August 2018
Beginner to intermediate
382 pages
7h 14m
Chinese
Content preview from Perl语言入门
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哈希
｜
121
效，否则就会导致新哈希中有重复键，而哈希的键必须唯一。所以在不断赋值的过程
中，最后那个键的值替换了之前的赋值，成为最终的结果。
我们知道，键
-
值对在哈希松绑之后的顺序是无法预知的，所以也就无法预知最后哪
些键
-
值对会被覆盖。这个技巧最好是在确定原始哈希的值是唯一的情况下使用，比
如前面提到的
IP
地址和主机名相互检索的例子：
%ip_address = reverse %host_name;
现在，我们不但能按主机名查询
IP
地址，也能反过来按
IP
地址查询主机名。
胖箭头
使用列表对哈希赋值时往往难以区分键和值的成对关系。比如下面的代码中，我们要
仔细扫描并逐个默念：“键、值，键、值……”，才能搞清楚第
5
个元素
2.5
其实是
键，不是值：
%some_hash = ('foo', 35, 'bar', 12.4, 2.5, 'hello',
'wilma', 1.72e30, 'betty', "bye\n");
如果
Perl
能让我们将此类列表中的键与值成对组合，方便区别谁是谁，岂不更好？
Larry
也有感于此，因此他发明了胖箭头（
=>
）。对
Perl
而言，它只是逗号的另一种
写法，所以有时候也称它为胖逗号。也就是说，在任何需要逗号（
,
）的地方都可以用
胖箭头代替，这对
Perl
来说没什么区别。改写上面的代码如下：
my %last_name = ( #
哈希也可以是词法变量
'fred' => 'flintstone',
'dino' => undef,
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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.
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.