purrr::possively 函数可能不适用于 map2_chr 函数 [英] purrr::possibly function possibly not working with map2_chr function

问题描述

我怀疑这是 purrr 包中的一个错误，但想先检查我在 StackOverflow 中的逻辑，请.

在我看来，可能 函数在 map2_chr 函数中不起作用.我正在使用 purrr 版本 0.2.5

考虑这个例子:

库(dplyr)图书馆(咕噜咕噜)让 <- tibble(posn = 2:0,let_list = 列表(字母[1:5]、字母[1:5]、字母[1:5]))%>%一瞥()

返回

观察:3变量:2$ posn<int>2, 1, 0$lets_list <list>[<"a", "b", "c", "d", "e">, <"a", "b", "c", "d", "e">, <;"a", "b", "c", "d", "e">]

在此示例中，我想使用 mutate 创建另一列，以根据posn"中的值返回列表lets_list"中的元素.

让%>%变异(lets_sel = map2_chr(lets_list，posn，~.x[.y]))

失败并显示此错误消息，因为第三行的 posn = 0.

<代码>>让 %>%+ 变异(lets_sel = map2_chr(lets_list，posn，~.x[.y]))# mutate_impl(.data, dots) 中的错误:# 评估错误:结果 3 不是长度为 1 的原子向量.

将 possably 函数与 map2_chr 一起使用也会返回错误.

让%>%变异(lets_sel = map2_chr(lets_list，posn，可能(~.x[.y]，NA_character_)))# mutate_impl(.data, dots) 中的错误:# 评估错误:结果 3 不是长度为 1 的原子向量.

然而，map2 函数工作正常:

<代码>>让 %>%+ 变异(lets_sel = map2(lets_list，posn，可能(~.x[.y]，NA_character_)))# 小费:3 x 3posnlets_listlets_sel<int><列表><列表>1 2 <chr [5]><chr [1]>2 1 <chr [5]><chr [1]>3 0 <chr [5]><chr [0]>

一种解决方法是先使用 map2，然后使用 map_chr，但我怀疑这是一个错误.

<代码>>让 %>%+ mutate(lets_sel = map2(lets_list, posn, ~.x[.y]),+lets_sel = map_chr(lets_sel, 可能(~.x[1], NA_character_)))# 小费:3 x 3posnlets_listlets_sel<int><列表><chr>1 2 <chr [5]>乙2 1 <chr [5]>一种3 0 <chr [5]>不适用

我在这里遗漏了什么吗?谢谢.

解决方案

possably() 不起作用，因为使用 0 进行索引不会引发错误；它只返回一个长度为 0 的向量:

nth_letter <- function(n) 字母[n]可能(nth_letter，未返回")(0)#>字符(0)第n个字母(0)#>字符(0)

在这种情况下，用 NA 替换无效索引可能会更容易(使用例如 dplyr::na_if() 或普通的旧 ifelse 如果真正的问题更复杂)得到你想要的:

让%>%变异(lets_sel = map2_chr(lets_list, na_if(posn, 0), ~ .x[.y]))#># 小费:3 x 3#>posnlets_listlets_sel#><int><列表><chr>#>1 2 <chr [5]>乙#>2 1 <chr [5]>一种#>3 0 <chr [5]><不适用>

由 reprex 包 (v0.2.0.9000) 于 2018 年 8 月 7 日创建.

I suspect that this is a bug in the purrr package, but would like to check my logic in StackOverflow first, please.

It seems to me that the possibly function is not working inside the map2_chr function. I'm using purrr version 0.2.5

Consider this example:

library(dplyr)
library(purrr)

lets <- tibble(posn = 2:0,
               lets_list = list(letters[1:5], letters[1:5], letters[1:5])) %>% 
  glimpse()

returns

Observations: 3
Variables: 2
$ posn      <int> 2, 1, 0
$ lets_list <list> [<"a", "b", "c", "d", "e">, <"a", "b", "c", "d", "e">, <"a", "b", "c", "d", "e">]

In this example, I want to create another column using mutate to return the element in the list "lets_list" based on the value in "posn".

lets %>% 
  mutate(lets_sel = map2_chr(lets_list, posn, ~.x[.y]))

fails with this error message as the third row have posn = 0.

> lets %>% 
+   mutate(lets_sel = map2_chr(lets_list, posn, ~.x[.y]))
#    Error in mutate_impl(.data, dots) : 
#      Evaluation error: Result 3 is not a length 1 atomic vector.

Using the possibly function with map2_chr returns an error too.

lets %>% 
  mutate(lets_sel = map2_chr(lets_list, posn, possibly(~.x[.y], NA_character_)))
# Error in mutate_impl(.data, dots) : 
#  Evaluation error: Result 3 is not a length 1 atomic vector.

However, the map2 function works fine:

> lets %>% 
+   mutate(lets_sel = map2(lets_list, posn, possibly(~.x[.y], NA_character_)))
# A tibble: 3 x 3
   posn lets_list lets_sel 
  <int> <list>    <list>   
1     2 <chr [5]> <chr [1]>
2     1 <chr [5]> <chr [1]>
3     0 <chr [5]> <chr [0]>

A workaround solution is to use map2 and then map_chr, but I suspect that this is a bug.

> lets %>% 
+   mutate(lets_sel = map2(lets_list, posn, ~.x[.y]),
+          lets_sel = map_chr(lets_sel, possibly(~.x[1], NA_character_)))
    # A tibble: 3 x 3
       posn lets_list lets_sel
      <int> <list>    <chr>   
    1     2 <chr [5]> b       
    2     1 <chr [5]> a       
    3     0 <chr [5]> NA      

Am I missing something here? Thanks.

解决方案

possibly() doesn't work because indexing with 0 doesn't throw an error; it just returns a length 0 vector:

nth_letter <- function(n) letters[n]

possibly(nth_letter, "not returned")(0)
#> character(0)

nth_letter(0)
#> character(0)

In this case it would probably be easier to replace invalid indices with NA (using e.g. dplyr::na_if(), or plain old ifelse if the real problem is more complex) to get what you are after:

lets %>% 
  mutate(lets_sel = map2_chr(lets_list, na_if(posn, 0), ~ .x[.y]))
#> # A tibble: 3 x 3
#>    posn lets_list lets_sel
#>   <int> <list>    <chr>   
#> 1     2 <chr [5]> b       
#> 2     1 <chr [5]> a       
#> 3     0 <chr [5]> <NA>

Created on 2018-08-07 by the reprex package (v0.2.0.9000).

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