在 R 的上下文中波浪号是什么？

Question

  mtcars %>%
  group_by(cyl) %>%
  group_map(~ head(.x, 2L)) 

Can anyone explain last line of code part? I know about the pipe but what is ~ head(.x, 2L) ?

Answer 1

It's a shorthand for an anonymous function that is applied to every group. .x is automatically the input in purrr style anonymous functions (and additionally .y for map2 functions).

But you can use a traditional anonymous functions as well:

mtcars %>%
  group_by(cyl) %>%
  group_map(., function(x) head(x, 2L)) # the `.` is just for illustration and can be omitted with the %>% 

Or you can write a named function and use it in group_map() :

new_fun <- function(x) {
   head(x, 2L)
}
mtcars %>%
  group_by(cyl) %>%
  group_map(new_fun)

The function you show ( head(.x, 2L) ) is applied once to every group in the data. You can check how many groups you have with:

mtcars %>%
  group_by(cyl) %>%
  n_groups()
#> [1] 3

For each of these groups, the first two rows of the data is printed:

mtcars %>%
  group_by(cyl) %>%
  group_map(~ head(.x, 2L)) 
#> [[1]]
#> # A tibble: 2 x 10
#>     mpg  disp    hp  drat    wt  qsec    vs    am  gear  carb
#>   <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1  22.8  108     93  3.85  2.32  18.6     1     1     4     1
#> 2  24.4  147.    62  3.69  3.19  20       1     0     4     2
#> 
#> [[2]]
#> # A tibble: 2 x 10
#>     mpg  disp    hp  drat    wt  qsec    vs    am  gear  carb
#>   <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1    21   160   110   3.9  2.62  16.5     0     1     4     4
#> 2    21   160   110   3.9  2.88  17.0     0     1     4     4
#> 
#> [[3]]
#> # A tibble: 2 x 10
#>     mpg  disp    hp  drat    wt  qsec    vs    am  gear  carb
#>   <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl> <dbl>
#> 1  18.7   360   175  3.15  3.44  17.0     0     0     3     2
#> 2  14.3   360   245  3.21  3.57  15.8     0     0     3     4