for循环导致无限加载等待

Question

我正在编写一个 PHP 脚本，其中包含以下 for 循环：

for($x=count($content->data->parameters[0]->weather->children());$content->data->weather->{"weather-conditions"}[$x]['weather-type']==NULL;--$x){
    echo $x."\n";
}

其中 $content 是 SimpleXMLElement 的一个实例，它解析 XML 如下：

<dwml xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="1.0" xsi:noNamespaceSchemaLocation="http://graphical.weather.gov/xml/DWMLgen/schema/DWML.xsd">
<data>
<parameters applicable-location="point1">
<location>
<weather time-layout="k-p3h-n41-3">
<name>Weather Type, Coverage, and Intensity</name>
<weather-conditions>
<value coverage="slight chance" intensity="light" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="slight chance" intensity="light" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="chance" intensity="light" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="likely" intensity="light" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="definitely" intensity="light" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="definitely" intensity="light" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="definitely" intensity="moderate" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
<value coverage="areas" intensity="none" additive="and" weather-type="blowing snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="definitely" intensity="moderate" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
<value coverage="areas" intensity="none" additive="and" weather-type="blowing snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="definitely" intensity="moderate" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
<value coverage="areas" intensity="none" additive="and" weather-type="blowing snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="definitely" intensity="moderate" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
<value coverage="areas" intensity="none" additive="and" weather-type="blowing snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="chance" intensity="light" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
<value coverage="areas" intensity="none" additive="and" weather-type="blowing snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="chance" intensity="light" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
<value coverage="areas" intensity="none" additive="and" weather-type="blowing snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="slight chance" intensity="light" weather-type="snow showers" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="slight chance" intensity="light" weather-type="snow showers" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="slight chance" intensity="light" weather-type="snow showers" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="slight chance" intensity="light" weather-type="snow showers" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions/>
<weather-conditions/>
<weather-conditions/>
<weather-conditions/>
<weather-conditions/>
<weather-conditions/>
<weather-conditions/>
<weather-conditions/>
<weather-conditions/>
<weather-conditions/>
<weather-conditions/>
<weather-conditions/>
<weather-conditions/>
<weather-conditions/>
<weather-conditions>
<value coverage="slight chance" intensity="light" weather-type="rain" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="slight chance" intensity="light" weather-type="rain" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="slight chance" intensity="light" weather-type="rain" qualifier="none">
<visibility xsi:nil="true"/>
</value>
<value coverage="slight chance" intensity="light" additive="and" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="slight chance" intensity="light" weather-type="rain" qualifier="none">
<visibility xsi:nil="true"/>
</value>
<value coverage="slight chance" intensity="light" additive="and" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="slight chance" intensity="light" weather-type="rain" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="slight chance" intensity="light" weather-type="rain" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="chance" intensity="light" weather-type="rain" qualifier="none">
<visibility xsi:nil="true"/>
</value>
<value coverage="chance" intensity="light" additive="and" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="chance" intensity="light" weather-type="rain" qualifier="none">
<visibility xsi:nil="true"/>
</value>
<value coverage="chance" intensity="light" additive="and" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="chance" intensity="light" weather-type="rain" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="chance" intensity="light" weather-type="rain" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
<weather-conditions>
<value coverage="chance" intensity="light" weather-type="rain" qualifier="none">
<visibility xsi:nil="true"/>
</value>
<value coverage="chance" intensity="light" additive="and" weather-type="snow" qualifier="none">
<visibility xsi:nil="true"/>
</value>
</weather-conditions>
</weather>
</parameters>
</data>
</dwml>

