Detailed explanation of the usage of php foreach

php Usage of foreach: 1. Use it through the syntax "foreach (array_expression as $value)"; 2. Use it through the syntax "foreach (array_expression as $key => $value)".

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Usage and examples of foreach in PHP

Foreach is often used in PHP, and to use foreach, you must use an array. Therefore, in this article, we will talk about arrays and foreach at the same time.

Foreach has two syntaxes:

The first one: traverse the given array statement array_expression array. Each time through the loop, the value of the current cell is assigned to $value and the pointer inside the array is moved forward one step (so the next cell will be obtained in the next loop).

foreach (array_expression as $value)

Second type: Same as above, at the same time, the key name of the current unit will also be assigned to the variable $key in each loop.

foreach (array_expression as $key => $value)

Let’s explain them one by one!

1. One-dimensional ordinary array and foreach

We first write a one-dimensional array, as follows:

$a = array('Tom','Mary','Peter','Jack');

1. We use the first one foreach method to output.

foreach ($a as $value) {
  echo $value."<br/>";
}

The final result is:

Tom
Mary
Peter
Jack

2. We use the second foreach method to output.

foreach ($a as $key => $value) {
  echo $key.&#39;,&#39;.$value."<br/>";
}

The final result is:

0,Tom
1,Mary
2,Peter
3,Jack

Summary: Obviously, we see that there is only one more $key, and the value of this $key is the serial number 1 , 2, 3, 4 and so on!

2. One-dimensional associative array and foreach

The one-dimensional associative array is as follows:

$b = array('a'=>'Tom','b'=>'Mary','c'=>'Peter','d'=>'Jack');

Some people also like to write it like this, as follows:

$b = array(
  'a'=>'Tom',
  'b'=>'Mary',
  'c'=>'Peter',
  'd'=>'Jack'
);

1. We use the first foreach method to output the same as above.

foreach ($b as $value) {
  echo $value."<br/>";
}

The final result is:

Tom
Mary
Peter
Jack

2. We use the second foreach method to output.

foreach ($b as $key => $value) {
  echo $key.&#39;,&#39;.$value."<br/>";
}

The final result is:

a,Tom
b,Mary
c,Peter
d,Jack

Summary: Obviously, in a one-dimensional associative array, $key is the associated serial number, that is, the corresponding a, b, c, d.

3. Two-dimensional ordinary array and foreach

When traversing a two-dimensional array, it is a little more troublesome. Why? Because the traversed value is an array. Since it is an array, you can perform various operations on the array!

Let’s first look at a basic two-dimensional array, as follows:

$c = array(
  array('1','Tom'),
  array('2','Mary'),
  array('3','Peter'),
  array('4','Jack')
);

1. We use the first foreach method:

foreach ($c as $value) {
  print_r($value);
  echo "<br/>";
}

gets the following result:

Array ( [0] => 11 [1] => Tom )
Array ( [0] => 22 [1] => Mary )
Array ( [0] => 33 [1] => Peter )
Array ( [0] => 44 [1] => Jack )

2. We use the second foreach method:

foreach ($c as $key => $value) {
  echo &#39;$key=&#39;.$key."<br/>";
  print_r($value);
  echo "<br/>";
}

gets the following result:

$key=0
Array ( [0] => 11 [1] => Tom )
$key=1
Array ( [0] => 22 [1] => Mary )
$key=2
Array ( [0] => 33 [1] => Peter )
$key=3
Array ( [0] => 44 [1] => Jack )

Summary: As can be seen from the above, for a basic two-dimensional array, $key is the serial number, such as 0\1\2\3 and so on!

4. Associative two-dimensional arrays and foreach

Explain that associative two-dimensional arrays are used a lot in actual projects. Why? Generally, the data extracted from the database is associated with two-dimensional arrays. If you learn to associate two-dimensional arrays, you will have mastered a large part of it in actual PHP practice!

