Do array subscripts in php have to be consecutive?

PHP array subscripts may not be consecutive. In PHP, the subscript of an array can be an integer or a string; the order of the elements of the array is not determined by the subscript, but by the order in which they are "joined". The subscripts of PHP arrays can be set arbitrarily without order or continuity. For example, "array(2=>1,4=>1.1,3=>5,7=>'abc',0=> ;true); ".

The operating environment of this tutorial: windows7 system, PHP8.1 version, DELL G3 computer

Array basics:

In php, the subscript of the array can be an integer or a string

In php, the order of the elements of the array is not determined by the subscript, but by its " The order of joining" determines

Definition:

$arr = array(元素1,元素2,...);

Example 1:

$arr1 = array(1,1.1,5,'abc',true,false); //可以存储任何数据，此时为默认下标
$arr2 = array(2=>1,4=>1.1,3=>5,7=>'abc',0=>true); //下标可任意设定(无需顺序，无需连续)
$arr3 = array(2=>1,1.1,1=>5,'abc',0=>true); //可以加下标，也可以不加，不加下标则为默认下标

//默认下标规则：前面已经用过的最大数字下标+1
//这个数字的下标分别是：2，3，1，4，0

Example 2:

$arr1 = array(2=>1,'dd'=>5,1=>1.1,'abc',0=>true); //混合下标，同样遵循默认下标规则
$arr2 = array(-2=>1,'dd'=>5,1.1,'abc',true); //负数下标不算在整数下标中，而只当作字符下标
//则这个数组最后三项的下标为：0,1,2

Example 3:

$arr1 = array(2.7=>1,'dd'=>5,1=>1.1,'abc',true); //浮点数下标会自动转换为整数，且直接去掉小数部分
$arr2 = array("2.7"=>1,'dd'=>5,"11"=>1.1,'abc',true); //纯数字字符串下标，当作数字看待
$arr3 = array(2=>1,'dd'=>5,true=>1.1,'abc',false=>true); //布尔值当作下标，则true为1，false为0
$arr4 = array(2=>1,'dd'=>5,2=>1.1,'abc',true); //如果下标跟前面的重复，则单纯覆盖前面同名下标的值

## Other forms of defining arrays:

$arr1[] = 1;
$arr1[] = 5;
$arr1[] = 1.1;
......  //直接在变量后面使用[]，就成为数组，并依次赋值

$arr2['aa'] = 1;
$arr2['bb'] = 5;
$arr2[5] = 1.1;

......  //这种形式写的下标，其实跟使用array语法结构几乎一样

Classification of arrays:

From key-value relationship Divided into:

• Associative array: usually refers to an array whose subscript is a string, and the string can roughly express the meaning of the data
  Example ：

$person = array("name" => "poe", "age" => 18, "edu" => "大学毕业");

• Index array: usually refers to an array's subscript strictly starting from 0 and consecutive numeric subscripts--similar to js array

Divided from the array level:

• One-dimensional array: Each element value in an array is An ordinary value (non-array value)
  Example:

$person = array("name" => "poe", "age" => 18, "edu" => "大学毕业");

• Two-dimensional array: Each item in an array is another One-dimensional array.

$person = array(
"name" => array("xiaohua","xiaofang),
"age" => array(18,22),
"edu" => array("大学毕业","小学",)
);

• Multidimensional array: and so on. . .
  General syntax form of multi-dimensional arrays:

$v1 = 数组名[下标][下标][......]

Traversal of arrays:

Basic syntax for traversal:

foreach($arr as [$key =>] $value) {
//这里就可以对$key and $value进行所有可能的操作 -- 因为他们就是一个变量
//$key代表每次取得元素的下标，可能是数字，也可以是字符串
//$value代表每次取得元素的值，可能是各种类型
//此循环结构会从数组的第一项一直遍历到最后一项，然后结束
}

Array pointers and traversal principles:

Each array has a "pointer" inside it, which determines The elements obtained when the array is currently evaluated

During the foreach traversal process, they all rely on this pointer.

Example:

$arr1 = array(2=>1,'dd'=>5,1=>1.1,'abc',0=>true);

In addition to the position setting of the foreach loop, the pointer also has some other functions that also depend on the pointer:

1:

$v1 = current($arr1); //Get the value of the element pointed to by the current pointer in $arr1. If it does not point to an element, it is false

2:

$v1 = key($arr1 ); //Get the subscript of the element pointed to by the current pointer in $arr1. . . . . . . . . . . . .

3：

$v1 = next($arr1); //Move the pointer to the "next element", and then get the value of the next element

4 ：

$v1 = prev($arr1); //Move the pointer to the "previous element", and then get the value of the previous element

5：

$v1 = reset($arr1); //Move the pointer to the "first element" and get the value of the element

6:

$v1 = end($arr1); //Move the pointer to the "last element" and get the value of the element

7:

$v1 = each($arr1); //Get the subscript of the current element and value, then move the pointer to the next position

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