Learning Perl 学习笔记 Ch6 哈希

1. 用键访问哈希的元素：$hash{$key}，哈希元素因赋值而诞生
   demo6-1:

#!/usr/bin/perl
$key="hello";
$hash{$key}="world";
print "Hello, $hash{$key}!\n";

./demo6-1 
Hello, world!

访问哈希表里不存在的元素会得到undef

2. 哈希的键必须是字符串，值可以是任何标量
3. 要访问整个哈希需用保留字符%, 哈希可以很容易的转换成{键，值，键，值……}这样的列表，这个过程叫做哈希松绑 ： @list = %hash
demo6-2:

#!/usr/bin/perl
$capital_of_country{"China"} = "Beijing";
$capital_of_country{"America"} = "Washington DC";
$capital_of_country{"Britain"} = "London";
$capital_of_country{"France"} = "Paris";
$capital_of_country{"Japan"} = "Tokyo";
$capital_of_country{"Korea"} = "Seoul";

@list_of_capitals = %capital_of_country;
print "@list_of_capitals\n";

./demo6-2
America Washington DC China Beijing Korea Seoul Britain London France Paris Japan Tokyo
./demo6-2
Korea Seoul America Washington DC China Beijing Britain London France Paris Japan Tokyo
./demo6-2
Japan Tokyo America Washington DC China Beijing France Paris Korea Seoul Britain London

• 注意到这里每次执行打印的顺序都不一样，这是因为哈希算法基于一个随机的因子来计算实际存储的位置，所以哈希松绑后的顺序并不固定，但是键值对的关系不会改变，即一个键的后面，必然是哈希表中它对应的值

4. 同样也可以用相反的方式给哈希赋值 ：%hash = @list（列表必须有偶数个元素，才能转成『键/值』对）
   demo6-3：

#!/usr/bin/perl
@list_of_capitals = qw /China Beijing America WashingtonD.C Britain London France Paris Japan Tokyo Korea Seoul/;

%capital_of_country = @list_of_capitals;

foreach $key (keys %capital_of_country){
    print "Country:$key Capital:$capital_of_country{$key}\n";
}

• 这里删掉了Washington和DC间的空格，因为qw会把空格识别为列表元素的分隔符
• keys 函数返回哈希表的键列表
• print 里使用了哈希元素的内插，但是对整个哈希表的内插是不支持的
• 哈希表的键必须是唯一的，如果有重名的键值对，后者将覆盖前者

./demo6-3
Country:Japan Capital:Tokyo
Country:Korea Capital:Seoul
Country:China Capital:Beijing
Country:Britain Capital:London
Country:France Capital:Paris
Country:America Capital:WashingtonD.C

结合哈希松绑和哈希赋值，可以实现简便的哈希表键值互换
%key_to_value = reverse %value_to_key

5. 哈希赋值的另一种方式是使用胖箭头=>，这样可以更清楚的表示哈希表中『键/值』对的关系：
   demo6-4:

#!/usr/bin/perl
%capital_of_country = (
 China => "Beijing",
 America => "WashingtonD.C",
 Britain => "London", 
 France => "Paris",
 Japan => "Tokyo",
 Korea => "Seoul",
 );

foreach $key (keys %capital_of_country){
    print "Country:$key Capital:$capital_of_country{$key}\n";
}

./demo6-4
Country:France Capital:Paris
Country:Britain Capital:London
Country:Japan Capital:Tokyo
Country:China Capital:Beijing
Country:America Capital:WashingtonD.C
Country:Korea Capital:Seoul

• 注意第二行使用的是圆括号(，而不是花括号{，使用花括号的场景是访问哈希表中的元素，而哈希表赋值使用的是和列表赋值一样的圆括号（考虑下哈希解绑应该更容易理解）
• 哈希赋值的最后一行有一个额外的逗号，这是正确的语法

6. keys函数返回哈希表的键列表，values函数返回哈希表中的值列表，虽然返回的列表中元素顺序是不固定的，但是对应位置的元素是成对的。keys[3]的元素必定是values[3]的键
   demo6-5：

#!/usr/bin/perl
%capital_of_country = (
 China => "Beijing",
 America => "WashingtonD.C",
 Britain => "London", 
 France => "Paris",
 Japan => "Tokyo",
 Korea => "Seoul",
 );

$index = 0;
@countries = keys %capital_of_country;
@capitals = values %capital_of_country;
while($index < @countries){
    print "$countries[$index]'s capital is $capitals[$index]\n";
    $index += 1;
}

./demo6-5
America's capital is WashingtonD.C
Japan's capital is Tokyo
China's capital is Beijing
France's capital is Paris
Britain's capital is London
Korea's capital is Seoul

7. 哈希的遍历可以用each函数或者foreach循环结构

• each函数内置定位器,每次调用都以数组的形式返回哈希表中的一对"键/值":
  ($key, $value) = each %hash
  *当each函数遍历完哈希表后会返回空数组
  *keys函数和values函数会重置这个定位器
  demo6-6:

#!/usr/bin/perl
%capital_of_country = (
 China => "Beijing",
 America => "WashingtonD.C",
 Britain => "London", 
 France => "Paris",
 Japan => "Tokyo",
 Korea => "Seoul",
 );

while(($key, $value) = each %capital_of_country){
    print "$key\'s capital is $value\n";
}

./demo6-6
America's capital is WashingtonD.C
Korea's capital is Seoul
France's capital is Paris
China's capital is Beijing
Japan's capital is Tokyo
Britain's capital is London

• foreach循环结构遍历keys函数返回的键列表

8. exists函数判断一个键是否存在于哈希表中,并以布尔值返回,delete函数则根据键删除哈希表中对应的元素
   demo6-7:

#!/usr/bin/perl
$key = "test";
if(exists $hash{$key}){
    print "1: key exists: $hash{$key}\n";
}else{
    print "1: key doesn't exist.\n";
}

$hash{$key} = "success";
if(exists $hash{$key}){
    print "2: key exists: $hash{$key}\n";
}else{
    print "2: key doesn't exist.\n";
}

$hash{$key} = undef;
if(exists $hash{$key}){
    print "3: key exists: $hash{$key}\n";
}else{
    print "3: key doesn't exist.\n";
}

delete $hash{$key};
if(exists $hash{$key}){
    print "4: key exists: $hash{$key}\n";
}else{
    print "4: key doesn't exist.\n";
}

./demo6-7
1: key doesn't exist.
2: key exists: success
3: key exists: 
4: key doesn't exist.

将哈希中的值设置为undef并不同于删除,exists依然会返回真

9. 环境变量哈希%ENV返回系统环境变量信息:
   demo6-8:

#!/usr/bin/perl
while(($key, $value) = each %ENV){
 print "$key:$value\n";
}

./demo6-8
PWD:/home/jingjie/workspace/LearningPerl/ch6
LC_PAPER:zh_CN.UTF-8
GTK_IM_MODULE:ibus
GJS_DEBUG_OUTPUT:stderr
GNOME_TERMINAL_SERVICE::1.67
TEXTDOMAINDIR:/usr/share/locale/
LESSCLOSE:/usr/bin/lesspipe %s %s
XDG_SESSION_TYPE:x11
JRE_HOME:/usr/local/java/jdk1.8.0_201/jre  
GJS_DEBUG_TOPICS:JS ERROR;JS LOG
LC_NUMERIC:zh_CN.UTF-8
LC_MONETARY:zh_CN.UTF-8
GDMSESSION:ubuntu
LC_ADDRESS:zh_CN.UTF-8
XMODIFIERS:@im=ibus
VTE_VERSION:5202
XDG_CURRENT_DESKTOP:ubuntu:GNOME
WINDOWPATH:1
LC_NAME:zh_CN.UTF-8
_:./demo6-8
CLASSPATH:.:/usr/local/java/jdk1.8.0_201/lib:/usr/local/java/jdk1.8.0_201/jre  /lib  
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