为什么在数组之前需要星号？ [英] Why do I need asterisk before an array?

问题描述

我不知道这是一个哈希问题还是一个数组问题，但我不明白为什么在第三个示例中需要使用星号（*）符号来获取用数据填充的散列。没有它，它会输出一个空的散列。

 ＃ -  *  -  coding：utf-8  -  *  -  
 require 'pp'
 pp [[：first_name，'Shane']，[：last_name，'Harvie']] 
＃=> [[：first_name，Shane]，[：last_name，Harvie]] 
 
 [[：first_name，'Shane']，[：last_name，'Harvie']]。flatten 
＃=> [：first_name，Shane，：last_name，Harvie] 
 
 Hash [* [[：first_name，'Shane']，[：last_name，'Harvie']]。flatten] 
＃=> {：first_name =>Shane，：last_name =>Harvie} 
 
 Hash [[[：first_name，'Shane']，[：last_name，'Harvie']]。 flatten] 
＃=> {} 
  

以下是该方法的解释。

  ------------------------------------- -------------------------- Hash :: [] 
 Hash [[key => |，value] *] => ;哈希
 
从Ruby 1.8 
 ---------------------------------- -------------------------------------- 
创建一个填充给定的新哈希对象。相当于
的文字+ {_key_，_value_，...} +。键和值出现在
对中，所以必须有偶数个参数。 
 
 Hash [a，100，b，200]＃=> {a=> 100，b=> 200} 
哈希[a=> 100，b=> 200]＃=> {a=> 100，b=> 200} 
 {a=> 100，b=> 200}＃=> {a=> 100，b=> 200} 
  

解决方案 * ）将否则会将数组转换为哈希中赋值的列表。您必须给 [] 运算符一个实际列表，以将其转换为散列的键/值对。 （请参阅下面的链接，以获取splat操作符的简短描述（实际上可以这样做）（将数组展开为列表）或反向（将列表收集到数组中）。）

按照上面的方式，您给 Hash [] 一个奇数项，即单个数组。 （想想 [[：first_name，'Shane']，[：last_name，'Harvie']]。flatten 产生了什么，它会产生 [： firstname，'Shane'，：last_name，'Havie'] 。）正如你引用的文档所说， [] 操作符必须有一个偶数元素的数量。请注意，以下（尽管无用）确实有效：

 >> Hash [[[：first_name，'Shane']，[：last_name，'Harvie']]。flatten，1] 
 => {[：first_name，Shane，：last_name，Harvie] => 1} 
  

（我不清楚为什么当你使用上面的代码时，你没有得到奇数个Hash参数错误 - 就像你尝试 Hash [1]



一个简单的例子可能会使它更清晰。首先，传入一个项目，一个数组。使用 * 打开 Hash [] 项目列表：



 >> Hash [[''foo'，'bar'，'bizz'，'buzz']] 
 => {} 
>>哈希[* ['foo'，'bar'，'bizz'，'buzz']] 
 => {foo=>bar，bizz=>buzz} 
  

请参阅此博客帖子以获得更完整的解释。您可能还会发现这篇关于图片的简短说明运营商有用。

I have no idea if this is a Hash issue or an Array issue, but I don't figure out why asterisk (*) sign is required in the third example to get a hash filled with data. Without it, it outputs an empty hash.

# -*- coding: utf-8 -*-
require 'pp'
pp [[:first_name, 'Shane'], [:last_name, 'Harvie']]
# => [[:first_name, "Shane"], [:last_name, "Harvie"]]

pp [[:first_name, 'Shane'], [:last_name, 'Harvie']].flatten
# => [:first_name, "Shane", :last_name, "Harvie"]

pp Hash[*[[:first_name, 'Shane'], [:last_name, 'Harvie']].flatten]
# => {:first_name=>"Shane", :last_name=>"Harvie"}

pp Hash[[[:first_name, 'Shane'], [:last_name, 'Harvie']].flatten]
# => {}

Below is an explanation of the method.

--------------------------------------------------------------- Hash::[]
     Hash[ [key =>|, value]* ]   => hash

     From Ruby 1.8
------------------------------------------------------------------------
     Creates a new hash populated with the given objects. Equivalent to
     the literal +{ _key_, _value_, ... }+. Keys and values occur in
     pairs, so there must be an even number of arguments.

        Hash["a", 100, "b", 200]       #=> {"a"=>100, "b"=>200}
        Hash["a" => 100, "b" => 200]   #=> {"a"=>100, "b"=>200}
        { "a" => 100, "b" => 200 }     #=> {"a"=>100, "b"=>200}

解决方案

The splat operator (that is, the *) turns what would otherwise be an array into a list for assignment within the hash. You have to give the [] operator an actual list to turn into the key/value pairs of a hash. (See below for a link to a short description of the splat operator which actually can do this (unwind an array into a list) or the reverse (gather a list into an array).)

The way you did it above, you give Hash[] an odd number of items, namely a single array. (Think of what [[:first_name, 'Shane'], [:last_name, 'Harvie']].flatten produces. It yields [:first_name, 'Shane', :last_name, 'Havie'].) As the docs you quoted say, the [] operator must have an even number of elements. Note that the following (though useless) does work:

>> Hash[[[:first_name, 'Shane'], [:last_name, 'Harvie']].flatten, 1]
=> {[:first_name, "Shane", :last_name, "Harvie"]=>1}

(It's unclear to me why you don't get the "odd number of arguments for Hash" error when using the code you have above - as you do if you try Hash[1].)

A simpler example may make it clearer. First, passing in one item, an array. The opening up the array with * to hand Hash[] a list of items:

>> Hash[['foo', 'bar', 'bizz', 'buzz']]
=> {}
>> Hash[*['foo', 'bar', 'bizz', 'buzz']]
=> {"foo"=>"bar", "bizz"=>"buzz"}

See this blog post for a fuller explanation. You might also find this short write-up about the splat operator useful.

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