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HomeBackend DevelopmentPHP TutorialPHP中递归函数的一个常见逻辑问题

首先,我们得知道递归函数是什么东西,通俗来讲也就是自己调用自己本身的函数。

现在需要设计一段代码,解决1到10叠加的问题。

代码A:

<?php //递归函数	$num=10;	function add($sum){		static $tot;		if($sum>=1){			$tot+=$sum;			add(--$sum);		}else{			return $tot;		}	}	echo add($num);?>
代码B:

<?php //递归函数	$num=10;	function add($sum){		static $tot;		if($sum>=1){			$tot+=$sum;			return add(--$sum);		}else{			return $tot;		}	}	echo add($num);?>
A中打印不出想要的结果,B中可以实现。而A和B代码中的唯一差别就是在if中添加了一个return。

下面就开始分析整个递归的过程,完全可以去看看盗梦空间加深理解了:(以不能实现的A代码为例)

1.将10带入函数中,if语句判断之后,$tot开始叠加。

2.这里重点来了:将参数减一之后重新带入函数中。(没有返回值!!!)

3.接着上面的过程一直循环,一直到$sum=1,这从外到内,由10到1,这10层的循环都没有返回值。

4.$sum=0的时候,要求有返回值。

就好像,盗梦空间中,主角在11层梦境中醒来,但是前面10层的梦境都处于睡眠阶段。这主角你觉得醒的过来吗,明显不可能。他只能卡在第11层梦境,永远醒不过来。

唯一能让主角醒过来的办法也就是他的每一层梦醒都醒过来,一直回到现实生活中。而相对与A代码,B代码则从第一层开始就要求有返回值,一直到11层都要有返回值。所以11层醒过来之后激活了第10层,然后一层一层的激活,最后就顺利醒过来了,这个最后的结果也就正确输出了。

怪不得他们说盗梦空间用递归函数很好解释,想来确实有点。

剩下的就是自己再深入理解了,一上来确实会有点晕。

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