PHP foreach 中“as $key => $value”和“as $value”的区别

Difference between "as $key => $value" and "as $value" in PHP foreach

提问by handmdmr

I have a database call and I'm trying to figure out what the $key => $valuedoes in a foreachloop.

我有一个数据库调用，我试图找出什么$key => $value做的一个foreach循环。

The reason I ask is because both these codes output the same thing, so I'm trying to understand why it's written this way. Here's the code:

我问的原因是因为这两个代码输出相同的东西，所以我试图理解为什么它是这样写的。这是代码：

1)In foreach use $key => $value

1）在foreach中使用 $key => $value

foreach($featured as $key => $value){
  echo $value['name'];
}

this outputs the same as:

这输出与：

2)In foreach use only $value

2）仅在 foreach 中使用 $value

foreach($featured as $value) {
  echo $value['name'];
}

So my question is, what is the difference between $key => $valueor just $valuein the foreachloop. The array is multidimensional if that makes a difference, I just want to know why to pass $keyto $valuein the foreachloop.

所以我的问题是，是什么样的区别$key => $value，或只是$value在foreach循环。如果这有所不同，该数组是多维的，我只想知道为什么$key要$value在foreach循环中传递给。

回答by djiango

Well, the $key => $valuein the foreach loop refers to the key-value pairs in associative arrays, where the key serves as the index to determine the value instead of a number like 0,1,2,... In PHP, associative arrays look like this:

好吧，$key => $valueforeach 循环中的 指的是关联数组中的键值对，其中键作为索引来确定值而不是像 0,1,2 之类的数字，...在 PHP 中，关联数组看起来像这个：

$featured = array('key1' => 'value1', 'key2' => 'value2', etc.);

In the PHP code: $featuredis the associative array being looped through, and as $key => $valuemeans that each time the loop runs and selects a key-value pair from the array, it stores the key in the local $keyvariable to use inside the loop block and the value in the local $valuevariable. So for our example array above, the foreach loop would reach the first key-value pair, and if you specified as $key => $value, it would store 'key1'in the $keyvariable and 'value1'in the $valuevariable.

在 PHP 代码中：$featured是循环通过的关联数组，as $key => $value意味着每次循环运行并从数组中选择一个键值对时，它会将键存储在局部$key变量中以在循环块内部使用，并且在局部$value变量。因此，对于我们上面的例子阵列，foreach循环将达到第一个键值对，如果你指定的as $key => $value，它会存储'key1'在$key变量，'value1'在$value变量。

Since you don't use the $keyvariable inside your loop block, adding it or removing it doesn't change the output of the loop, but it's best to include the key-value pair to show that it's an associative array.

由于您不在$key循环块内使用该变量，因此添加或删除它不会改变循环的输出，但最好包含键值对以表明它是一个关联数组。

Also note that the as $key => $valuedesignation is arbitrary. You could replace that with as $foo => $barand it would work fine as long as you changed the variable references inside the loop block to the new variables, $fooand $bar. But making them $keyand $valuehelps to keep track of what they mean.

另请注意，as $key => $value指定是任意的。您可以将其替换as $foo => $bar为，只要您将循环块内的变量引用更改为新变量，它就可以正常工作，$foo并且$bar. 但是制作它们$key并$value有助于跟踪它们的含义。

回答by Charaf JRA

Let's say you have an associative array like this:

假设您有一个像这样的关联数组：

$a = array(
    "one" => 1,
    "two" => 2,
    "three" => 3,
    "seventeen" => array('x'=>123)
);

In the first iteration : $key="one"and $value=1.

在第一次迭代中： $key="one"和$value=1。

Sometimes you need this key ,if you want only the value , you can avoid using it.

有时你需要这个键，如果你只想要值，你可以避免使用它。

In the last iteration : $key='seventeen'and $value = array('x'=>123)so to get value of the first element in this arrayvalue, you need a key, x in this case: $value['x'] =123.

在最后一次迭代 : 中$key='seventeen'，$value = array('x'=>123)要获取此值中第一个元素的array值，key在这种情况下，您需要 a , x: $value['x'] =123。

回答by Ahmad Baktash Hayeri

A very important place where it is REQUIREDto use the key => valuepair in foreachloop is to be mentioned. Suppose you would want to add a new/sub-element to an existing item (in another key) in the $featuresarray. You should do the following:

需要提到一个非常重要的地方，它需要key => value在foreach循环中使用对。假设您想向数组中的现有项目（在另一个键中）添加一个新/子元素。您应该执行以下操作：$features

foreach($features as $key => $feature) {
    $features[$key]['new_key'] = 'new value';  
} 

Instead of this:

取而代之的是：

foreach($features as $feature) {
    $feature['new_key'] = 'new value';  
} 

The bigdifference here is that, in the first case you are accessing the array's sub-value via the main array itself with a keyto the element which is currently being pointed to by the array pointer.

