2020-08-21 列表，数组，哈希嵌套

列表，数组和哈希的嵌套
Perl的数据结构中最常用到的两种类型数组和哈希。

1. 什么是数组（Perl语言入门（第五版）的解释）？
   如果说Perl的标量代表的是单数(singular)，那么在Perl里代表复数(plural)的就是列表和数组。
   列表(list)指的是标量的有序集合，而数组(array)则是存储列表的变量。在Perl里，这两个术语常常混用。不过更精确地说，列表指的是数据，而数组指的是变量。列表的值不一定要放在数组里，但每个数组变量都一定包含一个列表（即使该列表可能是空的）。

2. 什么是哈希（Perl语言入门（第五版）的解释）？
   所谓的哈希就是一种数据结构，和数组的相同之处在于：可以容纳很多值（没有上限），并能随机存取。而区别在于：不像数组是以数字来检索，哈希是以名字来检索。也就是说检索用的键不是数字，而是保证唯一的字符串。
   另一种看待哈希的方法，是将它想象成一大桶的数据，其中每个数据都有关联的标签。可以伸手到桶里任意取出一张标签，查看附着的数据是什么。但是桶里没有所谓的第一个元素，只有一堆数据。然而数组里，第一个元素为元素0.然后是元素1、2，等等。但是哈希里没有顺序，因此也没有第一，有的只是一些键/值对。这些键和值都是任意的标量，但是键总是会被转换成字符串。假如你以数字表达式50/20为键，那么它会被转换为一个含有三个字符的字符串"2.5"。

了解完了什么是数组、列表和哈希，下面我们开始分析它们是如何嵌套使用的：

Ⅰ. 列表嵌套列表（列表嵌套列表即为二维列表）
a. 列表的声明
方法一：
@list = (
["banana","apple","orange","pear"],
["cauliflower","lettuce","tomato","cucumber"],
["orange juice","apple juice","red tea","red wine"]
)

方法二：
@fruit = ("banana","apple","orange","pear")
@vegetable = ("cauliflower","lettuce","tomato","cucumber")
@drink = ("orange juice","apple juice","red tea","red wine")

正确方法：
@list = ([@fruit],[@vegetable],[@drink])
错误方法：
@list = (@fruit, @vegetable, @drink)
错误剖析：如果不用[]把每个子列表括起来，那么这个列表@list就相当于@fruit, @vegetable, @drink这三个列表拼接而成的一个一维列表相当于@list = ("banana","apple","orange","pear","cauliflower","lettuce","tomato","cucumber","orange juice","apple juice","red tea","red wine")

方法三：
正确方法：
list[0] = ("banana","apple","orange","pear")
错误剖析：这样的结果是$list[0] = "pear" 并且会报警告

b. 列表的访问
list[0] and @list[0] 两种表示都可以，但是 @list[0] 会报警告

Ⅱ. 列表嵌套哈希
a. 列表的声明
方法一：
@list = (
{
'fruit' => 'banana',
'vegetable' => 'cauliflower',
'drink' => 'orange juice'
},
{
'fruit' => 'apple',
'vegetable' => 'lettuce',
'drink' => 'apple juice'
},
{
'fruit' => 'orange',
'vegetable' => 'tomato',
'drink' => 'red tea'
},
{
'fruit' => 'pear',
'vegetable' => 'cucumber',
'drink' => 'red wine'
}
)

方法二：
%group1 = (
'fruit' => 'banana',
'vegetable' => 'cauliflower',
'drink' => 'orange juice'
)
%group2 = (
'fruit' => 'apple',
'vegetable' => 'lettuce',
'drink' => 'apple juice'
)
%group3 = (
'fruit' => 'orange',
'vegetable' => 'tomato',
'drink' => 'red tea'
)
%group4 = (
'fruit' => 'pear',
'vegetable' => 'cucumber',
'drink' => 'red wine'
)

正确方法：
@list = ({%group1}, {%group2}, {%group3}, {%group4})
错误方法：
@list = (%group1, %group2, %group3, %group4)
错误剖析：如果不用{}把每个子列表括起来，那么这个列表@list就相当于%group1, %group2, %group3, %group4 这四个哈希拼接而成的一个一维列表，相当于@list = ( 'fruit', 'banana', 'drink', 'orange juice', 'vegetable', 'cauliflower', 'fruit', 'apple', 'drink', 'apple juice', 'vegetable', 'lettuce', 'fruit', 'orange', 'drink', 'red tea', 'vegetable', 'tomato', 'fruit', 'pear', 'drink', 'red wine', 'vegetable', 'cucumber')

