在使用dplyr的自定义函数中不能识别默认参数

Question

接受这个函数foo()。我希望它有一个默认的cyl参数，因为这是它通常要处理的字段的名称。

library(tidyverse)

foo <- function(x = cyl){
    case_when(
        x == 6 ~ TRUE,
        x == 8 ~ FALSE,
        x == 4 ~ NA
    )
}

# works: 
mtcars %>% 
    mutate(cyl_refactor = foo(cyl)) %>% 
    select(cyl, cyl_refactor)

但我感到惊讶的是，除非我显式地提供默认参数，否则该函数将无法工作。参见下面失败的代码

# fails:
mtcars %>% 
    mutate(cyl_refactor = foo()) %>% 
    select(cyl, cyl_refactor)

Error: Problem with `mutate()` column `cyl_refactor`. ℹ `cyl_refactor = foo()`. x object 'cyl' not found

似乎只有在有数据参数时才会处理默认参数，如下所示。

foo2 <- function(data, x = cyl){
    data %>% 
        mutate(cyl_refactor = case_when(
        {{x}} == 6 ~ TRUE,
        {{x}} == 8 ~ FALSE,
        {{x}} == 4 ~ NA
    ))
}

mtcars %>% 
    foo2() %>% 
    select(cyl, cyl_refactor)

我确信，我对准引号的知识还存在一些空白，但我想了解如何在foo()中使用默认参数。

Answer 1

我们可以用missing和cur_data_all做这件事

foo <- function(x = cyl){
   if(missing(x)) x <- cur_data_all()[["cyl"]]
   
    case_when(
        x == 6 ~ TRUE,
        x == 8 ~ FALSE,
        x == 4 ~ NA
    )
}

-testing

> out1 <- mtcars %>% 
+     mutate(cyl_refactor = foo(cyl)) %>% 
+     select(cyl, cyl_refactor)
> out2 <- mtcars %>% 
+     mutate(cyl_refactor = foo()) %>% 
+     select(cyl, cyl_refactor)
> 
> identical(out1, out2)
[1] TRUE

Answer 2

这里有一个会“起作用”的，尽管我不推荐它

foo <- function(x = cyl){
  x <- enquo(x)
  eval.parent(rlang::quo_squash(rlang::quo(case_when(
    !!x == 6 ~ TRUE,
    !!x == 8 ~ FALSE,
    !!x == 4 ~ NA
  ))))
}

# Both run without error
mtcars %>% 
  mutate(cyl_refactor = foo(cyl)) %>% 
  select(cyl, cyl_refactor)

mtcars %>% 
  mutate(cyl_refactor = foo()) %>% 
  select(cyl, cyl_refactor)

问题是，为了使case_when工作，您不能只传递列名而不传递数据。为了在本例中“查找”数据，我在调用链上使用了eval.parent()来尝试查找cyl变量。

最好在直接传入输入数据的地方创建适当的函数(而不是他们自己查找的变量名称)。