xml2：在同一行中的指定文本之后抓取数组中的文本

Question

我有 XML 和一堆 envelope 元素。每个里面都有一个数组。数组中的每一行都有 2 个元素。第一个是标识符，第二个是我要抓取的文本。我需要行的第一个值来识别正确的行，以便我可以获取正确的值。

在下面的示例中，我在用代码 610954 表示的行中有 'food'。我想获取此代码后的 2 个元素（c('pizza', 'burger')。同样，代码 605380 表示 'drinks'。我想获取 c('coke', 'pepsi')。如何使用xml2包要干这个？

library(xml2)
library(magrittr)

myxml <- read_xml('
<inside>
 <envelope>
  <card-entries type="array">
     <card-entry>
       <card-id type="integer">605380</card-id>
       <value>coke</value>
     </card-entry>
     <card-entry>
       <card-id type="integer">610954</card-id>
       <value>pizza</value>
     </card-entry>  
   </card-entries>
 </envelope>
 <envelope>
  <card-entries type="array">
     <card-entry>
       <card-id type="integer">605380</card-id>
       <value>pepsi</value>
     </card-entry>
     <card-entry>
       <card-id type="integer">610954</card-id>
       <value>burger</value>
     </card-entry>  
   </card-entries>
 </envelope>
</inside>
'
)

## as far as I can parse it (but not specific enough)
myxml %>%
    xml_find_all('//envelope/card-entries[@type="array"]/card-entry') %>%
    xml_text()

food <- -CODE THAT GIVES HERE c('pizza', 'burger')- # 610954
drinks <- -CODE THAT GIVES HERE c('coke', 'pepsi')- # 605380

Answer 1

你原来的方法可以这样修改来得到饮料：

myxml %>%
  xml_find_all('//envelope/card-entries[@type="array"]/card-entry[card-id = "605380"]/value') %>%
  xml_text()
#[1] "coke"  "pepsi"

但您可以采用多种其他方法

# get following sibling called value
myxml %>% 
  # foods
  xml_find_all('//card-id[text()="610954"]/following-sibling::value') %>%
  xml_text()
#[1] "pizza"  "burger"

# get following::value[1] - Specify [1] or you would get all following values, 
# including "pepsi".  With value[1] you get only the following value.
myxml %>% 
  # foods
  xml_find_all('//card-id[text()="610954"]/following::value[1]') %>%
  xml_text()
#[1] "pizza"  "burger"

# look for value nodes with a preceding sibling with the appropriate card-id
myxml %>% 
  # drinks
  xml_find_all('//value[preceding-sibling::card-id[text()="605380"]]') %>%
  xml_text()
#[1] "coke"  "pepsi"

# Get value node that is a child of card-entry nodes with the appropriate card-id.
# specifically looking in envelope elements
myxml %>% 
  # drinks
  xml_find_all('//envelope/card-entries/card-entry[card-id = "605380"]/value') %>%
  xml_text()
#[1] "coke"  "pepsi"

# less specific
myxml %>% 
  xml_find_all('//card-entry[card-id = "605380"]/value') %>%
  xml_text()
#[1] "coke"  "pepsi"

Answer 2

怎么样：

library(tidyverse)
library(stringr)
myxml %>%
  xml_find_all('//envelope/card-entries[@type="array"]/card-entry') %>%
  xml_text() %>% 
  map(.f = str_sub, start = c(1, 7), end = c(6, 1000000L)) %>% 
  reduce(rbind) %>% 
  as_tibble() %>% 
  mutate(type = ifelse(V1 == 605380, yes = "drinks", no = "food"))

然后您可以轻松地将饮料和食物分开。

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