Using tidyeval's braces with a dplyr::group_by

Question

I wanted to pass a vector of strings to purrr::map to generate a list of tables.

library(tidyverse)
library(magrittr)

names(mtcars) %>% 
  extract(8:10) %>% 
  map(
   function(i)

     mtcars %>% 
     group_by({{i}}) %>% 
     tally
   )

but this returns objects grouped on a character string, not the variable name.

This works

names(mtcars) %>% 
  extract(8:10) %>% 
  map(
    function(i)

      mtcars %>% 
      group_by(get(i)) %>% 
      tally
    )

but I was hoping for a solution with a more idiomatically tidy approach.

Answer 1

Use !!sym(i) instead. I don't fully understand why this works, but I guess you need to create a symbol from your string first and then have to quasi-quote it in order to be able to replace it with your placeholder in the function. If this makes sense.

library(tidyverse)

names(mtcars)[8:10] %>% 
  map(
    function(i)

      mtcars %>% 
      group_by(!!sym(i)) %>% 
      tally
  )
#> [[1]]
#> # A tibble: 2 x 2
#>      vs     n
#>   <dbl> <int>
#> 1     0    18
#> 2     1    14
#> 
#> [[2]]
#> # A tibble: 2 x 2
#>      am     n
#>   <dbl> <int>
#> 1     0    19
#> 2     1    13
#> 
#> [[3]]
#> # A tibble: 3 x 2
#>    gear     n
#>   <dbl> <int>
#> 1     3    15
#> 2     4    12
#> 3     5     5

^{Created on 2019-11-25 by the reprex package (v0.2.1)}