代码无法使用R中的purrr包中的映射

Question

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)

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

Error: Index 1 must have length 1

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))

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

Answer 1

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

---

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

Answer 2

If you wanted to use split() , I usually split on my grouping factor and then just map an anonymous function for what I want to do for a single tibble/dataframe in the newly created list:

df1 %>% 
    split(.$type1) %>% 
    map(~ mutate(., z = x + y) %>% # chain like you would a single tib
        select(z) %>%
        unlist(T,F))
$a
[1] 12 14 16 18 20

$b
[1] 22 24 26 28 30