Then first list the associated two-dimensional array, as follows:

$d = array(
  array('id'=>'11','name'=>'Tom'),
  array('id'=>'22','name'=>'Mary'),
  array('id'=>'33','name'=>'Peter'),
  array('id'=>'44','name'=>'Jack')
);

1. Use the first method code:

foreach ($d as $value) {
  print_r($value);
  echo "<br/>";
}

The results obtained are as follows:

Array ( [id] => 11 [name] => Tom )
Array ( [id] => 22 [name] => Mary )
Array ( [id] => 33 [name] => Peter )
Array ( [id] => 44 [name] => Jack )

Obviously, the difference between association and non-association is: non-association is preceded by 0/1, etc., while association displays the specific name id/name, etc.

2. Code using the second method:

foreach ($d as $key => $value) {
  echo &#39;$key=&#39;.$key."<br/>";
  print_r($value);
  echo "<br/>";
}

The results obtained are as follows:

$key=0
Array ( [id] => 11 [name] => Tom )
$key=1
Array ( [id] => 22 [name] => Mary )
$key=2
Array ( [id] => 33 [name] => Peter )
$key=3
Array ( [id] => 44 [name] => Jack )

Summary: $key here is still 0/ 1/2/3.

5. Practical application in the project

Explanation: In the project, there are many changes in the array, of course foreach plays an important role! Of course, you can also use while, each, etc. methods, but foreach is the most convenient! Let’s briefly talk about some common project practices!

Practical 1: Convert a two-dimensional associative array into a one-dimensional ordinary array

Still the fourth list of associative two-dimensional arrays, as follows:

$d = array(
  array('id'=>'11','name'=>'Tom'),
  array('id'=>'22','name'=>'Mary'),
  array('id'=>'33','name'=>'Peter'),
  array('id'=>'44','name'=>'Jack')
);

Now we only need one column: name Of course, we can use the following methods to implement it,

foreach ($d as $key => $value) {
  echo ($value['name']);
  echo "<br/>";
}

But sometimes we have to list it as a one-dimensional array, so we have the following method:

//获取name列作为一维数组
$nameArr = array(); //name列
foreach ($d as $key => $value) {
  $nameArr[] = $value['name'];
}
print_r($nameArr);

Above, by assigning an empty array value, foreach this empty array is equal to our value, and we get a new array! The result of the above code is as follows:

Array
(
  [0] => Tom
  [1] => Mary
  [2] => Peter
  [3] => Jack
)

This array is obviously: a one-dimensional ordinary array, as follows:

$d = array('Tom','Mary','Peter','Jack');

Okay, let’s convert the two-dimensional That’s all for turning an associative array into a one-dimensional ordinary array!

Practice 2: Two-level classification and infinite-level classification

Obviously, the data we take out from the database is a two-dimensional array, and it is a two-dimensional associative array. So, how do we extract the parent category? How to get out the subcategories corresponding to the parent category?

The first thing to explain is: almost all categories are a database schema, so it is very necessary for us to understand its structure and how to retrieve the corresponding data!

For the second-level classification, for the convenience of explanation, I found a better example from the Internet, that is "news classification"!

Okay, without further ado, let’s get down to business! Let's write an array first.

//从数据库中取出的分类数据
$original_array = array(
  array('id' => 1,'pid' => 0,'name' => '新闻分类'),
  array('id' => 2,'pid' => 0,'name' => '最新公告'),
  array('id' => 3,'pid' => 1,'name' => '国内新闻'),
  array('id' => 4,'pid' => 1,'name' => '国际新闻'),
  array('id' => 5,'pid' => 0,'name' => '图片分类'),
  array('id' => 6,'pid' => 5,'name' => '新闻图片'),
  array('id' => 7,'pid' => 5,'name' => '其它图片')
);

At the same time, the database looks like this.

