October 2022
Beginner to intermediate
390 pages
7h 5m
Chinese
Content preview from Perl语言入门（第8版）
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哈希
｜
123
key
,
…
)
这样的新列表，达成键值互换。当结果存回
%inverse_hash
时，我们就能以
原本在
%any_hash
里的值进行检索，现在它已经成为
%inverse_hash
的键，而按它
找到的值则是
%any_hash
里的某个键。现在我们能按“值”（现在是键）来找“键”
（现在是值）了。
当然，思维缜密的读者可能已经猜到，这种技巧只能在原始哈希的值不重复的情况下
才能奏效，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？ ...
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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.