What is the method to implement recursion in php

The way PHP implements recursion is: 1. Use references as parameters, the code is [$result[]=$a;test($a,$result)]; 2. Use global variables; 3. Use Static variable, code is [static $count=0;].

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The method for php to implement recursion is:

1. Use references as parameters

Regardless of whether references are parameters or not, you must first understand what a reference is? A reference simply means that two variables with different names point to the same storage address. Originally, each variable had its own storage address, and assignment and deletion went their own way. Okay now, the two variables share a storage address. $a=&$b; . What it actually means is that $a has to share the same room with $b regardless of its original storage address. Therefore any change to the stored address value will affect both values.​

Functions originally do their own thing, even if they are functions with the same name. Recursive functions consider taking references as parameters and becoming a bridge to form data sharing between two functions. Although the two functions seem to operate on different addresses, they actually operate on the same memory address.

function test($a=0,&$result=array()){
$a++;
if ($a<10) {
 $result[]=$a;
 test($a,$result);
}
echo $a;
return $result;
 
}

The above example is very simple. Use a<10 as the judgment condition. If the condition is true, assign a to result[]; pass the reference of result into the function, and the a generated by each recursion will be Add to result array result. Therefore, the $result array generated in this example is Array ( [0] => 1 [1] => 2 [2] => 3 [3] => 4 [4] => 5 [5] = > 6 [6] => 7 [7] => 8 [8] => 9 ).

What is more interesting in this example is the value of echo a. I believe many people think it is 12345678910, but actually it is not, it is 1098765432. why? Because the function has performed the next function recursion before executing echoa. The actual execution of echo a is when the condition a<10 is not met, echo a returns result. For the upper layer, after executing the recursive function, it starts executing echo $a of this layer, and so on.

Example 2,

PHP’s reference allows two variables to point to the same content, for example $a = &$b; This means that $a and $b point to the same variable .

In the following example, because $data is passed by reference, the data will always be accumulated.

function recursion(&$data = [], $i = 0)
{
  if ($i < 10) {
    $data[] = $i;
    $i++;
    $this->recursion($data, $i);
  }
  return $data;
}
// 调用
$this->recursion();  // [0,1,2,3,4,5,6,7,8,9]

2. Use global variables

Use global variables to complete recursive functions. Please make sure you understand what global variables are. The global variable declared within the function is just a reference to the external variable with the same name. The scope of the variable is still within the scope of this function. Changing the values ​​of these variables will naturally change the values ​​of external variables with the same name. But once & is used, the variable with the same name is no longer a reference with the same name. It is not necessary to understand such a deep level to use global variables to implement recursive functions. You can understand recursive functions naturally by maintaining the original view of global variables.

function test($a=0,$result=array()){
 global $result;
 $a++;
 if ($a<10) {
  $result[]=$a;
  test($a,$result);
 }
 return $result;
}

global Declaring variables within a function is nothing but a reference to an external variable with the same name. The scope of the variable is still within the scope of this function. Changing the values ​​of these variables will naturally change the values ​​of external variables with the same name.

function recursion($data = [], $i = 0)
{
  global $data;
  if ($i < 10) {
    $data[] = $i;
    $i++;
    $this->recursion($data, $i);
  }
  return $data;
}
  
// 调用
$this->recursion();  // [0,1,2,3,4,5,6,7,8,9]

3. Use static variables

We often see static in classes, and today we use it in recursive functions. Remember the role of static: initialize the variable only the first time the function is called, and retain the variable value.

Give me an example:

function test(){
static $count=0;
echo $count;
 
$count++;
}
test();
test();
test();
test();
test();

What is the execution result of this code? Is it 00000? Definitely not. It's 01234. First, when calling test() for the first time, static initializes $count. After each subsequent execution, the value of $count will be retained and no longer initialized, which is equivalent to directly ignoring the sentence static $count=0;.

Therefore, the effect of applying static to a recursive function can be imagined. Variables that need to be used as "bridges" between recursive functions are initialized using static, and the value of the "bridge variables" will be retained for each recursion.

function test($a=0){
 static $result=array();
 $a++;
 if ($a<10) {
  $result[]=$a;
  test($a);
 }
 return $result;
}

Static variables are only initialized on the first call. Exists only in the local function scope, but its value is not lost when the program execution leaves this scope.

function recursion($i = 0)
{
  static $data = [];
  if ($i < 10) {
    $data[] = $i;
    $i++;
    $this->recursion($i);
  }
  return $data;
}
  
// 调用
$this->recursion();  // [0,1,2,3,4,5,6,7,8,9]

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