Examples of three methods to implement recursion in PHP

Recursive functions are often used by everyone. They call themselves directly or indirectly within the function body. They must make conditional judgments before calling themselves, otherwise they will continue to be called indefinitely. The previous chapter explains what php recursive functions are and introduces a simple example to illustrate . In this section we will introduce several ways to implement recursive functions in PHP. Three basic ways are listed here. Understanding it requires a certain level of basic knowledge, including an understanding of global variables, references, and static variables, as well as an understanding of their scope. Recursive functions are also a good technique for solving infinite levels of classification.

1. Use references as parameters

References just mean 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. Now two variables need to share a storage address. $a=&$b;. What it actually means is that $a has to share a 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.

Let’s take a look at it through an example:

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

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 added to the 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 not executed the next function recursion before echo $a. 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, the echo $a of this layer starts to be executed, and so on.

2. Use global variables

To use global variables to complete recursive functions, please first 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 will no longer be a reference with the same name.

Let’s give an example below:

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

3. Use static variables

Here Let's use static in the class. Today we will use it in the recursive function. Remember the role of static: initialize the variable only the first time the function is called, and retain the variable value.

Let’s use an example to illustrate:

<?php
function test(){
 static $count=0;
 echo $count;
 $count++;
}
test();
test();
test();
test();
test();
?>

Explanation: What is the execution result of this piece of code? You may think it is 00000, but the answer is of course no. The output result is 01234. First, when you call test() for the first time, static initializes $count. After each execution, the value of $count will be retained and will not be initialized again. This is equivalent to directly ignoring the sentence static $count=0;. So 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.

Summary

The so-called recursive function focuses on how to handle the function call itself and how to ensure that the required results can be reasonably "transmitted" between functions. The above are the three basic ways to implement recursion in PHP. Friends can try various codes by themselves in order to better and more proficiently master these methods and lay a good foundation for future learning and development.

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