[英]In R XML Package, what is the difference between xmlParse and xmlTreeParse?
When would I want to use the xmlParse
function versus the xmlTreeParse
function? 我什么时候想要使用
xmlParse
函数而不是xmlTreeParse
函数? Also, when are parameter values useInternalNodes=TRUE
or asText=TRUE
useful? 另外,何时参数值
useInternalNodes=TRUE
或asText=TRUE
有用吗?
For example: 例如:
library("XML")
nct_url <- "http://clinicaltrials.gov/ct2/show/NCT00112281?resultsxml=true"
xml_doc <- xmlParse(nct_url, useInternalNodes=TRUE)
vs. 与
doc <- xmlTreeParse(getURL(nct_url), useInternalNodes=TRUE)
top <- xmlRoot(doc)
top[["keyword"]]
xmlValue(top[["start_date"]])
xmlValue(top[["location"]])
People seem to use the xmlTreeParse
function for getting a non-repeating node via the $doc$children$... traversal. 人们似乎使用
xmlTreeParse
函数通过$ doc $ children $ ...遍历来获取非重复节点。 But I am not sure I understand when each approach is best. 但我不确定每种方法最好的时候都能理解。 Parsing XML is one of the reasons to almost abandon R and learn Python.
解析XML是几乎放弃R并学习Python的原因之一。 Lack of for-dummies examples without being forced to buy a book.
在没有被迫买书的情况下缺乏傻瓜的例子。
Here some feedback after using XML package. 这里有一些使用XML包后的反馈。
xmlParse
is a version of xmlTreeParse
where argument useInternalNodes
is set to TRUE. xmlParse
是xmlTreeParse
一个版本,其中参数useInternalNodes
设置为TRUE。 xmlTreeParse
. xmlTreeParse
。 This can be not very efficient and unnecessary if you want just to extract partial part of the xml document. xmlParse
. xmlParse
。 But you should know some xpath
bases to manipulate the result. xpath
基础来操纵结果。 asText=TRUE
if you have a text not a file or an url as input. asText=TRUE
。 Here an example where I show the difference between the 2 functions: 这里有一个例子,我展示了两个函数之间的区别:
txt <- "<doc>
<el> aa </el>
</doc>"
library(XML)
res <- xmlParse(txt,asText=TRUE)
res.tree <- xmlTreeParse(txt,asText=TRUE)
Now inspecting the 2 objects: 现在检查2个对象:
class(res)
[1] "XMLInternalDocument" "XMLAbstractDocument"
> class(res.tree)
[1] "XMLDocument" "XMLAbstractDocument"
You see that res is an internal document. 您看到res是内部文档。 It is pointer to a C object.
它是指向C对象的指针。 res.tree is an R object.
res.tree是一个R对象。 You can get its attributes like this :
你可以得到这样的属性:
res.tree$doc$children
$doc
<doc>
<el>aa</el>
</doc>
For res, you should use a valid xpath
request and one of theses functions ( xpathApply
, xpathSApply
, getNodeSet
) to inspect it. 对于res,您应该使用有效的
xpath
请求和其中一个函数( xpathApply
, xpathSApply
, getNodeSet
)来检查它。 for example: 例如:
xpathApply(res,'//el')
Once you create a valid Xml Node , you can apply xmlValue
, xmlGetAttr
,..to extract node information. 创建有效的Xml节点后,可以应用
xmlValue
, xmlGetAttr
,..来提取节点信息。 So here this 2 statements are equivalent: 所以这两个陈述是等价的:
## we have already an R object, just apply xmlValue to the right child
xmlValue(res.tree$doc$children$doc)
## xpathSApply create an R object and pass it to
xpathSApply(res,'//el',xmlValue)
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