使用R中purrr包中的map的代码不起作用 [英] Code not working using map from purrr package in R

问题描述

我正在学习purrr包中的map函数，并且下面的代码不起作用：

I'm learning the map function in purrr package and have the following code not working:

library(purrr)
library(dplyr)

df1 = data.frame(type1 = c(rep('a',5),rep('b',5)),
             x = 1:10,
             y = 11:20) 

df1 %>% 
  group_by(type1) %>% 
  nest() %>% 
  map(.$data,with(.x, x + y))

df1 %>% 
  group_by(type1) %>% 
  nest() %>% 
  map(.$data,function(df) df$x + df$y)

对于最后两个代码块，错误返回为：

For the last two block of code, the errors return as:

错误：索引1的长度必须为1

Error: Index 1 must have length 1

相反，以下两个代码块运行良好，

By contrary, the following two blocks of code work well,

df1 %>% 
  group_by(type1) %>% 
  nest() %>% .$data %>% 
  map(.,~with(.x, .x$x + .x$y))


df1 %>% 
  group_by(type1) %>% 
  nest() %>% .$data %>% 
  map(.,~with(.x, .x$x + .x$y))

有人可以帮助我了解错误以及如何解决这些错误吗？

Can anyone help me to understand the errors and how to fix them?

推荐答案

您需要在<$ c周围添加括号$ c> map 表达式，因为。不会在函数中显示为单独的参数占位符，因此 magrittr 管道正在应用第一参数规则，您可以在此处了解更多信息；并且还使用〜构造一个函数，该函数正是 map 所期望的：

You need to add braces around the map expression, since . doesn't appear as a separate argument placeholder in the function so magrittr pipe is applying the first-argument rule which you can read more about here; and also use ~ to construct a function which is what map is expecting:

df1 %>% 
    group_by(type1) %>% 
    nest() %>% 
    { map(.$data, ~ with(.x, x + y)) }

#[[1]]
#[1] 12 14 16 18 20

#[[2]]
#[1] 22 24 26 28 30

---

类似于第二种方法：

---

Similarly for the second method:

df1 %>% 
    group_by(type1) %>% 
    nest() %>% 
    { map(.$data,function(df) df$x + df$y) }
#[[1]]
#[1] 12 14 16 18 20

#[[2]]
#[1] 22 24 26 28 30

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