这里的最大区别在于，在第一种情况下，您通过主数组本身访问数组的子值，并使用指向当前由数组指针指向的元素的键。

While in the second (which doesn't work for this purpose) you are assigning the sub-value in the array to a temporary variable $featurewhich is unset after each loop iteration.

在第二个（不适用于此目的）中，您将数组中的子值分配给一个临时变量$feature，该变量在每次循环迭代后都未设置。

回答by sargas

The difference is that on the

不同的是，在

foreach($featured as $key => $value){
 echo $value['name'];
}

you are able to manipulate the value of each iteration's $keyfrom their key-value pair. Like @djiango answered, if you are not manipulating each value's $key, the result of the loop will be exactly the same as

您可以$key从它们的键值对中操作每个迭代的值。就像@djiango 回答的那样，如果您不操作每个值的$key，则循环的结果将与

foreach($featured as $value) {
  echo $value['name']
}

Source:You can read it from the PHP Documentation:

来源：您可以从PHP 文档中阅读：

The first form loops over the array given by array_expression. On each iteration, the value >of the current element is assigned to $value and the internal array pointer is advanced by >one (so on the next iteration, you'll be looking at the next element).*

The second form will additionally assign the current element's key to the $key variable on >each iteration.

第一种形式循环遍历由 array_expression 给出的数组。在每次迭代中，当前元素的值 > 被分配给 $value 并且内部数组指针前移 >one（因此在下一次迭代中，您将查看下一个元素）。*

第二种形式将在每次迭代时额外将当前元素的键分配给 $key 变量。

---

---

If the data you are manipulating is, say, arrays with custom keys, you could print them to screen like so:

如果您操作的数据是带有自定义键的数组，您可以将它们打印到屏幕上，如下所示：

$array = ("name" => "Paul", "age" => 23);

$array = ("name" => "Paul", "age" => 23);

foreach($featured as $key => $value){
 echo $key . "->" . $value;
}

Should print:

应该打印：

name->Paul

name->Paul

age->23

age->23

And you wouldn't be able to do that with a foreach($featured as $value)with the same ease. So consider the format above a convenient way to manipulate keys when needed.

而且您将无法轻松地做到这一点foreach($featured as $value)。因此，将上述格式视为一种在需要时操作键的便捷方式。

Cheers

干杯

回答by aynber

Say you have an array like this:

假设你有一个这样的数组：

$array = (0=>'123',1=>'abc','test'=>'hi there!')

In your foreach loop, each loop would be:

在您的 foreach 循环中，每个循环将是：

$key = 0, $value = '123'
$key = 1, $value = 'abc'
$key = 'test', $value = 'hi there!'

It's great for those times when you need to know the array key.

当您需要知道数组键时，这非常有用。

回答by 3seconds

if the array looks like:

如果数组看起来像：

• $featured["fruit"] = "orange";
• $featured["fruit"] = "banana";
• $featured["vegetable"] = "carrot";

• $featured["fruit"] = "orange";
• $featured["fruit"] = "banana";
• $featured["vegetable"] = "carrot";

the $key will hold the type (fruit or vegetable) for each array value (orange, banana or carrot)

$key 将保存每个数组值（橙色、香蕉或胡萝卜）的类型（水果或蔬菜）

回答by Thomas Martin Klein

here $keywill contain the $keyassociated with $valuein $featured. The difference is that now you have that key.

这里$键将包含$键关联$值的功能$。不同之处在于，现在您拥有了那把钥匙。

array("thekey"=>array("name"=>"joe"))

here $valueis

这里$value是

array("name"=>"joe")

$keyis "thekey"

$key是“thekey”

回答by optimiertes

Sample Array: Left ones are the keys, right one are my values

示例数组：左边是键，右边是我的值

$array = array(
        'key-1' => 'value-1', 
        'key-2' => 'value-2',
        'key-3' => 'value-3',
        );

Example A: I want only the values of $array

示例 A：我只想要 $array

foreach($array as $value) {    
    echo $value; // Through $value I get first access to 'value-1' then 'value-2' and to 'value-3'     
}

Example B: I want each value AND key of $array

示例 B：我想要的每个值 AND 键 $array

foreach($array as $key => $value) {                 
    echo $value; // Through $value I get first access to 'value-1' then 'value-2' and to 'value-3'  

    echo $key; // Through $key I get access to 'key-1' then 'key-2' and finally 'key-3'    

    echo $array[$key]; // Accessing the value through $key = Same output as echo $value;
    $array[$key] = $value + 1; // Exmaple usage of $key: Change the value by increasing it by 1            
}