ruby - 无法为变量 : undefined method `[]' for nil:NilClass (NoMethodError) 赋值

Question

我完全被这个难住了。

我有以下代码:

puts block.at_xpath("*/img")["width"].to_i

但是当我把它改成

width = block.at_xpath("*/img")["width"].to_i

我收到这个错误:

NokogiriTUT.rb:70:in `blockProcessor': undefined method `[]' for nil:NilClass (NoMethodError)

当我将看跌期权放在那里时，它会返回预期值。

更新:

        def blockProcessor(block)
    header = block.xpath('td[@class="default"]/*/span[@class="comhead"]')
    array = header.text.split
    if array[0] != nil #checks to make sure we aren't at the top of the parent list
        ### Date and Time ###
        if array[2] == 'hours' || array[2] == 'minutes'
            date = Time.now
        else
            days = (array[1].to_i * 24 * 60 * 60)
            date = Time.now - days
        end

        ##Get Comment##
        comment = block.at_xpath('*/span[@class="comment"]')

        hash = comment.text.hash
        #puts hash

        ##Manage Parent Here##

            width = block.at_xpath("*/img")["width"].to_i



            prevlevel = @parent_array[@parent_array.length-1][1]
            if width == 0 #has parents
                parentURL = header.xpath('a[@href][3]').to_s
                parentURL = parentURL[17..23]
                parentURL =  "http://news.ycombinator.com/item?id=#{parentURL}"
                parentdoc = Nokogiri::HTML(open(parentURL))
                a = parentdoc.at_xpath("//html/body/center/table/tr[3]/td/table/tr")
                nodeparent = blockProcessor(a) 
                @parent_array = []
                node = [hash, width, nodeparent] #id, level, parent
                @parent_array.push node     
            elsif width > prevlevel 
                nodeparent =  @parent_array[@parent_array.length-1][0]
                node = [hash, width, nodeparent]
                @parent_array.push node 
            elsif width == prevlevel
                nodeparent =  @parent_array[@parent_array.length-1][2]
                node = [hash, width, nodeparent]
                @parent_array.push node
            elsif width < prevlevel
                until prevlevel == w do
                    @parent_array.pop
                    prevlevel = @parent_array[@parent_array.length-1][1]
                end
                nodeparent = @parent_array[@parent_array.length-1][2]
                node = [hash, width, nodeparent]
                @parent_array.push node
            end
        puts "Author: #{array[0]} with hash #{hash} with parent: #{nodeparent}"

        ##Handles Any Parents of Existing Comments ##
        return hash
    end
end
   end

这是它作用于的 block 。

 <tr>
    <td><img src="http://ycombinator.com/images/s.gif" height="1" width="0"></td>
    <td valign="top"><center>
    <a id="up_3004849" href="vote?for=3004849&amp;dir=up&amp;whence=%2f%78%3f%66%6e%69%64%3d%34%6b%56%68%71%6f%52%4d%38%44"><img src="http://ycombinator.com/images/grayarrow.gif" border="0" vspace="3" hspace="2"></a><span id="down_3004849"></span>
    </center></td>
    <td class="default">
    <div style="margin-top:2px; margin-bottom:-10px; "><span class="comhead"><a href="user?id=patio11">patio11</a> 12 days ago  | <a href="item?id=3004849">link</a> | <a href="item?id=3004793">parent</a> | on: <a href="item?id=3004471">Ask HN: What % of your job interviewees pass FizzB...</a></span></div>
    <br><span class="comment"><font color="#000000">Every time FizzBuzz problems come up among engineers, people race to solve them and post their answers, then compete to see who can write increasingly more nifty answers for a question which does not seek niftiness at all.<p>I'm all for intellectual gamesmanship, but these are our professional equivalent of a doctor being asked to identify the difference between blood and water.  You can do it.  <i>We know</i>.  Demonstrating that you can do it is not the point of the exercise.  We do it to have a cheap-to-administer test to exclude people-who-cannot-actually-program-despite-previous-job-titles from the expensive portions of the hiring process.</p></font></span><p><font size="1"><u><a href="reply?id=3004849&amp;whence=%2f%78%3f%66%6e%69%64%3d%34%6b%56%68%71%6f%52%4d%38%44">reply</a></u></font></p>
    </td>
</tr>

Answer 1

您的基本问题是您不了解 XPath。 (您在那里有很好的伙伴关系；XPath 非常令人困惑。)您的选择器根本不匹配您认为它们匹配的内容。尤其是那个爆炸的

*/img

应该是

//img

或类似的东西。

现在，因为 xpath 选择器不匹配任何东西，这个 Ruby 语句的值

block.at_xpath("*/img")

为零。而且 nil 不支持 []，因此当您尝试对其调用 ["width"] 时，Ruby 会提示 undefined method [] for nil :NilClass 错误。

至于为什么只有当您将它分配给一个变量时它才会爆炸……是的，这实际上不是发生的事情。您可能还更改了其他内容。

---

现在，请允许我提出一些其他希望有建设性的代码批评:

• 您的问题显然是为了让人难以回答而设计的。以后，请隔离有问题的代码，不要只粘贴您的整个家庭作业(或此屏幕抓取器的用途)。

• 如果你把它做成一个单独的可运行的 Ruby 文件，我们可以在我们的计算机上逐字执行，那就更好了，例如:

.

require "nokogiri"
doc = Nokogiri.parse <<-HTML
<tr>
  <td><img src="http://ycombinator.com/images/s.gif" height="1" width="0"></td>
  <td valign="top"><center>
    <a id="up_3004849" href="vote?for=3004849&amp;dir=up&amp;whence=%2f%78%3f%66%6e%69%64%3d%34%6b%56%68%71%6f%52%4d%38%44"><img src="http://ycombinator.com/images/grayarrow.gif" border="0" vspace="3" hspace="2"></a><span id="down_3004849"></span>
  </center></td>
  <td class="default">
    <div style="margin-top:2px; margin-bottom:-10px; ">
      <span class="comhead">
        <a href="user?id=patio11">patio11</a> 12 days ago  | <a href="item?id=3004849">link</a> | <a href="item?id=3004793">parent</a> | on: <a href="item?id=3004471">Ask HN: What % of your job interviewees pass FizzB...</a>
      </span>
    </div>
    <br><span class="comment"><font color="#000000">Every time FizzBuzz problems come up among engineers, people race to solve them and post their answers, then compete to see who can write increasingly more nifty answers for a question which does not seek niftiness at all.<p>I'm all for intellectual gamesmanship, but these are our professional equivalent of a doctor being asked to identify the difference between blood and water.  You can do it.  <i>We know</i>.  Demonstrating that you can do it is not the point of the exercise.  We do it to have a cheap-to-administer test to exclude people-who-cannot-actually-program-despite-previous-job-titles from the expensive portions of the hiring process.</p></font></span><p><font size="1"><u><a href="reply?id=3004849&amp;whence=%2f%78%3f%66%6e%69%64%3d%34%6b%56%68%71%6f%52%4d%38%44">reply</a></u></font></p>
  </td>
</tr>
HTML

width = doc.at_xpath("*/img")["width"].to_i

这样我们就可以用我们的计算机进行调试，而不仅仅是用我们的头脑。

• 您现在正在编写 Ruby，而不是 Java，因此请遵守 Ruby 的 spacing and naming conventions :文件名是 snake_case，缩进是 2 个空格，没有制表符等。阅读格式错误的代码真的很困难——“错误”意味着“不标准”。

• 在任何有描述性注释(### 日期和时间###)的地方都是提取方法的机会(def date_and_time(array)) 并使您的代码更清晰、更易于调试。