使用purrr:map()从不规则列表中提取数据

Question

给定一个包含多个元素的列表，目标是将它们放入数据框架中。purr包中的map_df函数对于常规列表非常有用，但对于不规则列表则会出现错误。

例如，遵循这教程，可以完成以下工作：

library(purrr)
library(repurrrsive) # The data comes from this package


map_dfr(got_chars, magrittr::extract, c("name", "culture", "gender", "id", "born", "alive"))

 A tibble: 30 x 6
   name               culture  gender    id born                                   alive
   <chr>              <chr>    <chr>  <int> <chr>                                  <lgl>
 1 Theon Greyjoy      Ironborn Male    1022 In 278 AC or 279 AC, at Pyke           TRUE 
 2 Tyrion Lannister   ""       Male    1052 In 273 AC, at Casterly Rock            TRUE 
 3 Victarion Greyjoy  Ironborn Male    1074 In 268 AC or before, at Pyke           TRUE 
 4 Will               ""       Male    1109 ""                                     FALSE
 5 Areo Hotah         Norvoshi Male    1166 In 257 AC or before, at Norvos         TRUE 
 6 Chett              ""       Male    1267 At Hag's Mire                          FALSE
 7 Cressen            ""       Male    1295 In 219 AC or 220 AC                    FALSE
 8 Arianne Martell    Dornish  Female   130 In 276 AC, at Sunspear                 TRUE 
 9 Daenerys Targaryen Valyrian Female  1303 In 284 AC, at Dragonstone              TRUE 
10 Davos Seaworth     Westeros Male    1319 In 260 AC or before, at King's Landing TRUE 
# … with 20 more rows

但是，如果从列表中删除元素，则该函数将失败。

got_chars[[1]]["gender"]<-NULL
map_dfr(got_chars, magrittr::extract, c("name", "culture", "gender", "id", "born", "alive"))

#Error: Argument 3 is a list, must contain atomic vectors

所需的输出将是缺失元素的NA值。什么是优雅的解决方案？我怀疑这个解决方案包括使用purrr:possibly()，但我还没有弄清楚。

Answer 1

一种方法是定义一个partial()ly指定的pluck()，它提取感兴趣的名称，如果缺少，返回NA。将修改后的pluck()传递给双映射，内部映射遍历要提取的名称，外部映射遍历got_chars列表：

v <- set_names(c("name", "culture", "gender", "id", "born", "alive"))
map_dfr( got_chars, ~map(v, partial(pluck, .x, .default=NA)) )
# # A tibble: 30 x 6
#    name             culture  gender    id born                             alive
#    <chr>            <chr>    <chr>  <int> <chr>                            <lgl>
#  1 Theon Greyjoy    Ironborn NA      1022 In 278 AC or 279 AC, at Pyke     TRUE 
#  2 Tyrion Lannister ""       Male    1052 In 273 AC, at Casterly Rock      TRUE 
#  3 Victarion Greyj… Ironborn Male    1074 In 268 AC or before, at Pyke     TRUE 
#  4 Will             ""       Male    1109 ""                               FALSE
#  5 Areo Hotah       Norvoshi Male    1166 In 257 AC or before, at Norvos   TRUE 
#  6 Chett            ""       Male    1267 At Hag's Mire                    FALSE
#  7 Cressen          ""       Male    1295 In 219 AC or 220 AC              FALSE
#  8 Arianne Martell  Dornish  Female   130 In 276 AC, at Sunspear           TRUE 
#  9 Daenerys Targar… Valyrian Female  1303 In 284 AC, at Dragonstone        TRUE 
# 10 Davos Seaworth   Westeros Male    1319 In 260 AC or before, at King's … TRUE 
# # … with 20 more rows

为了澄清，.x在got_chars上迭代，因为它存在于用~指定的lambda函数中，因此它对应于外部map。内部map的函数是用partial()指定的，它将当前查看的got_chars元素(即.x)作为pluck()的第一个参数。修改后的pluck()接受要提取的名称作为其(新的)第一个参数，因此它可以按原样传递到内部映射，而不需要任何额外的~。

