August 2018
Beginner to intermediate
382 pages
7h 14m
Chinese
Content preview from Perl语言入门
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74
｜
第
4
章
那么第二个（不完善的）子程序中
$fred
与
$barney
相加的结果怎样了？我们并没有
将总和存储起来，所以
Perl
会丢弃它。启用警告信息时，
Perl
会注意到将两数相加的
结果丢弃是毫无用处的，并显示“
a useless use of addition in a void context
”之类的信
息来警告你。其中的术语“
void context
（空上下文）”表示运算结果未被使用，既
没有存储到变量里，也未被任何函数使用。
“最后执行的表达式”的准确含义真的就是最后执行的表达式，而非程序代码的最后
一行。比如下面这个子程序，它会返回
$fred
和
$barney
两者中值较大者：
sub larger_of_fred_or_barney {
if ($fred > $barney) {
$fred;
} else {
$barney;
}
}
最后执行的表达式是自成一行的
$fred
或
$barney
，所以它们将充当子程序的返回
值。必须等到执行阶段得知这些变量的内容后，我们才会知道返回值到底是
$fred
还
是
$barney
。
目前为止都还只是些示例而已。接下来会介绍如何在每次调用子程序时传入不同的
值，而不再依靠全局变量。使用传入参数才是我们最终的实际用法。
参数
假如子程序
larger_of_fred_or_barney
不强迫我们一定用全局变量
$fred
和
$barney
，
那么使用上会更灵活。在目前状况下，假如我们想从
$wilma
和
$betty ...
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I wanted to learn C and C++, but it didn't click for me until I picked up an O'Reilly book. When I went on the O’Reilly platform, I was astonished to find all the books there, plus live events and sandboxes so you could play around with the technology.Addison B.
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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.