什么是“拥抱运算符"?`{{}}`? [英] What is the "embracing operator" `{{ }}`?
问题描述
我刚刚遇到了拥抱运算符"{{ }} 在 tidyverse 风格指南.
I just came across the "embracing operator" {{ }} in section 2.2.3 of the tidyverse style guide.
拥抱运算符 {{ }} 在 R 中有什么作用?
What does the embracing operator {{ }} do in R?
推荐答案
它叫做 curly-curly 运算符(参见?"{{}}").
在传递必须在其他上下文中进行评估之前就地替换的参数时，这很有用.
It's useful when passing an argument that has to be substituted in place before being evaluated in another context.
看这个简单的例子(虽然有点尴尬，因为我们可以在这里调用函数时简单地引用cyl"):
See this simple example (although a bit awkward as we could simple quote the "cyl" when calling the function here):
library(dplyr)
# does not work
get_var <- function(data, column) {
data %>% select(column)
}
get_var(mtcars, cyl)
#> Error: object 'cyl' not found
# works
get_var <- function(data, column) {
data %>% select({{ column }})
}
get_var(mtcars, cyl)
#> cyl
#> Mazda RX4 6
#> Mazda RX4 Wag 6
#> Datsun 710 4
#> Hornet 4 Drive 6
#> Hornet Sportabout 8
#> ...
由 reprex 包 (v0.3.0) 于 2020 年 7 月 8 日创建
Created on 2020-07-08 by the reprex package (v0.3.0)
或者一个更好的例子:
library(dplyr)
# does not work
get_var <- function(data, column, value) {
data %>% filter(column == value)
}
get_var(mtcars, cyl, 6)
#> Error: Problem with `filter()` input `..1`.
#> x object 'cyl' not found
#> i Input `..1` is `column == value`.
# works
get_var <- function(data, column, value) {
data %>% filter({{ column }} == value)
}
get_var(mtcars, cyl, 6)
#> mpg cyl disp hp drat wt qsec vs am gear carb
#> Mazda RX4 21.0 6 160.0 110 3.90 2.620 16.46 0 1 4 4
#> Mazda RX4 Wag 21.0 6 160.0 110 3.90 2.875 17.02 0 1 4 4
#> Hornet 4 Drive 21.4 6 258.0 110 3.08 3.215 19.44 1 0 3 1
#> Valiant 18.1 6 225.0 105 2.76 3.460 20.22 1 0 3 1
#> Merc 280 19.2 6 167.6 123 3.92 3.440 18.30 1 0 4 4
#> Merc 280C 17.8 6 167.6 123 3.92 3.440 18.90 1 0 4 4
#> Ferrari Dino 19.7 6 145.0 175 3.62 2.770 15.50 0 1 5 6
由 reprex 包 (v0.3.0) 于 2020 年 7 月 8 日创建
Created on 2020-07-08 by the reprex package (v0.3.0)
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