August 2018
Beginner to intermediate
382 pages
7h 14m
Chinese
Content preview from Perl语言入门
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哈希
｜
119
$foo = 'bar';
print $family_name{ $foo . 'ney' }; #
打印
'rubble'
若对某个已存在的哈希元素赋值，就会覆盖之前的值：
$family_name{'fred'} = 'astaire'; #
给已有的元素赋上新值
$bedrock = $family_name{'fred'}; #
得到
"astaire"
，之前的值已抛弃
这和数组与标量的情形类似，如果对
$pebbles[17]
或者
$dino
赋值，就会覆盖之前
的值。如果对
$family_name{"fred"}
赋值，同样会覆盖之前的值。
哈希元素会因赋值而诞生：
$family_name{'wilma'} = 'flintstone'; #
增加一个新的键
-
值对
$family_name{'betty'} .= $family_name{'barney'}; #
在需要的时候动态创建该元素
这种特性称为自动延展（
autovivification
），我们在《
Intermediate Perl
》一书中有讨
论。这和数组与标量的情形如出一辙，假如以前不存在
$pebbles[17]
或
$dino
，赋值
之后这些变量就会出现；假如以前不存在
$family_name{"betty"}
，现在就有了。
访问哈希表里不存在的值会得到
undef
：
$granite = $family_name{'larry'}; #
没有
larry
这个键，所以值为 ...
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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.