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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输入与输出
｜
93
看起来好像和
while
循环的行为没啥差别，真是这样吗？
不是的，差别在于读入数据的时机以及耗费的资源。在
while
循环里，
Perl
每次
读取一行输入，把它存到某个变量并执行循环体，而后再尝试获取后续输入。但
在
foreach
循环里，行输入操作符会按列表上下文处理，所以要做的第一件事就是把
所有输入都读进来，按列表返回。数据量小的话没关系，但假如输入数据来自
400MB
大小的
Web
服务器日志文件，就要准备相应大小的内存来安顿！所以没啥特殊情
况，尽量还是用
while
循环吧，一次处理一行。
来自钻石操作符的输入
还有另外一种读取输入的方法，就是使用钻石操作符
<>
。它能让程序在处理调用参
数时，提供类似于标准
Unix
工具程序的功能
。如果想用
Perl
编写类似
cat
、
sed
、
awk
、
sort
、
grep
、
lpr
之类的工具程序，一边从标准输入拿数据，一边输出结果到标
准输出，那么钻石操作符将会是你的好帮手。但如果要支持复杂的参数，单单靠钻石
操作符显然是不行的。
钻石操作符是
Larry
的女儿
Heidi
命名的。有一天
Randal
拿着他新写的
Perl
培训材料去
Larry
家给他看的时候，提出这个操作符一直没有一个叫得出的名
字。
Larry
也一筹莫展，但这时他
8
岁的女儿
Heidi
灵机一动，说：“这就是钻
石，爸爸。”于是就有了这个名字。谢谢
Heidi
！
程序的调用参数（
invocation argument
）
通常是命令行中跟在程序名后面的几个“单 ...
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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.