How to loop through arrays using foreach and for

The

foreach syntax structure is used to traverse the array.

foreach()

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

Syntax:

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 the pointer inside the array moves 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:

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

Run this example output:

18
20
25

Use array key value

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

Run the example output:

wang: 18
li: 20
zhang: 25

Tips

When foreach starts executing, the pointer inside the array will automatically point to the first unit, 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.

<?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);
?>

Output of running 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:

<?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);
?>

Output of running example :

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

Traverse Multidimensional array

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

一 Two-dimensional array Mixed example:

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

Run this example output:

18
20
小张
25

Traversal of multi-dimensional array The most appropriate processing method must be adopted based on the actual data structure.

Extended pre-reading

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() loop to traverse the array

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

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

The output of the running example is as follows:

18
20
25

Tips

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

//使用array()语句结构将联系人列表中所有数据声明为一个二维数组,默认下标是顺序数字索引    
$contact1 = array(                                             //定义外层数组    
array(1,'高某','A公司','北京市','(010)987654321','gm@Linux.com'),//子数组1    
array(2,'洛某','B公司','上海市','(021)123456789','lm@apache.com'),//子数组2    
array(3,'峰某','C公司','天津市','(022)24680246','fm@mysql.com'),  //子数组3    
array(4,'书某','D公司','重庆市','(023)13579135','sm@php.com')     //子数组4    
);   //以HTML表格的形式输出二维数组中的每个元素    
echo '<table border="1" width="600" align="center">';    
echo '<caption><h1>联系人列表</h1></caption>';    
echo '<tr bgcolor="#dddddd">';    
echo '<th>编号</th><th>姓名</th><th>公司</th><th>地址</th><th>电话</th><th>EMALL</th>';    
echo '</tr>';    //使用双层for语句嵌套二维数组$contact1,以HTML表格的形式输出
    //使用外层循环遍历数组$contact1中的行    
    for($row=0;$row<count($contact1);$row++)
    {        echo '<tr>';        //使用内层循环遍历数组$contact1 中 子数组的每个元素,使用count()函数控制循环次数        
        for($col=0;$col<count($contact1[$row]);$col++)
            {            
             echo '<td>'.$contact1[$row][$col].'</td>';
            }        
         echo '</tr>';
    }    
         echo '</table>';
$contact1 = array(                   //定义外层数组
    array(1,'高某','A公司','北京市','(010)987654321','gm@Linux.com'),//子数组1
    array(2,'洛某','B公司','上海市','(021)123456789','lm@apache.com'),//子数组2
    array(3,'峰某','C公司','天津市','(022)24680246','fm@mysql.com'),  //子数组3
    array(4,'书某','D公司','重庆市','(023)13579135','sm@php.com')     //子数组4
);
foreach($contact1 as $key=>$s){
//echo  $key;//以每个数组的键值作为表名
    foreach($s as $row){
    echo  $row;
    }
}

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