Teach you step by step how to learn PHP recursive functions

Recursion is when a function calls itself within its function body. Executing a recursive function will call itself repeatedly, entering a new level each time. Recursive functions must have an end condition. Let’s follow the editor to learn recursion.

The idea of ​​recursion is to transform a relatively complex problem (original problem) into smaller problems (sub-problems) that are similar to the original problem to solve. Once each small problem is solved, the final big problem will naturally be solved. The recursive method only needs a small amount of programs to describe the multiple repeated calculations required in the problem-solving process, greatly reducing the amount of program code. Of course, recursive functions are not perfect and have certain disadvantages, that is, recursive functions do not operate efficiently.
The number of recursion levels in PHP is not unlimited. This is related to the memory limit of the program. PHP5 allows a program to use 128M of memory, so when the number of recursion levels exceeds When the 128M memory is exhausted, the program will generate a fatal error and exit. PHP7 is allowed to use 256M of memory.

A recursive function is to call itself inside the function. Executing a recursive function will call itself repeatedly, entering a new level each time. Recursive functions must have an end condition. When the function keeps recursing until it encounters a wall and returns, the wall is the end condition. Therefore, recursion must have two elements, the ending condition and the recursion relationship.

Note: When the function is called and executed, space will be allocated in the memory to store temporary data. Then there is no connection between functions by default during execution (except for static Variables, passed by reference, global variables). The variables inside are all local variables by default and have no influence on each other. Let’s take a look at an example to deepen our impression:

<?php
    function demo($num){
        //
        if($num == 1 || $num == 2){
            return 1;
        }else{
            return demo($num - 1) + demo($num - 2);
        }
    }

    echo &#39;数列第 10 位是：&#39;.demo(10);//数列第 10 位是：55
?>

Does this example look familiar? This is the Fibonacci sequence. When we calculate the Fibonacci sequence, if we use other methods, it is still better. It is somewhat difficult, but using recursive functions is much more efficient.

Recommended tutorial: "PHP Video Tutorial"

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