说明：数据库的分类就是这个样子的！取出来的数组也是这个样子的！一般这样子的！

//从数据库中取出的分类数据
$original_array = array(
  array(
    'id' => 1,
    'pid' => 0,
    'name' => '新闻分类'
  ),
  array(
    'id' => 2,
    'pid' => 0,
    'name' => '最新公告'
  ),
  array(
    'id' => 3,
    'pid' => 1,
    'name' => '国内新闻'
  ),
  array(
    'id' => 4,
    'pid' => 1,
    'name' => '国际新闻'
  ),
  array(
    'id' => 5,
    'pid' => 0,
    'name' => '图片分类'
  ),
  array(
    'id' => 6,
    'pid' => 5,
    'name' => '新闻图片'
  ),
  array(
    'id' => 7,
    'pid' => 5,
    'name' => '其它图片'
  )
);

那么首先我们得知道我们想要的结果是什么样子呢？这一点：我们必要知道！（以前我对这方面了解比较不深，又常用开源程序，因此导致我不怎么会写这方面了）

我们最终想要的结果是这样子的！（不怕大家笑话，这一点我请一个朋友帮的忙才解决的！）

 

//整理后的分类数据
$output_array = array(
  array(
    'id' => 1,
    'pid' => 0,
    'name' => '新闻分类',
    'children' => array(
      array(
        'id' => 3,
        'pid' => 1,
        'name' => '国内新闻'
      ),
      array(
        'id' => 4,
        'pid' => 1,
        'name' => '国际新闻'
      ),
    ),
  ),
  array(
    'id' => 2,
    'pid' => 0,
    'name' => '最新公告',
  ),
  array(
    'id' => 5,
    'pid' => 0,
    'name' => '图片分类',
    'children' => array(
      array(
        'id' => 6,
        'pid' => 5,
        'name' => '新闻图片'
      ),
      array(
        'id' => 7,
        'pid' => 5,
        'name' => '其它图片'
      ),
    ),
  ),
);

很明显，这里数组多了一个字段，就是 children！

那么，怎么 从 $original_array 变为 $output_array呢？这里有我一个朋友做的函数，当然也用到 foreach！

函数如下：

 

//整理函数
/**
 * 生成无限级树算法
 * @author Baiyu 2014-04-01
 * @param array $arr        输入数组
 * @param number $pid        根级的pid
 * @param string $column_name    列名,id|pid父id的名字|children子数组的键名
 * @return array $ret
 */
function make_tree($arr, $pid = 0, $column_name = 'id|pid|children') {
  list($idname, $pidname, $cldname) = explode('|', $column_name);
  $ret = array();
  foreach ($arr as $k => $v) {
    if ($v [$pidname] == $pid) {
      $tmp = $arr [$k];
      unset($arr [$k]);
      $tmp [$cldname] = make_tree($arr, $v [$idname], $column_name);
      $ret [] = $tmp;
    }
  }
  return $ret;
}

那们怎么使用呢？

 

//整理函数的使用
$output_array = make_tree($original_array);

完整使用方法如下：

 

$output_array =make_tree($arr, 0, 'id|pid|children')

函数之后，我们这样调用就得到了一级分类与二级分类！

foreach ($output_array as $key => $value) {
  echo &#39;<h2 id="value-name">&#39;.$value[&#39;name&#39;].&#39;</h2>&#39;;
  foreach ($value[&#39;children&#39;] as $key => $value) {
    echo $value[&#39;name&#39;].&#39;,&#39;;
}

结果如下：

附：$output_array 这个数组，我们使用print_r，就可以得到如下的结果！

 

Array
(
  [0] => Array
    (
      [id] => 1
      [pid] => 0
      [name] => 新闻分类
      [children] => Array
        (
          [0] => Array
            (
              [id] => 3
              [pid] => 1
              [name] => 国内新闻
              [children] => Array
                (
                )
 
            )
 
          [1] => Array
            (
              [id] => 4
              [pid] => 1
              [name] => 国际新闻
              [children] => Array
                (
                )
 
            )
 
        )
 
    )
 
  [1] => Array
    (
      [id] => 2
      [pid] => 0
      [name] => 最新公告
      [children] => Array
        (
        )
 
    )
 
  [2] => Array
    (
      [id] => 5
      [pid] => 0
      [name] => 图片分类
      [children] => Array
        (
          [0] => Array
            (
              [id] => 6
              [pid] => 5
              [name] => 新闻图片
              [children] => Array
                (
                )
 
            )
 
          [1] => Array
            (
              [id] => 7
              [pid] => 5
              [name] => 其它图片
              [children] => Array
                (
                )
 
            )
 
        )
 
    )
 
)

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