August 2018
Beginner to intermediate
382 pages
7h 14m
Chinese
Content preview from Perl语言入门
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列表与数组
｜
67
即使是普通的赋值运算（对标量或列表赋值），都可以有不同的上下文：
@list = @people; #
得到
3
个人的姓名列表
$n = @people; #
得到人数
3
但请不要立刻得出结论，认为在标量上下文中一定会得到元素个数而非元素列表。许
多能返回列表的表达式会有各种丰富有趣的行为。
其实“产生列表”的表达式和“产生标量”的表达式之间并无不同，任何表达式都可
以产生列表或标量，根据上下文而定。因此当我们说“产生列表的表达式”时，我们
指的是该表达式通常被用在列表上下文中。所以当它们不小心被用在标量上下文中时
（像
reverse
或是
@fred
），你可能会因此感到惊讶。
不光如此，从我们积累的经验来看，仅仅通过对表达式形式上的判断，是无法归纳出
一个通用法则来的。每一个表达式都可能有它自己的特定规则。所以，实际上能够概
括的规则也就是：哪种上下文更有意义，就应该是哪种上下文。但其实记住这条规则
并没什么用。
Perl
是一种会尽量帮你完成常见任务的语言，往往它的选择就是你想要
的结果。
在标量上下文中使用产生列表的表达式
有些经常被用来生成列表的表达式如果用在标量上下文中，结果会怎样？这要看表达
式的缔造者怎么说，这个人基本上就是
Larry
了，他会在说明文档里详细解释。其实
学习
Perl
的大部分时间都是在学习
Larry
的思维方式。因此，只要能以他的思考方式
出发，就能大致推猜
Perl
接下来会怎么做。但在初学阶段，请多翻多读说明文档。
某些表达式在标量上下文中不会返回任何值。比如， ...
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I’ve been on the O’Reilly platform for more than eight years. I use a couple of learning platforms, but I'm on O'Reilly more than anybody else. When you're there, you start learning. I'm never disappointed.Amir M.
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.