What are the three ways to traverse an array in php

3 ways to traverse an array: 1. Use for statement to traverse, syntax "for($i=0;$i$v){//loop code}"; 3. Use while with each() and list() to traverse, the syntax is "while(list($k, $v)=each($arr)){//Loop code}".

The operating environment of this tutorial: Windows 7 system, PHP version 8.1, DELL G3 computer

Traversing arrays in PHP There are three commonly used methods:

• Use the for statement to loop through the array;

• Use the foreach statement to traverse the array;

• Combined use of list(), each() and while loop to traverse the array.

The most efficient of these three methods is to use the foreach statement to traverse the array. The foreach structure has been introduced since PHP4. It is a statement specially designed for traversing arrays in PHP. It is recommended that everyone use it. Let’s first introduce these methods respectively.

Method 1: Use the for statement to traverse

The for loop will pre-define the variable that controls the number of loops in the for statement, so the for loop The statement can perform loop operations according to a known number of loops, which is suitable for situations where the number of times the script needs to be run is clearly known.

The syntax format of the for loop is as follows:

for (初始化语句; 循环条件; 变量更新--自增或自减) {
    语句块;   
}

Statement analysis:

• Initialization statement (expression 1): mainly to initialize a The variable value is used to set a counter, which is the value at the beginning of the loop; this statement is only executed during the first loop and will not be executed again in the future.

• Loop condition (expression 2): Restriction condition for loop execution, used to control whether to execute the code in the loop body; if the condition is TRUE, the loop continues, if the condition is FALSE , the loop ends and exits the loop immediately.
  

• Variable update (expression 3): an expression with an increment or decrement operation. Every time the loop is executed, the value of the counter is immediately modified so that the loop The conditions gradually become "untenable".
  

• Statement block: Several codes that need to be executed when the condition is judged to be true.

Example

<?php
header("Content-type:text/html;charset=utf-8");
$array= array(2,"数学","梨子",3.14,"榴莲",12);
var_dump($array);
echo "遍历数组：<br>";
for ($i=0; $i < count($array); $i++) { 
    echo $i."：".$array[$i] . "<br/>";
} 
?>

Output result:

Method 2: Use foreach statement to traverse

foreach is a statement specially designed for traversing arrays. It is a commonly used method when traversing arrays. It provides great convenience in traversing arrays; it can also be used after PHP5 Traverse objects (foreach can only be applied to arrays and objects).

The foreach statement traverses the array regardless of the array subscript, and can be used for discontinuous index arrays and associative arrays with strings as subscripts.

The foreach statement has two syntax formats:

• Grammar format 1:

foreach ($array as $value){
    语句块;
}

Traverse the given $array array and assign the value of the current array to $value in each loop.

Example

<?php
header("Content-type:text/html;charset=utf-8");
$array= array(2,"数学","梨子",3.14,"榴莲",12);
var_dump($array);
echo "遍历数组：<br>";
foreach ($array as $val){
    echo "值是：" . $val ;
    echo "<br/>";
}
?>

• Syntax format 2:

foreach ($array as $key => $value){
    语句块;
}

Traverse the given $array array, in each loop, the value of the current array will be assigned to $value, and the key name will be assigned to $key.

Example:

<?php
header("Content-type:text/html;charset=utf-8");
$array= array(2,"数学","梨子",3.14,"榴莲",12);
var_dump($array);
echo "遍历数组：<br>";
foreach ($array as $key => $value) { 
    echo "键名为：".$key."，键值为：".$value . "<br/>";
} 
?>

Explanation:

The foreach statement uses the pointer inside the array every time it loops It will move forward one step, so that the next array element will be obtained in the next loop, until it traverses to the end of the array, stops traversing and exits the loop.

Method 3: Use the while statement each() list() to traverse

while loop statement is a while statement type loop statement, first judge the loop condition. When the condition is satisfied, the loop body will be executed. If it is not satisfied, it will stop.

each() function Returns the current key value in the array and moves the array pointer forward. After each() is executed, the array pointer will stay at the next element in the array or at the last element when the end of the array is reached.

list() function is used to assign values ​​to a set of variables in one operation.

while loop each() function list() function can traverse the array, let’s take a look at the code example below:

<?php
header("Content-type:text/html;charset=utf-8");
$array= array(2,"数学","梨子",3.14,"榴莲",12);
var_dump($array);
echo "遍历数组：<br>";
while( list($key, $val) = each($array) ) {
    echo "$key => $val<br>";
}
?>

Output result:

Explanation:

The while loop statement is a when type loop statement. The loop condition is first judged. When the condition is met, the loop body is executed. Stop when not satisfied.

• 作用：反复执行某一项操作，直到指定的条件不成立。

• 特点：先判断表达式，当表达式结果为真时执行相应的语句。

while循环的一般形式为：

while(表达式){    //表达式为循环条件
    PHP语句块;  //PHP语句块为循环体
}

语句解析：

• 先计算“表达式”的值，当值为真时， 执行循环体内的“PHP语句块”；

  说明：“表达式”的计算结果是布尔类型（ TRUE 或 FALSE）的，如果是其他类型的值也会自动转换为布尔类型的值（因为PHP是弱语言类型，会根据变量的值，自动把变量转换为正确的数据类型）。
  

  “语句块”就是由{ }包围的一个或多个语句的集合；如果语句块中只有一个语句，也可以省略{ }。
  

• 执行结束后，返回到表达式，再次计算表达式的值进行判断，当表达式值为真，则继续执行“语句块”……这个过程会一直重复

• 直到表达式的值为假时才跳出循环，执行while 下面的语句。

while语句的流程图如下所示：

通常“表达式”是使用比较运算符或者逻辑运算符计算后的值

注：“死循环”--无法靠自身的控制终止的循环

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