Answer 2

一个固有的问题是[ (或其别名magrittr::extract)在缺少我们试图提取的元素时的行为：

list(a = 1)["b"]
# $<NA>
# NULL

magrittr::extract(list(a = 1), "b")
# $<NA>
# NULL

我们可以界定：

extract_if_present <- function(x, y) {
  x[intersect(y, names(x))]
}

表现为：

extract_if_present(list(a = 1), "b")
# named list()

然后用缺少的元素进行行绑定“只起作用”：

map_dfr(
  got_chars_mutilated,
  extract_if_present,
  c("name", "culture", "gender", "id", "born", "alive")
)
# # A tibble: 30 x 6
#    name               culture     id born                                   alive gender
#    <chr>              <chr>    <int> <chr>                                  <lgl> <chr> 
#  1 Theon Greyjoy      Ironborn  1022 In 278 AC or 279 AC, at Pyke           TRUE  NA    
#  2 Tyrion Lannister   ""        1052 In 273 AC, at Casterly Rock            TRUE  Male  
#  3 Victarion Greyjoy  Ironborn  1074 In 268 AC or before, at Pyke           TRUE  Male  
#  4 Will               ""        1109 ""                                     FALSE Male  
#  5 Areo Hotah         Norvoshi  1166 In 257 AC or before, at Norvos         TRUE  Male  
#  6 Chett              ""        1267 At Hag's Mire                          FALSE Male  
#  7 Cressen            ""        1295 In 219 AC or 220 AC                    FALSE Male  
#  8 Arianne Martell    Dornish    130 In 276 AC, at Sunspear                 TRUE  Female
#  9 Daenerys Targaryen Valyrian  1303 In 284 AC, at Dragonstone              TRUE  Female
# 10 Davos Seaworth     Westeros  1319 In 260 AC or before, at King's Landing TRUE  Male  
# # … with 20 more rows

列的顺序有点混乱，取决于行的顺序和它们遗漏的内容。

Answer 3

喜欢那个教程！在本教程的末尾，作者说：

在编程时，以通常的方式显式指定类型和构建数据框架是更安全的，但更麻烦。

您可以使用更详细的方法将默认值设置为NA。

got_chars %>% {
  tibble(
    name = map_chr(., "name"),
    culture = map_chr(., "culture"),
    gender = map_chr(., "gender", .default = NA),
    id = map_chr(., "id"),
    born = map_chr(., "born"),
    alive = map_chr(., "alive")
  )
}
# # A tibble: 30 x 6
# name               culture    gender id    born                                     alive
# <chr>              <chr>      <chr>  <chr> <chr>                                    <chr>
#   1 Theon Greyjoy      "Ironborn" NA     1022  "In 278 AC or 279 AC, at Pyke"           TRUE 
# 2 Tyrion Lannister   ""         Male   1052  "In 273 AC, at Casterly Rock"            TRUE 
# 3 Victarion Greyjoy  "Ironborn" Male   1074  "In 268 AC or before, at Pyke"           TRUE 
# 4 Will               ""         Male   1109  ""                                       FALSE
# 5 Areo Hotah         "Norvoshi" Male   1166  "In 257 AC or before, at Norvos"         TRUE 
# 6 Chett              ""         Male   1267  "At Hag's Mire"                          FALSE
# 7 Cressen            ""         Male   1295  "In 219 AC or 220 AC"                    FALSE
# 8 Arianne Martell    "Dornish"  Female 130   "In 276 AC, at Sunspear"                 TRUE 
# 9 Daenerys Targaryen "Valyrian" Female 1303  "In 284 AC, at Dragonstone"              TRUE 
# 10 Davos Seaworth     "Westeros" Male   1319  "In 260 AC or before, at King's Landing" TRUE