r - 从 %>% 管道运算符获取倒数第二个输入

Question

我想从我刚刚创建的列中提取一个因子 内一个管道。
我可以使用以下代码执行此操作:

library("dplyr")
library("magrittr")
library("janitor")

iris <- iris %>% janitor::clean_names()    
iris %>% filter(species %in% c("setosa","versicolor")) %>% group_by(species) %>% 
             summarise(mean_sepal_length = mean(sepal_length)) %>% ungroup() %>%
             mutate(species = factor(species, levels = (iris %>% group_by(species) %>%  #<- works but messy
                                                            summarise(mean_sepal_length = mean(sepal_width)) %>% 
                                                            ungroup() %>% arrange(mean_sepal_length) %$% species))) %>% 
                    arrange(species)

我想知道是否有“更清洁”的方法来做到这一点。就像是:

iris %>% filter(species %in% c("setosa","versicolor")) %>% group_by(species) %>% 
     summarise(mean_sepal_length = mean(sepal_length)) %>% ungroup() %>%
     mutate(species = factor(species, levels = (. %>% arrange(mean_sepal_length) %$% species))) %>% 
            arrange(species)

哪里 .是 倒数第二个参数 而不是给管道的最后一个参数？
这会引发错误，因为管道的最后一个参数是 mutate 语句:

Error: Problem with `mutate()` input `species`. x 'match' requires vector arguments i Input `species` is `factor(...)`.

我认为这根本不是管道运算符(operator)的工作方式，所以这可能是不可能的。

Answer 1

为了让第二个选项起作用，我们可以包装 .内{}

library(dplyr)
library(magrittr)
iris %>% 
 filter(species %in% c("setosa","versicolor")) %>%
 group_by(species) %>% 
 summarise(mean_sepal_length = mean(sepal_length)) %>% 
 ungroup() %>%
 mutate(species = factor(species, 
      levels = ({.} %>%
                  arrange(mean_sepal_length) %$% species))) %>%
 arrange(species)
# A tibble: 2 x 2
#  species    mean_sepal_length
#  <fct>                  <dbl>
#1 setosa                  5.01
#2 versicolor              5.94