编辑：解析此文件后，这是var_dump($content->data->parameters[0]->weather); 产生的结果：

object(SimpleXMLElement)#4 (3) {
  ["@attributes"]=>
  array(1) {
    ["time-layout"]=>
    string(11) "k-p3h-n41-3"
  }
  ["name"]=>
  string(37) "Weather Type, Coverage, and Intensity"
  ["weather-conditions"]=>
  array(41) {
    [0]=>
    object(SimpleXMLElement)#2 (1) {
      ["value"]=>
      object(SimpleXMLElement)#45 (2) {
        ["@attributes"]=>
        array(4) {
          ["coverage"]=>
          string(13) "slight chance"
          ["intensity"]=>
          string(5) "light"
          ["weather-type"]=>
          string(4) "snow"
          ["qualifier"]=>
          string(4) "none"
        }
        ["visibility"]=>
        object(SimpleXMLElement)#46 (0) {
        }
      }
    }
    [1]=>
    object(SimpleXMLElement)#5 (1) {
      ["value"]=>
      object(SimpleXMLElement)#45 (2) {
        ["@attributes"]=>
        array(4) {
          ["coverage"]=>
          string(13) "slight chance"
          ["intensity"]=>
          string(5) "light"
          ["weather-type"]=>
          string(4) "snow"
          ["qualifier"]=>
          string(4) "none"
        }
        ["visibility"]=>
        object(SimpleXMLElement)#46 (0) {
        }
      }
    }
    [2]=>
    object(SimpleXMLElement)#6 (1) {
      ["value"]=>
      object(SimpleXMLElement)#45 (2) {
        ["@attributes"]=>
        array(4) {
          ["coverage"]=>
          string(6) "chance"
          ["intensity"]=>
          string(5) "light"
          ["weather-type"]=>
          string(4) "snow"
          ["qualifier"]=>
          string(4) "none"
        }
        ["visibility"]=>
        object(SimpleXMLElement)#46 (0) {
        }
      }
    }
    [3]=>
    object(SimpleXMLElement)#7 (1) {
      ["value"]=>
      object(SimpleXMLElement)#45 (2) {
        ["@attributes"]=>
        array(4) {
          ["coverage"]=>
          string(6) "likely"
          ["intensity"]=>
          string(5) "light"
          ["weather-type"]=>
          string(4) "snow"
          ["qualifier"]=>
          string(4) "none"
        }
        ["visibility"]=>
        object(SimpleXMLElement)#46 (0) {
        }
      }
    }
    [4]=>
    object(SimpleXMLElement)#8 (1) {
      ["value"]=>
      object(SimpleXMLElement)#45 (2) {
        ["@attributes"]=>
        array(4) {
          ["coverage"]=>
          string(10) "definitely"
          ["intensity"]=>
          string(5) "light"
          ["weather-type"]=>
          string(4) "snow"
          ["qualifier"]=>
          string(4) "none"
        }
        ["visibility"]=>
        object(SimpleXMLElement)#46 (0) {
        }
      }
    }
    [5]=>
    object(SimpleXMLElement)#9 (1) {
      ["value"]=>
      object(SimpleXMLElement)#45 (2) {
        ["@attributes"]=>
        array(4) {
          ["coverage"]=>
          string(10) "definitely"
          ["intensity"]=>
          string(5) "light"
          ["weather-type"]=>
          string(4) "snow"
          ["qualifier"]=>
          string(4) "none"
        }
        ["visibility"]=>
        object(SimpleXMLElement)#46 (0) {
        }
      }
    }
    [6]=>
    object(SimpleXMLElement)#10 (1) {
      ["value"]=>
      array(2) {
        [0]=>
        object(SimpleXMLElement)#45 (2) {
          ["@attributes"]=>
          array(4) {
            ["coverage"]=>
            string(10) "definitely"
            ["intensity"]=>
            string(8) "moderate"
            ["weather-type"]=>
            string(4) "snow"
            ["qualifier"]=>
            string(4) "none"
          }
          ["visibility"]=>
          object(SimpleXMLElement)#47 (0) {
          }
        }
        [1]=>
        object(SimpleXMLElement)#46 (2) {
          ["@attributes"]=>
          array(5) {
            ["coverage"]=>
            string(5) "areas"
            ["intensity"]=>
            string(4) "none"
            ["additive"]=>
            string(3) "and"
            ["weather-type"]=>
            string(12) "blowing snow"
            ["qualifier"]=>
            string(4) "none"
          }
          ["visibility"]=>
          object(SimpleXMLElement)#47 (0) {
          }
        }
      }
    }
    [7]=>
    object(SimpleXMLElement)#11 (1) {
      ["value"]=>
      array(2) {
        [0]=>
        object(SimpleXMLElement)#46 (2) {
          ["@attributes"]=>
          array(4) {
            ["coverage"]=>
            string(10) "definitely"
            ["intensity"]=>
            string(8) "moderate"
            ["weather-type"]=>
            string(4) "snow"
            ["qualifier"]=>
            string(4) "none"
          }
          ["visibility"]=>
          object(SimpleXMLElement)#47 (0) {
          }
        }
        [1]=>
        object(SimpleXMLElement)#45 (2) {
          ["@attributes"]=>
          array(5) {
            ["coverage"]=>
            string(5) "areas"
            ["intensity"]=>
            string(4) "none"
            ["additive"]=>
            string(3) "and"
            ["weather-type"]=>
            string(12) "blowing snow"
            ["qualifier"]=>
            string(4) "none"
          }
          ["visibility"]=>
          object(SimpleXMLElement)#47 (0) {
          }
        }
      }
    }
    [8]=>
    object(SimpleXMLElement)#12 (1) {
      ["value"]=>
      array(2) {
        [0]=>
        object(SimpleXMLElement)#45 (2) {
          ["@attributes"]=>
          array(4) {
            ["coverage"]=>
            string(10) "definitely"
            ["intensity"]=>
            string(8) "moderate"
            ["weather-type"]=>
            string(4) "snow"
            ["qualifier"]=>
            string(4) "none"
          }
          ["visibility"]=>
          object(SimpleXMLElement)#47 (0) {
          }
        }
        [1]=>
        object(SimpleXMLElement)#46 (2) {
          ["@attributes"]=>
          array(5) {
            ["coverage"]=>
            string(5) "areas"
            ["intensity"]=>
            string(4) "none"
            ["additive"]=>
            string(3) "and"
            ["weather-type"]=>
            string(12) "blowing snow"
            ["qualifier"]=>
            string(4) "none"
          }
          ["visibility"]=>
          object(SimpleXMLElement)#47 (0) {
          }
        }
      }
    }
    [9]=>
    object(SimpleXMLElement)#13 (1) {
      ["value"]=>
      array(2) {
        [0]=>
        object(SimpleXMLElement)#46 (2) {
          ["@attributes"]=>
          array(4) {
            ["coverage"]=>
            string(10) "definitely"
            ["intensity"]=>
            string(8) "moderate"
            ["weather-type"]=>
            string(4) "snow"
            ["qualifier"]=>
            string(4) "none"
          }
          ["visibility"]=>
          object(SimpleXMLElement)#47 (0) {
          }
        }
        [1]=>
        object(SimpleXMLElement)#45 (2) {
          ["@attributes"]=>
          array(5) {
            ["coverage"]=>
            string(5) "areas"
            ["intensity"]=>
            string(4) "none"
            ["additive"]=>
            string(3) "and"
            ["weather-type"]=>
            string(12) "blowing snow"
            ["qualifier"]=>
            string(4) "none"
          }
          ["visibility"]=>
          object(SimpleXMLElement)#47 (0) {
          }
        }
      }
    }
    [10]=>
    object(SimpleXMLElement)#14 (1) {
      ["value"]=>
      array(2) {
        [0]=>
        object(SimpleXMLElement)#45 (2) {
          ["@attributes"]=>
          array(4) {
            ["coverage"]=>
            string(6) "chance"
            ["intensity"]=>
            string(5) "light"
            ["weather-type"]=>
            string(4) "snow"
            ["qualifier"]=>
            string(4) "none"
          }
          ["visibility"]=>
          object(SimpleXMLElement)#47 (0) {
          }
        }
        [1]=>
        object(SimpleXMLElement)#46 (2) {
          ["@attributes"]=>
          array(5) {
            ["coverage"]=>
            string(5) "areas"
            ["intensity"]=>
            string(4) "none"
            ["additive"]=>
            string(3) "and"
            ["weather-type"]=>
            string(12) "blowing snow"
            ["qualifier"]=>
            string(4) "none"
          }
          ["visibility"]=>
          object(SimpleXMLElement)#47 (0) {
          }
        }
      }
    }
...//More in here, trivial.
    [40]=>
    object(SimpleXMLElement)#44 (1) {
      ["value"]=>
      array(2) {
        [0]=>
        object(SimpleXMLElement)#45 (2) {
          ["@attributes"]=>
          array(4) {
            ["coverage"]=>
            string(6) "chance"
            ["intensity"]=>
            string(5) "light"
            ["weather-type"]=>
            string(4) "rain"
            ["qualifier"]=>
            string(4) "none"
          }
          ["visibility"]=>
          object(SimpleXMLElement)#47 (0) {
          }
        }
        [1]=>
        object(SimpleXMLElement)#46 (2) {
          ["@attributes"]=>
          array(5) {
            ["coverage"]=>
            string(6) "chance"
            ["intensity"]=>
            string(5) "light"
            ["additive"]=>
            string(3) "and"
            ["weather-type"]=>
            string(4) "snow"
            ["qualifier"]=>
            string(4) "none"
          }
          ["visibility"]=>
          object(SimpleXMLElement)#47 (0) {
          }
        }
      }
    }
  }
}

由于末尾的元素有时会为空，并且我想要最新的非空天气项，我将 x 设置为天气数组中最后一项的索引，并尝试向后工作，但页面只是不加载，并且错误日志中没有任何内容。 编辑：我已将方法更改为此，以使其更易于理解：

for($x=0; $content->data->parameters[0]->weather->{"weather-conditions"}[count($content->data->parameters[0]->weather->children())-$x]['weather-type']==NULL; $x++){
    echo $x."\n";
}

编辑：如果不清楚，我需要通过子数组向后递归，而不是向前。

Answer 1

我可以在您的问题中找到一些不同的问题：

XML 无效。请在提问时提供工作数据，此处<location> 元素已打开且从未关闭。我的其余答案的前提是这个开始标签已被删除。

没有阻止无限循环。$x 低于 0 是没有意义的，但你不检查它。你应该添加这样一个检查，这样你就不会仅仅因为你在中断决策逻辑的另一部分犯了一些错误就创建了一个无限循环：

for (
    $x = count($content->data->parameters[0]->weather->children());
    $x >= 0 && $content->data->weather->{"weather-conditions"}[$x]['weather-type'] == NULL;
    --$x
) {
    echo $x . "\n";
}

使用（固定的）示例数据，现在将从 42 倒数到 0，但不低于 0。至少无限循环已被删除。清除这些技术问题后，您可以专注于问题领域：

由于最后的元素有时会是空的，我想要最新的非空天气项目

嗯，您的实际示例 XML 不包含 任何 空的 <weather> 元素（即没有任何子元素的 <weather> 元素。所以不清楚您在问什么。 p>

但是它包含空的 <weather-conditions/> 元素，因此作为其余答案的先决条件，我改为使用它。

XML 没有特定的顺序 - 至少不是设计使然 - 正如您的示例 XML 所示，空的 <weather-conditions/> 元素更多地位于文档的中间而不是末尾。您真正想要的是所有包含子元素的<weather-conditions> 元素（此处为<value>）。最好使用 xpath 查询来做到这一点。

因此，您首先查询 具有 <value> children 的所有元素，然后获取最后一个元素（按文档顺序）。所以我们开始：

list($lastCondition) = $content->xpath('
    (
        /*/data/parameters/weather/weather-conditions[value]
    )[position() = last()]
');

变量 $lastCondition 然后包含最后一个非空元素，这里是基于您的示例 XML 的 XML 输出：

<weather-conditions>
                    <value coverage="chance" intensity="light" weather-type="rain" qualifier="none">
                        <visibility xsi:nil="true"/>
                    </value>
                    <value coverage="chance" intensity="light" additive="and" weather-type="snow" qualifier="none">
                        <visibility xsi:nil="true"/>
                    </value>
                </weather-conditions>

正如您所看到的，您的问题有很多不清楚的地方，因此很难回答。最后，我希望这仍然有帮助。

使用的方法是SimpleXMLElement::xpath()，Xpath 1.0（该方法使用的）在W3C站点XML Path Language (XPath) Version 1.0上指定，Stackoverflow本身就包含很多示例，所以最好先在这里搜索。

Answer 2

原来我在层次结构中跳过了几个级别，这就是为什么检查字符串是否为 NULL 的条件总是返回 TRUE（因此是无限循环）。最终代码：

for($x=count($content->data->parameters[0]->weather->children()); $x>0; $x--){
    if($content->data->parameters[0]->weather->{"weather-conditions"}[$x]!=NULL){
        return $content->data->parameters[0]->weather->{"weather-conditions"}[$x][0]->value['weather-type'];
    }
}