PHP foreach 和参考

Question

我试图在 PHP 的嵌套 foreach 循环中使用指针来修改值。 但是，以下行似乎不起作用：

// Assign a the attribs value to the array
$link_row['value'] = $args[ $u_value ];

变量 $args[ $u_value ];已填充并且可以毫无问题地输出，但是当我将它添加到 $link_row 引用时，它似乎没有设置..

  foreach ($unique_links as $link_id => &$link_attr)
  {
     foreach($link_attr as &$link_row)
     {
        foreach($link_row as $u_attr => &$u_value)
        {
           if ($u_attr == 'attribute_name') 
           {               

              // Assign a the attribs value to the array
              $link_row['value'] = $args[ $u_value ];

              // If one of the values for the unique key is blank,  
              // we can remove the entire 
              // set from being checked
              if ( !isset($args[ $u_value ]) ) 
              {
                 unset($unique_links[$link_id] );
              }
           }
        }
     }
  }

Answer 1

在foreach 中使用变量后，您必须始终取消设置变量。即使您有两个foreach(..) 语句一个接一个。即使您在另一个foreach(..) 中有一个foreach(..)。 没有例外！

foreach ($unique_links as $link_id => &$link_attr)
{
 foreach($link_attr as &$link_row)
 {
    foreach($link_row as $u_attr => &$u_value)
    {
       if ($u_attr == 'attribute_name') 
       {               

          // Assign a the attribs value to the array
          $link_row['value'] = $args[ $u_value ];

          // If one of the values for the unique key is blank,  
          // we can remove the entire 
          // set from being checked
          if ( !isset($args[ $u_value ]) ) 
          {
             unset($unique_links[$link_id] );
          }
       }
    }
    unset($u_value);  // <- this is important
 }
 unset($link_row);  // <- so is this
}
unset($lnk_attr);  // <- and so is this, even if you reached the end of your program or the end of a function or a method and even if your foreach is so deeply indented or on such a long line that you're not sure what code might follow it, because another developer (maybe even you) will come back and read the code and he might not see that you used a reference in a foreach

这是不久前搞砸了一个大项目的另一段有趣的代码：

foreach ($data as $id => &$line) {
    echo "This is line {$id}: '{$line}'\n";
    $line .= "\n";
}

echo "And here is the output, one line of data per line of screen:\n";

foreach ($data as $id => &$line) {
    echo $line;
}

事实上有人在第一个foreach(..) 之后没有unset($line) 确实弄乱了数组中的数据，因为&$line 是一个引用，而第二个foreach(..) 在循环时为其分配了不同的值通过数据，它一直覆盖最后一行数据。

Answer 2

由于我看不到您的数组，我不知道哪些是整数，哪些是关联的，等等。

据我所知，没有理由引用 $u_value。它不会造成任何伤害，但无论哪种方式都没有区别。

更重要的是，任何时候你的第二个 if 条件为真，在你在线到达它之前你都会遇到错误

 $link_row['value'] = $args[ $u_value ];

也许你想使用

$link_row['value'] = isset($args[$u_value]) ? $args[ $u_value ] : "NOT PRESENT";

你提到的那行似乎工作得很好。

我的代码：

    $args = array(100,200,300,400,500);
    $unique_links = array (array(   'a' => array('attribute_name' => 1,'x' => 2, 'y' => 3, 'z' =>4), 
                                    'b' => array('attribute_name' => 3,'x' => 2, 'y' => 3, 'z' =>4),
                                    'c' => array('attribute_name' => 0,'x' => 2, 'y' => 3, 'z' =>4),
                                    'd' => array('attribute_name' => 7,'x' => 2, 'y' => 3, 'z' =>4),
                                    'e' => array('attribute_name' => 1,'x' => 2, 'y' => 3, 'z' =>4)

                                    ));                                 
    echo_r($unique_links);
    foreach ($unique_links as $link_id => &$link_attr)
    {
        foreach($link_attr as &$link_row)
        {
            foreach($link_row as $u_attr => $u_value)
            {
                echo "&nbsp&nbsp&nbsp&nbsp $u_attr is $u_value <br />";
                if ($u_attr == 'attribute_name')
                {

                    // Assign a the attribs value to the array
                    $link_row['value'] = isset($args[$u_value]) ? $args[ $u_value ] : "NOT PRESENT";

                    // If one of the values for the unique key is blank, we can remove the entire
                    // set from being checked
                    if ( !isset($args[ $u_value ]) )
                    {
                        //echo "want to kill: $link_id <br />";
                        //unset($unique_links[$link_id] );
                    }
                }
            }
            echo "<br />";
        }
    }
    echo_r($unique_links);

我的输出：

Array
(
[0] => Array
    (
        [a] => Array
            (
                [attribute_name] => 1
                [x] => 2
                [y] => 3
                [z] => 4
            )

        [b] => Array
            (
                [attribute_name] => 3
                [x] => 2
                [y] => 3
                [z] => 4
            )

        [c] => Array
            (
                [attribute_name] => 0
                [x] => 2
                [y] => 3
                [z] => 4
            )

        [d] => Array
            (
                [attribute_name] => 7
                [x] => 2
                [y] => 3
                [z] => 4
            )

        [e] => Array
            (
                [attribute_name] => 1
                [x] => 2
                [y] => 3
                [z] => 4
            )

    )

)

 attribute_name is 1
 x is 2
 y is 3
 z is 4
 value is 200

 attribute_name is 3
 x is 2
 y is 3
 z is 4
 value is 400

 attribute_name is 0
 x is 2
 y is 3
 z is 4
 value is 100

 attribute_name is 7
 x is 2
 y is 3
 z is 4
 value is NOT PRESENT

 attribute_name is 1
 x is 2
 y is 3
 z is 4
 value is 200

Array
(
[0] => Array
    (
        [a] => Array
            (
                [attribute_name] => 1
                [x] => 2
                [y] => 3
                [z] => 4
                [value] => 200
            )

        [b] => Array
            (
                [attribute_name] => 3
                [x] => 2
                [y] => 3
                [z] => 4
                [value] => 400
            )

        [c] => Array
            (
                [attribute_name] => 0
                [x] => 2
                [y] => 3
                [z] => 4
                [value] => 100
            )

        [d] => Array
            (
                [attribute_name] => 7
                [x] => 2
                [y] => 3
                [z] => 4
                [value] => NOT PRESENT
            )

        [e] => Array
            (
                [attribute_name] => 1
                [x] => 2
                [y] => 3
                [z] => 4
                [value] => 200
            )

    )

)

我注释掉 unnset 因为它似乎会杀死整个数组，而不仅仅是你想要的部分。我猜这是由于杀死了您当前正在迭代的部分而导致的一些奇怪行为。

Answer 3

我认为您可能会覆盖循环中的值。要对此进行测试，您可以执行以下操作

$link_row['value'] = $args[ $u_value ];

改成

$link_row[] = $args[ $u_value ];

然后在循环之外添加这个

echo "Link Row Value(s):<pre>".print_r($link_row,true)."</pre><br />\n";

这将显示所有正在转换/设置为 $link_row['value'] 的值，如果您看到多个索引，则这些值将被覆盖