如何使用xml数据进行矢量化？

let's say, I have this xml file:

比方说,我有这个xml文件:

<?xml version="1.0" encoding="UTF-8" ?>
<TimeSeries>
  <timeZone>1.0</timeZone>
  <series>
    <header/>
    <event date="2009-09-30" time="10:00:00" value="0.0" flag="2"></event>
    <event date="2009-09-30" time="10:15:00" value="0.0" flag="2"></event>
    <event date="2009-09-30" time="10:30:00" value="0.0" flag="2"></event>
    <event date="2009-09-30" time="10:45:00" value="0.0" flag="2"></event>
    <event date="2009-09-30" time="11:00:00" value="0.0" flag="2"></event>
    <event date="2009-09-30" time="11:15:00" value="0.0" flag="2"></event>
  </series>
  <series>
    <header/>
    <event date="2009-09-30" time="08:00:00" value="1.0" flag="2"></event>
    <event date="2009-09-30" time="08:15:00" value="2.6" flag="2"></event>
    <event date="2009-09-30" time="09:00:00" value="6.3" flag="2"></event>
    <event date="2009-09-30" time="09:15:00" value="4.4" flag="2"></event>
    <event date="2009-09-30" time="09:30:00" value="3.9" flag="2"></event>
    <event date="2009-09-30" time="09:45:00" value="2.0" flag="2"></event>
    <event date="2009-09-30" time="10:00:00" value="1.7" flag="2"></event>
    <event date="2009-09-30" time="10:15:00" value="2.3" flag="2"></event>
    <event date="2009-09-30" time="10:30:00" value="2.0" flag="2"></event>
  </series>
  <series>
    <header/>
    <event date="2009-09-30" time="10:00:00" value="0.0" flag="2"></event>
    <event date="2009-09-30" time="10:15:00" value="0.0" flag="2"></event>
    <event date="2009-09-30" time="10:30:00" value="0.0" flag="2"></event>
    <event date="2009-09-30" time="10:45:00" value="0.0" flag="2"></event>
    <event date="2009-09-30" time="11:00:00" value="0.0" flag="2"></event>
  </series>
</TimeSeries>

and let's say I want to do something with its series elements and that I would like to put in practice the advice 'vectorize the vectorizable'... I import the XML library and do the following:

并且假设我想用它的系列元素做一些事情,并且我想在实践中提出建议'矢量化矢量化'...我导入XML库并执行以下操作:

R> library("XML")
R> doc <- xmlTreeParse('/home/mario/Desktop/sample.xml')
R> TimeSeriesNode <- xmlRoot(doc)
R> seriesNodes <- xmlElementsByTagName(TimeSeriesNode, "series")
R> length(seriesNodes)
[1] 3
R> (function(x){length(xmlElementsByTagName(x[['series']], 'event'))}
+ )(seriesNodes)
[1] 6
R> 

and I don't understand why I should only get the result of applying the function to the first element: I had expected three values, just as the length of seriesNodes, something like this:

我不明白为什么我只应该将函数应用到第一个元素:我原本期望三个值,就像seriesNodes的长度一样,如下所示:

R> mapply(length, seriesNodes)
series series series 
     7     10      6 

oops! I already came with the answer: "use mapply":

哎呀!我已经得到了答案:“使用mapply”:

R> mapply(function(x){length(xmlElementsByTagName(x, 'event'))}, seriesNodes)
series series series 
     6      9      5 

but then I see the following problem: the R-inferno tells me that I'm "loop-hiding", not "vectorizing"! can I avoid looping at all? ...

但后来我看到了以下问题:R-inferno告诉我,我是“循环隐藏”,而不是“矢量化”!我可以避免循环吗? ...

2 个解决方案

#1

You could also use xpathApply or xpathSApply-- these functions extract node sets using an XPath specification and then execute a function each set. Both of these functions are provided by the XML package. In order to use these functions, the XML document must be parsed using xmlInternalTreeParse or with the useInternalNodes option of xmlTreeParse set to be true:

您还可以使用xpathApply或xpathSApply--这些函数使用XPath规范提取节点集,然后执行每个集合的函数。这两个函数都是由XML包提供的。要使用这些函数,必须使用xmlInternalTreeParse或将xmlTreeParse的useInternalNodes选项设置为true来解析XML文档:

require( XML )

countEvents <- function( series ){

  events <- xmlElementsByTagName( series, 'event' )
  return( length( events ) ) 

}

doc <- xmlTreeParse( "sample.xml", useInternalNodes = T )

xpathSApply( doc, '/TimeSeries/series', countEvents )
[1] 6 9 5

I don't know if it is any "faster", but the code is definitely cleaner and very explicit to anyone who knows the XPath syntax and how an apply function operates.

我不知道它是否“更快”,但对于知道XPath语法以及apply函数如何运行的人来说,代码肯定更清晰,更明确。

#2

Since seriesNodes is a list of nodes, there is no easy way to avoid the implicit looping. Simple operations like getting the length are not computationally intensive, so I wouldn't lose any sleep over not being able to vectorise.

由于seriesNodes是一个节点列表,因此没有简单的方法可以避免隐式循环。像获取长度这样的简单操作不是计算密集型的,因此我不会因为无法进行矢量化而失去任何睡眠。

Note that you can use sapply(seriesNodes, length), instead of mapply, since there is only one argument to the length function.

请注意,您可以使用sapply(seriesNodes,length)而不是mapply,因为length函数只有一个参数。

The "proper R way" to do things is to use (s|m)apply calls to extract vectors of useful bits of data, then analyse those in the usual manner.

做事的“正确的R方式”是使用(s | m)应用调用来提取有用数据位的向量,然后以通常的方式分析它们。

Finally, if you really are desperate to vectorise counting events, use names(unlist(seriesNodes)) and then count the occurances of "series.children.event.name" in between each occurance of "series.name". This is undoubtedly uglier, and possibly slower than the sapply call.

最后,如果你真的急于矢量化计数事件,请使用名称(unlist(seriesNodes)),然后在每次出现“series.name”之间计算“series.children.event.name”的出现次数。这无疑是丑陋的,而且可能比公开呼叫要慢。