Question

我有这个Ruby地图：

FIXED_COUNTRY_TO_PHONE = { 
    'FI' => '+358501111',
    'RU' => '4019900780', 
    'SE' => '+4672345678',
    'UA' => '0123456789',
    'KZ' => '0123456789'  
  }.freeze

如果其他值不匹配，如何设置最终值，例如'*' => '1234567'？

Answer 1

FIXED_COUNTRY_TO_PHONE = Hash.new('1234567').merge({ 
    'FI' => '+358501111',
    'RU' => '4019900780', 
    'SE' => '+4672345678',
    'UA' => '0123456789',
    'KZ' => '0123456789'  
  }).freeze

但简单

DEFAULT = "1234567".freeze
FIXED_COUNTRY_TO_PHONE["FI"] || DEFAULT
#=> "+358501111"
FIXED_COUNTRY_TO_PHONE["??"] || DEFAULT
#=> "1234567"

看起来也不错

Answer 2

您可以使用Hash#default=设置默认值：

hash =  {
    'FI' => '+358501111',
    'RU' => '4019900780', 
    'SE' => '+4672345678',
    'UA' => '0123456789',
    'KZ' => '0123456789'  
  }
hash.default = '1234567'
hash['UK']
#=> '1234567'

Answer 3

There are two main ways of dealing with this problem. @llya assigns a default value to the hash, @fl00r applies the default when the hash is evaluated for a non-existing key, causing nil to be returned.

llya shows one way of implementing the default value. It is very clear but uses a few lines of code. Two other ways of doing that deserve mention (though I am not suggesting they are in any sense better than how llya has done it).

Set the default then add the key-value pairs:

hash = Hash.new('1234567').
            merge('FI'=>'+358501111', 'RU'=>'4019900780', 'SE'=>'+4672345678',
                  'UA'=>'0123456789', 'KZ'=>'0123456789').      
            freeze

hash['oh, my']
  #=> "1234567"

Use Object#tap

hash = { 'FI'=>'+358501111', 'RU'=>'4019900780', 'SE'=>'+4672345678',
         'UA'=>'0123456789', 'KZ'=>'0123456789'
       }.tap { |h| h.default = '1234567' }.
         freeze

hash['oh, my']
  #=> "1234567"

A variantof fl00r's || DEFALULT approach, which arguably reads better, is to use Hash#fetch:

hash = { 'FI'=>'+358501111', 'RU'=>'4019900780', 'SE'=>'+4672345678',
         'UA'=>'0123456789', 'KZ'=>'0123456789' }

DEFAULT = "1234567"

hash.fetch('oh, my', DEFAULT)
  #=> "1234567"

hash[key] || DEFAULT clearly cannot be used if hash has a key whose value may be nil or false, though that is not an issue here.

Both approaches have pros and cons. Setting a default value avoids the need for the retrieval of values (which may be done in multiple places) to be concerned about whether the hash contains a particular key, but on the other hand readers of those parts of the code may not be aware (or have forgotten) that the hash was defined with a default value. If the || DEFAULT approach is used there is the possibility that the coder may forget to include || DEFAULT in one or more places where the hash is referenced. In terms of code maintenance I would think it's a toss-up, provided the constant DEFAULT is defined in close proximity to the hash, and commented suitably.

It seems to me that it's generally best to use a default value, and wherever the hash is referenced remind readers that it has a default value, either with a comment or a suggestive name for the hash (e.g., applies_to_oranges_with_default).

如果其他不匹配，则匹配值

3 个答案: