初始化哈希 [英] Initializing hashes

问题描述

我经常写这样的内容：

  a_hash ['x']？ a_hash ['x'] + ='多一些文字'：a_hash ['x'] ='第一文字'
  

应该有更好的方法来做到这一点，但我找不到它。

解决方案

有两种方法可以用 Hash 创建初始值。

一种是将单个对象传递给 Hash.new 。这在很多情况下都能很好地工作，特别是如果对象是冻结的值，但是如果对象具有内部状态，这可能会产生意想不到的副作用。由于同一个对象在没有指定值的情况下在所有键之间共享，所以修改其中一个的内部状态会显示出来。

  a_hash = Hash.new初始值
 a_hash ['a']＃=> 初始值
＃op =方法不会修改内部状态（通常），因为它们为键分配了新的
＃值。 
 a_hash ['b'] + ='拥有b'＃=> b拥有的初始值
＃其他方法，如＃<<<<和#gsub修改字符串
 a_hash ['c']。gsub！（/ initial /，c's）
 a_hash ['d']的状态'<< 由d修改
 a_hash ['e']＃=> c的值由d修改
  

另一个初始化方法是传递 Hash。新的块，每次为没有值的密钥请求一个值时调用该块。这允许您为每个键使用不同的值。

  another_hash = Hash.new {new initial value} 
 another_hash ['a']＃=> new initial value
＃op =方法仍然按预期工作
 another_hash ['b'] + ='拥有b'
＃但是，如果您不指定修改值，它会丢失
＃，因为每次未分配密钥的值被要求
 another_hash ['c']<<< 由c拥有＃=> c拥有的新初始值
 another_hash ['c']＃=> 新的初始值
  

该块传递了两个参数：散列被要求输入一个值，使用的钥匙。这使您可以为该键分配一个值，以便每次给定特定键时都会显示相同的对象。

  yet_another_hash = Hash.new {| hash，key | hash [key] =＃{key}的初始值} 
 yet_another_hash ['a']＃=> a的初始值
 yet_another_hash ['b']＃=> b的初始值
 yet_another_hash ['c'] .gsub！（'initial'，'awesome'）
 yet_another_hash ['c']＃=> c's awesome value
 yet_another_hash＃=> {a=> a的初始值，b=> b的初始值，c=> c's awesome value} 
  

最后一种方法是我最常用的方法。它对缓存昂贵的计算结果也很有用。

I frequently write something like this:

a_hash['x'] ? a_hash['x'] += ' some more text' : a_hash['x'] = 'first text'

There ought to be a better way to do this, but I can't find it.

解决方案

There are two ways to create initial values with for a Hash.

One is to pass a single object in to Hash.new. This works well in many situations, especially if the object is a frozen value, but if the object has internal state, this may have unexpected side-effects. Since the same object is shared between all keys without an assigned value, modifying the internal state for one will show up in all.

a_hash = Hash.new "initial value"
a_hash['a'] #=> "initial value"
# op= methods don't modify internal state (usually), since they assign a new
# value for the key.
a_hash['b'] += ' owned by b' #=> "initial value owned by b"
# other methods, like #<< and #gsub modify the state of the string
a_hash['c'].gsub!(/initial/, "c's")
a_hash['d'] << " modified by d"
a_hash['e'] #=> "c's value modified by d"

Another initialization method is to pass Hash.new a block, which is invoked each time a value is requested for a key that has no value. This allows you to use a distinct value for each key.

another_hash = Hash.new { "new initial value" }
another_hash['a'] #=> "new initial value" 
# op= methods still work as expected
another_hash['b'] += ' owned by b'
# however, if you don't assign the modified value, it's lost,
# since the hash rechecks the block every time an unassigned key's value is asked for
another_hash['c'] << " owned by c" #=> "new initial value owned by c"
another_hash['c'] #=> "new initial value"

The block is passed two arguments: the hash being asked for a value, and the key used. This gives you the option of assigning a value for that key, so that the same object will be presented each time a particular key is given.

yet_another_hash = Hash.new { |hash, key| hash[key] = "#{key}'s initial value" }
yet_another_hash['a'] #=> "a's initial value"
yet_another_hash['b'] #=> "b's initial value"
yet_another_hash['c'].gsub!('initial', 'awesome')
yet_another_hash['c'] #=> "c's awesome value"
yet_another_hash #=> { "a" => "a's initial value", "b" => "b's initial value", "c" => "c's awesome value" }

This last method is the one I most often use. It's also useful for caching the result of an expensive calculation.

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