Perl编程实用技巧速查

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"Perl Notes for Professionals.pdf 是一本关于Perl编程语言的非官方免费教育书籍，包含90多页的专业提示和技巧。这本书旨在帮助初学者和高级学习者掌握Perl语言，书中涵盖了从基础到进阶的多个主题，如变量、注释、字符串插值、真值判断以及日期和时间处理等。" Perl是一种强大的脚本编程语言，广泛用于文本处理、系统管理、网络编程等多个领域。本书的第一章“Getting started with Perl Language”引导读者入门Perl，介绍了如何安装Perl环境，以及如何编写和运行第一个Perl程序。在第二章“Comments”中，读者将学习到Perl中的单行注释（以#开始）和多行注释（使用=begin和=end标记）。注释是程序中用来解释代码功能的重要部分，对于代码的可读性和维护性至关重要。第三章“Variables”深入讨论了Perl中的变量类型，包括标量（Scalar）、数组（Array）、数组引用（Array References）、标量引用（Scalar References）、类型glob、类型glob引用、文件句柄（Filehandles）和常量。Perl的变量使用特殊符号（称为Sigils）来标识其类型，例如$用于标量，@用于数组，%用于哈希。第四章“Interpolation in Perl”讲解了Perl中的字符串插值，这是Perl处理动态字符串的一种方式。插值允许在双引号内的字符串中插入变量和表达式，使得变量值在输出时被计算。第五章“True and False”列出了Perl中的真值和假值，这对于条件语句和逻辑操作的理解至关重要。Perl中，任何非零值或非空字符串都被视为真，而零、空字符串、未定义值或`undef`被视为假。第六章“Dates and Time”涉及日期和时间处理，Perl提供了多种处理日期和时间的方法，包括内置的函数和模块，可以帮助开发者进行日期运算、格式化输出和时间比较。本书还可能涵盖其他主题，如正则表达式、文件和目录操作、面向对象编程等，提供了一个全面的Perl学习资源，适合希望提升Perl技能的专业人士或有志于学习Perl的新手。通过深入阅读和实践书中的例子，读者可以逐步提高自己在Perl编程方面的专业能力。

GoalKicker.com – Perl® Notes for Professionals 11

Section 3.7: Hash References

Hash references are scalars which contain a pointer to the memory location containing the data of a hash. Because

the scalar points directly to the hash itself, when it is passed to a subroutine, changes made to the hash are not

local to the subroutine as with a regular hash, but instead are global.

First, let's examine what happens when you pass a normal hash to a subroutine and modify it within there:

use strict;

use warnings;

use Data::Dumper;

sub modify

{

my %hash = @_;

$hash{new_value} = 2;

print Dumper("Within the subroutine");

print Dumper(\%hash);

return;

}

my %example_hash = (

old_value => 1,

);

modify(%example_hash);

print Dumper("After exiting the subroutine");

print Dumper(\%example_hash);

Which results in:

$VAR1 = 'Within the subroutine';

$VAR1 = {

'new_value' => 2,

'old_value' => 1

};

$VAR1 = 'After exiting the subroutine';

$VAR1 = {

'old_value' => 1

};

Notice that after we exit the subroutine, the hash remains unaltered; all changes to it were local to the modify

subroutine, because we passed a copy of the hash, not the hash itself.

In comparison, when you pass a hashref, you are passing the address to the original hash, so any changes made

within the subroutine will be made to the original hash:

use strict;

use warnings;

use Data::Dumper;

sub modify

{

my $hashref = shift;

GoalKicker.com – Perl® Notes for Professionals 13

print $employee{shift};

# but this one will execute [shift][1], extracting first element from @_,

# and use result as a key

print $employee{+shift};

Like with arrays, you can access multiple hash elements at the same time. This is called a hash slice. The resulting

value is a list, so use the @ sigil:

my @words = @translations_of_hello{'spanish', 'german'}; # ('Hola', 'Hallo')

Iterate over the keys of an hash with keys keys will return items in a random order. Combine with sort if you wish.

for my $lang (sort keys %translations_of_hello) {

say $translations_of_hello{$lang};

}

If you do not actually need the keys like in the previous example, values returns the hash's values directly:

for my $translation (values %translations_of_hello) {

say $translation;

}

You can also use a while loop with each to iterate over the hash. This way, you will get both the key and the value at

the same time, without a separate value lookup. Its use is however discouraged, as each can break in mistifying

ways.

# DISCOURAGED

while (my ($lang, $translation) = each %translations_of_hello) {

say $translation;

}

Access to unset elements returns undef, not an error:

my $italian = $translations_of_hello{'italian'}; # undef

map and list ﬂattening can be used to create hashes out of arrays. This is a popular way to create a 'set' of values,

e.g. to quickly check whether a value is in @elems. This operation usually takes O(n) time (i.e. proportional to the

number of elements) but can be done in constant time (O(1)) by turning the list into a hash:

@elems = qw(x y x z t);

my %set = map { $_ => 1 } @elems; # (x, 1, y, 1, t, 1)

my $y_membership = $set{'y'}; # 1

my $w_membership = $set{'w'}; # undef

This requires some explanation. The contents of @elems get read into a list, which is processed by map. map accepts a

code block that gets called for each value of its input list; the value of the element is available for use in $_. Our

code block returns two list elements for each input element: $_, the input element, and 1, just some value. Once you

account for list ﬂattening, the outcome is that map { $_ => 1 } @elems turns qw(x y x z t) into (x => 1, y =>

1, x => 1, z => 1, t => 1).

As those elements get assigned into the hash, odd elements become hash keys and even elements become hash

values. When a key is speciﬁed multiple times in a list to be assigned to a hash, the last value wins. This eﬀectively

discards duplicates.

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