PHP array traversal foreach syntax structure and examples, foreach syntax_PHP tutorial

PHP array traversal foreach syntax structure and examples, foreach syntax

foreach()

PHP foreach() syntax structure is used to traverse operations or output arrays. foreach() can only be used to traverse arrays or objects. An error will occur when trying to use it for other data types or an uninitialized variable.

Grammar:

foreach (array as $value)
  statement
// 或者：
foreach (array as $key => $value)

statement

In the above syntax, each loop assigns the value of the current unit to $value and moves the pointer inside the array forward one step. In the second syntax format, the key name of the current unit is also assigned to the variable $key in each loop.

Example:

<&#63;php
$arr_age = array("wang"=>18, "li"=>20, "zhang"=>25);
foreach ($arr_age as $age) {
  echo $age,'<br />';
}
&#63;>

Output of running this example:

18
20
25

Use array keys

<&#63;php
$arr_age = array("wang"=>18, "li"=>20, "zhang"=>25);
foreach ($arr_age as $key=>$age) {
  echo $key,': ',$age,'<br />';
}
&#63;>

Output of running the example:

wang: 18
li: 20
zhang: 25

When foreach starts executing, the pointer inside the array will automatically point to the first element, which means there is no need to call reset() before the foreach loop.

Foreach operates on a copy of the specified array, not the array itself. Modifications to the returned array elements will not affect the original array (see example below), but when the foreach loop runs to the end, the internal pointer of the original array will point to the end of the array.

<&#63;php
$arr_age = array("wang"=>18, "li"=>20, "zhang"=>25);
foreach ($arr_age as $age) {
  $age = $age+10;
  echo $age,'<br />';
}
// 输出原数组
print_r($arr_age);
&#63;>

Output of running the example:

28
30
35
Array ( [wang] => 18 [li] => 20 [zhang] => 25 ) 

To modify the original array elements in foreach, you can do it by reference. Change the above example to:

<&#63;php
$arr_age = array("wang"=>18, "li"=>20, "zhang"=>25);
foreach ($arr_age as &$age) {
  $age = $age+10;
  echo $age,'<br />';
}
// 输出原数组
print_r($arr_age);
&#63;>

Output of running the example:

18
20
25

Array ( [wang] => 28 [li] => 30 [zhang] => 35 ) 

Traverse multidimensional arrays

The foreach syntax structure can only be used to traverse one-dimensional arrays. To traverse multi-dimensional arrays, generally use foreach nested recursion or divide the original array into one-dimensional arrays and then perform foreach traversal.

Example of one- and two-dimensional array mixing:

<&#63;php
$arr_age = array("wang"=>18, "li"=>20, "zhang"=>array("name"=>"小张", "age"=>25));
foreach ($arr_age as $age) {
  if(is_array($age)){
    foreach ( $age as $detail) {
    echo $detail,'<br />';
    }
  } else {
    echo $age,'<br />';
  }
}
&#63;>

The most appropriate processing method for traversing multi-dimensional arrays must be based on the actual data structure.

PHP arrays are implemented through HashTable tables, so foreach traverses the array according to the order in which elements are added. If you want to iterate by index size, you should use a for() loop.

for() loops through the array

If you are operating an array of continuous key values, you can also use a for() loop to traverse the array:

<&#63;php
$arr_age = array(18, 20, 25);
$num = count($arr_age);
for($i = 0; $i < $num; $i++){
  echo $arr_age[$i]."<br />";
}
&#63;>

You can also use list() and each() combined to traverse the array, but testing found that it is not as efficient as foreach().

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