August 2018
Beginner to intermediate
382 pages
7h 14m
Chinese
Content preview from Perl语言入门
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78
｜
第
4
章
然后
foreach
循环会遍历参数列表
@_
里剩余的元素。循环的控制变量默认为
$_
（别
忘了
@_
和
$_
没有任何关系，它们的名称相似纯属巧合）。循环第一次执行时，
$_
是
5
，而
if
进行比较时看到
$_
比
$max_so_far
还大，所以
$max_so_far
会被设
成
5
——新的高水位线。
第三次循环时，
$_
是
10
。这是新的最大值，所以它会被存入
$max_so_far
。
到第四次时，
$_
是
4
。这时
if
比较的结果为假，因为
$_
不比
$max_so_far
（即
10
）
大，所以会跳过
if
里的程序代码。
最后，
$_
是
6
，因此
if
里的程序代码又被跳过一次。这是最后一次执行循环，所以
整段循环就执行完了。
此时，
$max_so_far
就变成了我们的返回值。既然它是目前见过的最大值，并且我们
已经遍历过所有数字，那它一定是列表中最大的值：
10
。
空参数列表
现在，即使有超过两个的参数，改版后的
&max
算法也能给出正确结果。但假如没有
任何参数传入，又会发生什么呢？
乍听之下，这似乎有点杞人忧天。毕竟，怎么可能会有人调用
&max
却不传入任何参
数呢？不过，也许有人会写出下面这样的代码：
$maximum = &max(@numbers);
数组
@numbers
有时可能只是一个空列表。比如数组内容是通过程序从文件读入的，
但文件却是空的。所以，这种情况下
&max
会如何处理？
子程序的第一行会对参数数组
@_
（现在是空的）进行 ...
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I'm always learning. So when I got on to O'Reilly, I was like a kid in a candy store. There are playlists. There are answers. There's on-demand training. It's worth its weight in gold, in terms of what it allows me to do.