R xmlParse（）索引

Question

我有從xmlParse()生成的數據。我能夠獲得對被稱爲一個XmlNode參考root

> class(root) 
[1] "XMLInternalElementNode" "XMLInternalNode"  "XMLAbstractNode"  

的時候，我做

> root[[2]][[1]] 
<tr class="party-republican race-winner"><th rowspan="5" class="results-county">Autauga <span class="precincts-reporting">100.0% Reporting</span></th>&#13; 
           &#13; 
           <th scope="row" class="results-candidate">M. Romney</th>&#13; 
           <td class="results-party"><abbr title="Republican">GOP</abbr></td>&#13; 
           <td class="results-percentage">72.6%</td>&#13; 
           <td class="results-popular"> 17,366</td>&#13; 
          </tr> 

我嘗試引用在側價值標籤：

<td class="results-percentage"> 

但是這樣做root[[1]][[2]][["<td class='results-percentage'>]]返回null

我在做什麼錯誤，阻止我從交流聆聽72.6％的價值？

Answer 1

你應該給一個有效xpath，事情是這樣的：

//td[@class='results-percentage'] ## preeceeding by td and cotaining a certain class 

使用您的數據：

library(XML) 
dd <- xmlParse('<tr class="party-republican race-winner"><th rowspan="5" class="results-county">Autauga <span class="precincts-reporting">100.0% Reporting</span></th>&#13; 
    &#13; 
    <th scope="row" class="results-candidate">M. Romney</th>&#13; 
    <td class="results-party"><abbr title="Republican">GOP</abbr></td>&#13; 
    <td class="results-percentage">72.6%</td>&#13; 
    <td class="results-popular"> 17,366</td>&#13; 
    </tr> ',asText=TRUE) 

然後應用的XPath

getNodeSet(dd, "//td[@class='results-percentage']/text()")[[1]] 
72.6% 

或者使用xpathSApply：

xpathSApply(dd, "//td[@class='results-percentage']",xmlValue) 
[1] "72.6%"