Referring to individual variables in … with dplyr quos

问题

Reading the guide to programming with dplyr, I am able to refer to all ... variables at once. But how can I use them individually?

Here's a function that counts two variables. It succeeds using quos() and !!!:

library(dplyr) # version 0.6 or higher
library(tidyr)

# counts two variables
my_fun <- function(dat, ...){

  cols <- quos(...)

  dat <- dat %>%
    count(!!!cols)

  dat

}

my_fun(mtcars, cyl, am)

#> # A tibble: 6 x 3
#>     cyl    am     n
#>   <dbl> <dbl> <int>
#> 1     4     0     3
#> 2     4     1     8
#> 3     6     0     4
#> 4     6     1     3
#> 5     8     0    12
#> 6     8     1     2

Now I want to tidyr::spread the second variable, in this case the am column. When I add to my function:

result <- dat %>%
    tidyr::spread(!!!cols[[2]], "n", fill = 0)

I get:

Error: Invalid column specification

How should I refer to just the 2nd variable of the cols <- quos(...) list?

回答1:

It is not clear whether spread works with quosure or not. An option is to use spread_ with strings

my_fun <- function(dat, ...){

  cols <- quos(...)

  dat %>%
    select(!!! cols) %>% 
    count(!!! cols) %>%
    spread_(quo_name(cols[[2]]), "n", fill = 0)

   }

my_fun(mtcars, cyl, am)
# A tibble: 3 x 3
#   cyl   `0`   `1`
#* <dbl> <dbl> <dbl>
#1     4     3     8
#2     6     4     3
#3     8    12     2

回答2:

Use named parameters instead. If you're relying on doing different things to different elements of the ... list it would only make sense to be explicit so it's easier to understand what each input is doing and make it easier for you to manipulate.

来源：https://stackoverflow.com/questions/44166247/referring-to-individual-variables-in-with-dplyr-quos

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