble: 5 x 2
col_two col_three
<dbl> <dbl>
1 0.267 0.926
2 0.456 0.0902
3 0.659 0.707
4 0.421 0.0451
5 0.801 0.220
[[3]]
# A tibble: 5 x 2
col_two col_three
<dbl> <dbl>
1 0.437 0.649
2 0.256 0.466
3 0.331 0.594
4 0.586 0.558
5 0.625 0.444

Answer 3

We can use modify_if
modify_if(tib_list,.f = ~ .x %>% select(-col_one),
.p = seq_along(tib_list) != 1)
-output
[[1]]
# A tibble: 5 x 3
col_one col_two col_three
<dbl> <dbl> <dbl>
1 0.819 0.666 0.384
2 0.183 0.549 0.0211
3 0.374 0.240 0.252
4 0.359 0.913 0.792
5 0.515 0.402 0.217
[[2]]
# A tibble: 5 x 2
col_two col_three
<dbl> <dbl>
1 0.696 0.0269
2 0.433 0.147
3 0.235 0.743
4 0.589 0.748
5 0.635 0.851
[[3]]
# A tibble: 5 x 2
col_two col_three
<dbl> <dbl>
1 0.707 0.976
2 0.0966 0.130
3 0.574 0.572
4 0.854 0.680
5 0.819 0.582