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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其他控制结构
｜
193
print "fred is '$fred', barney is '$barney'\n" if $I_am_curious;
用这种“倒装句”编写程序可以把语句中重要的部分放在前面。上面那条语句的重
点是查看一些变量的值，而不是检查你是否好奇。当然，
$I_am_curious
这个变量是
我们杜撰出来的，并非内置的
Perl
变量。通常使用这个技巧的时候，变量会被命名
为
$TRACING
，或者用
constant
编译指令来声明一个全局常量。有些人喜欢将整个语
句写成一行，也可能在
if
之前加上些制表符使它向右边缩进一些，上面那个例子就
是如此。也有人喜欢将
if
修饰符放在下一行并缩进一段距离：
print "fred is '$fred', barney is '$barney'\n"
if $I_am_curious;
虽然这些带有修饰符的表达式都可以用块的形式重写，即用最传统的方法写，但反过
来，由传统写法往带有修饰符的写法改写却未必可以。修饰符的两边都只能写单个表
达式，因此不能写某事
if
某事
while
某事
until
某事
unless
某事
foreach
某事，因
为那样太让人困惑了。另外修饰符的左边也不能放多条语句。如果确实需要，还是建
议你回到传统写法，仍然写那些圆括号和花括号。
如同我们之前在
if
修饰符部分谈到的，右边的控制表达式总是先求值，和传统写法
的执行顺序是一样的。
在使用
foreach
修饰符的时候无法自选控制变量，必须使用 ...
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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.