方法三：
正确方法：
list[0] = {%group1};
错误方法：
list[0] = 'orange juice' 并且有警告
list[0] = ['fruit','banana','vegetable','cauliflower','drink','orange juice']

b. 列表的访问
list[0]{fruit} are the same.
$list[0] = {'fruit' => 'banana','drink' => 'orange juice','vegetable' => 'cauliflower'}

Ⅲ. 哈希嵌套哈希
a. 哈希的声明
方法一：
my %hash = (
group1 => {
'fruit' => 'banana',
'vegetable' => 'cauliflower',
'drink' => 'orange juice'
},
group2 => {
fruit => 'apple',
vegetable => 'lettuce',
drink => 'apple juice'
},
group3 => {
'fruit' => 'orange',
'vegetable' => 'tomato',
'drink' => 'red tea'
},
group4 => {
fruit => 'pear',
vegetable => 'cucumber',
drink => 'red wine'
}
);

%hash = (
'group1', {'fruit', 'banana', 'drink', 'orange juice', 'vegetable', 'cauliflower'},
'group2', {'fruit', 'apple', 'drink', 'apple juice', 'vegetable', 'lettuce'},
'group3', {'fruit', 'orange', 'drink', 'red tea', 'vegetable', 'tomato'},
'group4', {'fruit', 'pear', 'drink', 'red wine', 'vegetable', 'cucumber'}
);

方法二：
%group1 = (
'fruit' => 'banana',
'vegetable' => 'cauliflower',
'drink' => 'orange juice'
)
%group2 = (
'fruit' => 'apple',
'vegetable' => 'lettuce',
'drink' => 'apple juice'
)
%group3 = (
'fruit' => 'orange',
'vegetable' => 'tomato',
'drink' => 'red tea'
)
%group4 = (
'fruit' => 'pear',
'vegetable' => 'cucumber',
'drink' => 'red wine'
)

正确方法：
%hash = (
'group1' => {%group1},
'group2' => {%group2},
'group3' => {%group3},
'group4' => {%group4}
);
错误方法：
%hash = (%group1, %group2, %group3, %group4);
错误剖析：这个哈希相当于%hash = %group4
%hash = (
'group1' => %group1,
'group2' => %group2,
'group3' => %group3,
'group4' => %group4
);
错误剖析：这个哈希的结果相当于 %hash = ('tomato','group4','orange juice','vegetable','cauliflower','group2','orange','drink','fruit','pear','group1','fruit','banana','drink','drink','red wine','vegetable','cucumber','group3','fruit','red tea','vegetable')

方法三：
hash{'group1'} = {
'fruit' => 'banana',
'vegetable' => 'cauliflower',
'drink' => 'orange juice'
};
hash{'group1'}{'fruit'} = 'banana' and list{group1) = {'fruit' => 'banana','drink' => 'orange juice','vegetable' => 'cauliflower'}

Ⅳ. 哈希嵌套列表
a. 哈希的声明
方法一：
%hash = (
'fruit' => ["banana","apple","orange","pear"],
'vegetable' => ["cauliflower","lettuce","tomato","cucumber"],
'drink' => ["orange juice","apple juice","red tea","red wine"]
);

%hash = (
'fruit', ["banana","apple","orange","pear"],
'vegetable', ["cauliflower","lettuce","tomato","cucumber"],
'drink', ["orange juice","apple juice","red tea","red wine"]
);

方法二：
@fruit = ("banana","apple","orange","pear")
@vegetable = ("cauliflower","lettuce","tomato","cucumber")
@drink = ("orange juice","apple juice","red tea","red wine")

%hash = (
'fruit', [@fruit],
'vegetable', [@vegetable],
'drink', [@vegetable]
);

方法三：
hash{'fruit'} = ["banana","apple","orange","pear"]

b. 哈希的访问
hash{'fruit'} = ['banana','apple','orange','pear']

结语：
好了！数组、列表和哈希的相互嵌套的分析到这里就要告一段落了。
由于本人才学疏浅，文章中有不对的或者表示不清楚的欢迎大家指正，谢谢！

转自http://blog.sina.com.cn/s/blog_48a587210101fwkd.html