August 2018
Beginner to intermediate
382 pages
7h 14m
Chinese
Content preview from Perl语言入门
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200
｜
第
10
章
#
两个条件都为假，什么也不干（我们有点伤感）
}
Perl
在这里可能会走捷径。如果逻辑与操作符的左边表达式为假，整个表达式就不可
能为真，因为必须两边都为真才会得到真。因此这时不必再检查右边的表达式，从而
避免对其求值。针对下面的例子，请考虑
$hour
是
3
的时候会怎样：
if ( (9 <= $hour) && ($hour < 17) ) {
print "Aren't you supposed to be at work...?\n";
}
相似的地方还有，若逻辑或操作符的左边表达式为真，那么右边也会免于求值。请考
虑下面的例子中
$name
是
fred
会如何：
if ( ($name eq 'fred') || ($name eq 'barney') ) {
print "You're my kind of guy!\n";
}
因为这个特性，这种操作符被称为“短路（
short-circuit
）”逻辑操作符。它们只要有
可能就会走捷径来获得结果。事实上，依赖这种短路行为的代码很常见，比如求得平
均值的程序：
if ( ($n != 0) && ($total/$n < 5) ) {
print "The average is below five.\n";
}
这个例子里，右边的表达式只有在左边为真的时候才被求值，因此程序不会因为意
外的“除以零”而崩溃（我们会在第
16
章的“捕获错误”一节展示其他相关的例
子）。
短路操作符的返回值
和
C
这类的语言不同的地方在于，
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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.