Some applications of recursion (1) Traversing folders

Recursive call of function

The meaning of recursion

Recursion is actually just an algorithmic description, not a new syntax!

Sometimes, when we solve problems, we will encounter this situation. When we divide a big problem into several small problems according to a certain solution, we find that the solutions to these small problems are actually the same as the big problem just now. The solution is the same again!

Typical, such as: finding factorial!

10! = 10 * 9!

9! = 9 * 8!

8! = 8 * 7!

……

Syntactically, recursive calling of a function means that the function calls itself again during execution!

Two important points of recursion:

1, Recursive exit: refers to when to stop recursive calls

2, Recursion point: refers to when to start using recursive calls

When writing a recursive call, write the recursive exit first, and then the recursive point!

So, the characteristics of recursive calls are: the code is relatively simple to write, but when executed, consumes more memory resources !

It can also be said that The essence of recursion is to exchange space for time!

The following is a small case of traversing a folder recursively:

Code:

<?<span style="color: #000000;">php 
</span><span style="color: #008000;">/*</span><span style="color: #008000;">*
 * 定义遍历指定路径下文件与文件夹,通过递归的方法
 * @param  $dir string
 </span><span style="color: #008000;">*/</span><span style="color: #000000;">
function dirs($dir,$level</span>=<span style="color: #800080;">0</span><span style="color: #000000;">){
    </span><span style="color: #008000;">//</span><span style="color: #008000;">列出指定路径中的文件和目录</span>
    $files=<span style="color: #000000;">scandir($dir);

    </span><span style="color: #008000;">//</span><span style="color: #008000;">遍历所有的目录</span>
    <span style="color: #0000ff;">foreach</span>($files <span style="color: #0000ff;">as</span><span style="color: #000000;"> $file){

        </span><span style="color: #008000;">//</span><span style="color: #008000;">重复一个字符串</span>
        echo str_repeat(<span style="color: #800000;">'</span><span style="color: #800000;"> </span><span style="color: #800000;">'</span>,$level*<span style="color: #800080;">4</span><span style="color: #000000;">);

        </span><span style="color: #008000;">//</span><span style="color: #008000;">拼接路径</span>
        $tmpdir=$dir.<span style="color: #800000;">'</span><span style="color: #800000;">/</span><span style="color: #800000;">'</span><span style="color: #000000;">.$file;

        </span><span style="color: #008000;">//</span><span style="color: #008000;">判断是否是一个目录，文件夹</span>
        <span style="color: #0000ff;">if</span><span style="color: #000000;">(is_dir($tmpdir)){
            </span><span style="color: #008000;">//</span><span style="color: #008000;">让文件夹变成红色</span>
            echo <span style="color: #800000;">"</span><span style="color: #800000;"><font style='color:red;'>$tmpdir</font><br/><span style="color: #800000;">"</span><span style="color: #000000;">;
            </span><span style="color: #008000;">//</span><span style="color: #008000;">目录下有两个隐藏文件.和..，排除掉</span>
            <span style="color: #0000ff;">if</span>($file !=<span style="color: #800000;">'</span><span style="color: #800000;">.</span><span style="color: #800000;">'</span> && $file !=<span style="color: #800000;">'</span><span style="color: #800000;">..</span><span style="color: #800000;">'</span><span style="color: #000000;">){
                </span><span style="color: #008000;">//</span><span style="color: #008000;">通过递归的方法，调用自己，进行遍历</span>
                dirs($tmpdir,$level+<span style="color: #800080;">1</span>);<span style="color: #008000;">//</span><span style="color: #008000;">递归点</span>
<span style="color: #000000;">            }
        }</span><span style="color: #0000ff;">else</span><span style="color: #000000;">{
            </span><span style="color: #008000;">//</span><span style="color: #008000;">如果不是一个目录就直接显示这个文件</span>
            echo $file.<span style="color: #800000;">'</span><span style="color: #800000;"><br/></span><span style="color: #800000;">'</span><span style="color: #000000;">;
        }

    }
}

dirs(</span><span style="color: #800000;">'</span><span style="color: #800000;">d:/sphinx</span><span style="color: #800000;">'</span>);

Effect:

In the next article, I will use the recursive method to implement a small case of infinite classification.