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我正在努力抓取一个依赖 .atomsvc​​ 文件的公共数据源，以允许用户在 Excel 中设置数据馈送。我使用 XML 库在 R 中构建了一个非常脆弱的解析器来提取 URL。我想知道如何在 xml2 中做到这一点（最好以更简洁和优雅的方式）

这是我使用 XML 库的方法

# Crystal Reports Parser Sample
library(XML)
library(dplyr)

# Get the .atomsvc file from the Export to Data Feed Option on the PA DEP website
pa_string <- '<?xml version="1.0" encoding="utf-8" standalone="yes"?><service xmlns:atom="http://www.w3.org/2005/Atom" xmlns:app="http://www.w3.org/2007/app" xmlns="http://www.w3.org/2007/app"><workspace><atom:title>Oil_Gas_Well_Production</atom:title><collection href="http://www.depreportingservices.state.pa.us/ReportServer?%2FOil_Gas%2FOil_Gas_Well_Production&amp;P_PERIOD_ID=198&amp;P_COUNTY%3Aisnull=True&amp;P_CLIENT%3Aisnull=True&amp;P_PERMIT_NUM%3Aisnull=True&amp;P_OGO_NUM%3Aisnull=True&amp;P_PRODUCING%3Aisnull=True&amp;rs%3AParameterLanguage=&amp;rs%3ACommand=Render&amp;rs%3AFormat=ATOM&amp;rc%3ADataFeed=xAx0x2"><atom:title>Tablix1</atom:title></collection></workspace></service>'

pa_list <- pa_string %>% xmlParse() %>% xmlToList()
# Extract the URL
URL <- URLdecode(pa_list$workspace$collection$.attrs)

据我所知，这是 xml2 版本

# Crystal Reports xml2 Parser
library(xml2)
library(dplyr)

# Get the .atomsvc file from the Export to Data Feed Option on the PA DEP website
pa_string <- '<service xmlns:atom="http://www.w3.org/2005/Atom" xmlns:app="http://www.w3.org/2007/app" xmlns="http://www.w3.org/2007/app"><workspace><atom:title>Oil_Gas_Well_Production</atom:title><collection href="http://www.depreportingservices.state.pa.us/ReportServer?%2FOil_Gas%2FOil_Gas_Well_Production&amp;P_PERIOD_ID=198&amp;P_COUNTY%3Aisnull=True&amp;P_CLIENT%3Aisnull=True&amp;P_PERMIT_NUM%3Aisnull=True&amp;P_OGO_NUM%3Aisnull=True&amp;P_PRODUCING%3Aisnull=True&amp;rs%3AParameterLanguage=&amp;rs%3ACommand=Render&amp;rs%3AFormat=ATOM&amp;rc%3ADataFeed=xAx0x2"><atom:title>Tablix1</atom:title></collection></workspace></service>'

pa_list <- pa_string %>% read_xml() %>% as_list()

我不知道如何从这里提取 URL，或者这是否是思考如何思考这个问题的正确方法。非常感激任何的帮助！

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1 回答 1

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这是一种方法，通过从指定节点中提取属性：

library(xml2)
library(tidyverse)

pa_string <- '<?xml version="1.0" encoding="utf-8" standalone="yes"?><service xmlns:atom="http://www.w3.org/2005/Atom" xmlns:app="http://www.w3.org/2007/app" xmlns="http://www.w3.org/2007/app"><workspace><atom:title>Oil_Gas_Well_Production</atom:title><collection href="http://www.depreportingservices.state.pa.us/ReportServer?%2FOil_Gas%2FOil_Gas_Well_Production&amp;P_PERIOD_ID=198&amp;P_COUNTY%3Aisnull=True&amp;P_CLIENT%3Aisnull=True&amp;P_PERMIT_NUM%3Aisnull=True&amp;P_OGO_NUM%3Aisnull=True&amp;P_PRODUCING%3Aisnull=True&amp;rs%3AParameterLanguage=&amp;rs%3ACommand=Render&amp;rs%3AFormat=ATOM&amp;rc%3ADataFeed=xAx0x2"><atom:title>Tablix1</atom:title></collection></workspace></service>'

 pa_string %>% 
  read_xml() %>% 
  xml_find_all("//*[name()='collection']")%>%
  xml_attr("href") 
#output
[1] "http://www.depreportingservices.state.pa.us/ReportServer?%2FOil_Gas%2FOil_Gas_Well_Production&P_PERIOD_ID=198&P_COUNTY%3Aisnull=True&P_CLIENT%3Aisnull=True&P_PERMIT_NUM%3Aisnull=True&P_OGO_NUM%3Aisnull=True&P_PRODUCING%3Aisnull=True&rs%3AParameterLanguage=&rs%3ACommand=Render&rs%3AFormat=ATOM&rc%3ADataFeed=xAx0x2"

xpath:

#// - Recursive descent; searches for the specified element at any depth. 
#* -  Matches any element node
#[ ] - Applies a filter pattern.
#name()='collection' - self explanatory

更短：

pa_string %>% 
  read_xml() %>% 
  xml_find_all("//@href") #select all attributes with name `href`

由于只有一个具有属性的元素，因此还可以执行以下操作：

pa_string %>% 
  read_xml() %>% 
  xml_find_all("//@*") #Matches any attribute node
于 2017-09-29T18:47:18.100 回答