具有显式默认命名空间的XML文档的XPath和命名空间规范

我正在努力获得XPath表达式和命名空间规范的正确组合,如 XML(参数名称空间)所需,对于在顶部元素上定义了显式xmlns名称空间的XML文档.

UPDATE

感谢har07,我能够把它放在一起：

查询名称空间后,ns的第一个条目还没有名称,这就是问题所在：

nsDefs <- xmlNamespaceDefinitions(doc)
ns <- structure(sapply(nsDefs,function(x) x$uri),names = names(nsDefs))

> ns
                                             omegahat                          r 
    "http://something.org"  "http://www.omegahat.org" "http://www.r-project.org"

所以我们只需指定一个充当前缀的名称(这可以是任何有效的R名称)：

names(ns)[1] <- "xmlns"

现在,我们所要做的就是在XPath表达式中的任何地方使用默认名称空间前缀：

getNodeSet(doc,"/xmlns:doc//xmlns:b[@omegahat:status='foo']",ns)

对于那些对基于name()和namespace-uri()(以及其他)的替代解决方案感兴趣的人可能会发现this post很有帮助.

仅仅是为了参考：在我们找到解决方案之前,这是试错代码：

考虑一下？xmlParse中的示例：

require("XML")

doc <- xmlParse(system.file("exampleData","tagnames.xml",package = "XML"))

> doc
<?xml version="1.0"?>
<doc>
  <!-- A comment -->
  <a xmlns:omegahat="http://www.omegahat.org" xmlns:r="http://www.r-project.org">
    <b>
      <c>
        <b/>
      </c>
    </b>
    <b omegahat:status="foo">
      <r:d>
        <a status="xyz"/>
        <a/>
        <a status="1"/>
      </r:d>
    </b>
  </a>
</doc>
nsDefs <- xmlNamespaceDefinitions(getNodeSet(doc,"/doc/a")[[1]])
ns <- structure(sapply(nsDefs,names = names(nsDefs))
getNodeSet(doc,"/doc//b[@omegahat:status='foo']",ns)[[1]]

但是,在我的文档中,名称空间已在< doc>中定义.标签,所以我相应地调整了示例XML代码：

xml_source <- c(
  "<?xml version="1.0"?>","<doc xmlns:omegahat="http://www.omegahat.org" xmlns:r="http://www.r-project.org">","<!-- A comment -->","<a>","<b>","<c>","<b/>","</c>","</b>","<b omegahat:status="foo">","<r:d>","<a status="xyz"/>","<a/>","<a status="1"/>","</r:d>","</a>","</doc>"
)
write(xml_source,file="exampleData_2.xml")  
doc <- xmlParse("exampleData_2.xml")
nsDefs <- xmlNamespaceDefinitions(doc)
ns <- structure(sapply(nsDefs,names = names(nsDefs))    
getNodeSet(doc,"/doc",namespaces = ns)
getNodeSet(doc,namespaces = ns)[[1]]

一切都还行.但更重要的是,我的XML代码还具有默认命名空间(xmlns)的显式定义：

xml_source <- c(
  "<?xml version="1.0"?>","<doc xmlns="http://something.org" xmlns:omegahat="http://www.omegahat.org" xmlns:r="http://www.r-project.org">","</doc>"  
)
write(xml_source,file="exampleData_3.xml")  
doc <- xmlParse("exampleData_3.xml")
nsDefs <- xmlNamespaceDefinitions(doc)
ns <- structure(sapply(nsDefs,names = names(nsDefs))

过去的工作现在失败了：

> getNodeSet(doc,namespaces = ns)
list()
attr(,"class")
[1] "XMLNodeSet"
Warning message:
using http://something.org as prefix for default namespace http://something.org 

> getNodeSet(doc,"/xmlns:doc",namespaces = ns)
XPath error : Undefined namespace prefix
XPath error : Invalid expression
Error in xpathApply.XMLInternalDocument(doc,path,fun,...,namespaces = namespaces,: 
  error evaluating xpath expression /xmlns:doc
In addition: Warning message:
using http://something.org as prefix for default namespace http://something.org 
getNodeSet(doc,namespaces = matchNamespaces(doc,namespaces="xmlns",nsDefs = nsDefs)
)

这似乎让我更接近：

> getNodeSet(doc,+ namespaces = matchNamespaces(doc,nsDefs = nsDefs)
+ )[[1]]
<doc xmlns="http://something.org" xmlns:omegahat="http://www.omegahat.org" xmlns:r="http://www.r-project.org">
  <!-- A comment -->
  <a>
    <b>
      <c>
        <b/>
      </c>
    </b>
    <b omegahat:status="foo">
      <r:d>
        <a status="xyz"/>
        <a/>
        <a status="1"/>
      </r:d>
    </b>
  </a>
</doc> 

attr(,"class")
[1] "XMLNodeSet"

然而,现在我不知道如何进入子节点：

> getNodeSet(doc,"/xmlns:doc//b[@omegahat:status='foo']",ns)[[1]]
XPath error : Undefined namespace prefix
XPath error : Invalid expression
Error in xpathApply.XMLInternalDocument(doc,: 
  error evaluating xpath expression /xmlns:doc//b[@omegahat:status='foo']
In addition: Warning message:
using http://something.org as prefix for default namespace http://something.org 

> getNodeSet(doc,+ namespaces = c(
+ matchNamespaces(doc,nsDefs = nsDefs),+ matchNamespaces(doc,namespaces="omegahat",nsDefs = nsDefs)
+ )
+ )
list()
attr(,"class")
[1] "XMLNodeSet"

/xmlns:doc//xmlns:b[@omegahat:status='foo']

getNodeSet(doc,"/ns:doc//ns:b[@omegahat:status='foo']",c(ns="http://something.org"))