August 2018
Beginner to intermediate
382 pages
7h 14m
Chinese
Content preview from Perl语言入门
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98
｜
第
5
章
当
Perl
看到这行程序代码，它会遵照你的要求输出
5
。接着，
Perl
会从
print
取得返
回值
1
，再将它乘以
4
。然后它会丢掉这项乘积，因为你没告诉它接下来要做什么。
这时，你旁边就会有人看到，然后说：“嘿，
Perl
连数学都不会！应该输出
20
，而不
是
5
！”
问题在于
Perl
可以省略括号，而大多数人又容易忘记括号的归属。没有括号的时候，
print
是列表操作符，会把其后的列表里的所有东西全都输出来。一般来说，这就是
我们想要的。但是假如
print
后面紧跟着左括号，它就是一个函数调用，只会将括号
内的东西输出来。因为该行程序代码有括号，所以对
Perl
来说，就和下面的写法一
样：
( print(2+3) ) * 4; #
糟糕！
好在只要你启用了警告信息，
Perl
几乎总能帮你找出这类问题。所以请使用
-w
或加
上
use warnings
，至少在你开发程序与调试时启用它。现在，修复的办法就是加上适
当的括号：
print( (2+3) * 4 );
实际上，这条规则——“假如它看起来像函数调用，它就是函数调用”，不仅
对
print
适用，对
Perl
所有的列表函数同样适用，只不过
print
可能最容易让人注意
到这条规则。如果
print
（或其他函数名）后面接着一个左括号，请务必确定在函数
的所有参数之后也有相应的右括号。
用
printf
格式化输出
处理输出结果时，你也许希望使用控制能力比
print
更强的操作符。事实上，你可能
已经习惯用
C ...
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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.