Detailed explanation of usage examples of foreach array traversal in PHP array (picture)

What is foreach to traverse an array?

When we use foreach to traverse an array, we often make mistakes due to unclear concepts. Here is a brief introduction to commonly used foreach operations.

(PHP 4, PHP 5, PHP 7)

The foreach syntax structure provides a simple way to traverse an array. foreach can only be applied to arrays and objects. If you try to apply it to variables of other data types, or uninitialized variables, an error message will be issued.

Two syntaxes for each foreach to traverse an array:

foreach (array_expression as $value)
    statement
foreach (array_expression as $key => $value)
    statement

The first format traverses the given array_expression array. In each loop, the value of the current unit is assigned to $value and the pointer inside the array moves forward one step (so the next unit will be obtained in the next loop).

The second format does the same thing, except that the key name of the current unit will also be assigned to the variable $key in each loop.

Since PHP 5, it is also possible to traverse objects.

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

Note: Unless the array is referenced, foreach operates on a copy of the specified array, not the array itself. foreach has some side effects on array pointers. Do not rely on the value of an array pointer during or after a foreach loop unless it is reset.

Since PHP 5, it is easy to modify the elements of an array by adding & before $value. This method assigns by reference rather than copying a value. The reference to

<?php
header("Content-Type:text/html; charset=utf-8");
$arr = array(1, 2, 3, 4);
foreach ($arr as &$value) {
    $value = $value * 2;
}
// $arr is now array(2, 4, 6, 8)
unset($value); // 最后取消掉引用
?>

$value is only available if the array being traversed can be referenced (for example, it is a variable). The following code will not run:

<?php
foreach (array(1, 2, 3, 4) as &$value) {
    $value = $value * 2;
}
?>

Note: The $value reference of the last element of the array will still be retained after the foreach loop. It is recommended to use unset() to destroy it. foreach does not support the ability to suppress error messages using "@".

The following is an example of php foreach traversing an array

1. The array to be traversed is a one-dimensional array, which is relatively simple. If it is a single array, it is expressed in two ways, as follows:

<?php
header("Content-Type:text/html; charset=utf-8");
$array = array("PHP中文网","百度","搜狗","www.php.cn");
foreach ($array as $value){
    echo $value."<br>";
}
echo "<br>";
foreach ($array as $key =>$value){
    echo $key."--".$value."<br>";
}
?>

The output result is:

2. If it is a one-dimensional associative array, it is displayed as follows, Note that the key values ​​are different:

<?php
header("Content-Type:text/html; charset=utf-8");
$array = array("网站1"=>"PHP中文网","网站2"=>"百度","网站3"=>"搜狗","网址"=>"www.php.cn");
foreach ($array as $value){
    echo $value."<br>";
}
echo "<br>";
foreach ($array as $key =>$value){
    echo $key."--".$value."<br>";
}
?>

The traversal result is:

3. It is a little troublesome when traversing a two-dimensional array One point, the value at this time is an array, from which a variety of methods of operating arrays can be derived. Let’s traverse the basic two-dimensional array first:

<?php
header("Content-Type:text/html; charset=utf-8");
$array = array(
    array("1","PHP中文网"),
    array("2","www.php.cn"),
    array("3","百度"),
    array("4","搜狗")
);
foreach ($array as $value){
    print_r($value);
    echo "<br>";
}
?>

The traversal results are as follows:

Use the second method. When traversing the array, pay attention to the key The value of , do not confuse it with the array id value;

<?php
header("Content-Type:text/html; charset=utf-8");
$array = array(
    array("1","PHP中文网"),
    array("2","www.php.cn"),
    array("3","百度"),
    array("4","搜狗")
);
foreach ($array as $key =>$value){
    echo "key =".$key."<br>";
    print_r($value);
    echo "<br>";
}
?>

The traversal results are as follows:

In addition, there are often requests to change a certain column of the two-dimensional array It becomes a one-dimensional array. At this time, the corresponding fields in value need to be taken out and placed in a one-dimensional array. If it is operated with a for loop, it is more cumbersome. It is simpler to use foreach. As follows, get the id column and name column of the current array. , stored in one-dimensional arrays respectively:

<?php
header("Content-Type:text/html; charset=utf-8");
$array = array(
    array("id"=>"1","name"=>"PHP中文网"),
    array("id"=>"2","name"=>"www.php.cn"),
    array("id"=>"3","name"=>"百度"),
    array("id"=>"4","name"=>"搜狗")
); //二维数组定义
//获取某一列作为一维数组
$idArr = array();//id列
$nameARR = array();//name列
foreach($array as $key => $value){
    $arr = $value["id"];
    $idArr =$arr;  //存入ID列
    $name = $value["name"];
    $nameARR = $name;  //存入name列
    print_r($idArr);
    print_r($nameARR);
}
?>

The traversal result is:

[Related tutorial recommendations]

1. Recommended related topics: "php array (Array)"

2. Recommended related video courses: " Use a foreach loop to traverse: index and associative array》

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