用空数组意外行为初始化一个Hash

问题描述

我想用一个空的 Array 来初始化一个 Hash ，并且为每个新键推入一个特定的值数组。

以下是我试图做的：

  a = Hash.new（[]）
＃=> {} 
 a [1]<< asd
＃=> [asd] 
 a 
＃=> {
  

a 的预期输出为 {1 => [asd]} 但没有发生。

Ruby版本：

  ruby​​ 2.0 .0p598（2014-11-13 revision 48408）[x86_64-linux] 
  

解决方案

只要做

  a = Hash.new {| h，k | h [k] = []} 
 a [1]<< asd
 a＃=> {1 => [asd]} 
  

从文档。这真的解释了为什么你没有得到所需的结果。

new（obj）→new_hash
$ b

new {| hash，key | block}→new_hash

您可以手动测试：

  a = Hash.new（[]）
a [1] .object_id＃=> 2160424560 
 a [2] .object_id＃=> 2160424560 
  

现在使用 Hash 对象创建时，您可以看到对未知键的每次访问，并返回相同的默认对象。现在另一种方式，我的意思是方式：

  b = Hash.new {| h， K | []} 
 b [2] .object_id＃=> 2168989980 
 b [1] .object_id＃=> 2168933180 
  

因此，通过 block 表单，每个未知键访问，返回一个新的 Array 对象。

I want to initialize a Hash with an empty Array and for every new key push a certain value to that array.

Here's what I tried to do:

a = Hash.new([])
# => {} 
a[1] << "asd"
# => ["asd"]
a
# => {}

The expected output for a was {1 => ["asd"]} but that didn't happen. What am I missing here?

Ruby version:

ruby 2.0.0p598 (2014-11-13 revision 48408) [x86_64-linux]

解决方案

Just do

a = Hash.new { |h, k| h[k] = [] }
a[1] << "asd"
a # => {1=>["asd"]}

Read the below lines from the Hash::new documentation. It really explains why you didn't get the desired result.

new(obj) → new_hash

new {|hash, key| block } → new_hash

You can test by hand :

a = Hash.new([])
a[1].object_id # => 2160424560
a[2].object_id # => 2160424560

Now with the above style of Hash object creation, you can see every access to an unknown key, returning back the same default object. Now the other way, I meant block way :

b = Hash.new { |h, k| [] }
b[2].object_id # => 2168989980
b[1].object_id # => 2168933180

So, with the block form, every unknown key access, returning a new Array object.

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