通过符号而不是字符串访问哈希键[重复]

Question

我有以下代码。我不明白为什么table['something'] 与table[:something] 不同。

require 'json'
data = '[{"type":"local","db":"all","user":"postgres","addr":null,"method":"ident"},{"type":"local","db":"all","user":"all","addr":null,"method":"ident"},{"type":"host","db":"all","user":"all","addr":"0.0.0.0/0","method":"md5"},{"type":"host","db":"all","user":"all","addr":"::1/128","method":"md5"},{"type":"local","db":"communicator","user":"communicator","addr":" ","method":"trust"}]'
table = JSON.parse(data)
table.each do | auth |
  if (auth['user'])
    print( '\n\n\n' + auth[:user] )
  end
end

与print( '\n\n\n' + auth[:user] ) 在线，我收到一个错误：

TypeError: no implicit conversion of nil into String
        from (irb):88:in `+'

这意味着通过:key 访问table 与'key' 不同。 为什么？如何转换 table 以使其与 :key 而不是 'key' 一起使用？

Answer 1

符号 :key 不是字符串“键”，就像数字 3 不是字符串“3”一样。要将字符串转换为密钥，您可以使用 key.to_sym

关于其他两个 SO 问题之间的差异已被询问。 What's the difference between a string and a symbol in Ruby? 另见this文章

编辑 如果您无法控制源，则可以将键更改为符号的解决方案

require 'json'

data = '[{"type":"local","db":"all","user":"postgres","addr":null,"method":"ident"},{"type":"local","db":"all","user":"all","addr":null,"method":"ident"},{"type":"host","db":"all","user":"all","addr":"0.0.0.0/0","method":"md5"},{"type":"host","db":"all","user":"all","addr":"::1/128","method":"md5"},{"type":"local","db":"communicator","user":"communicator","addr":" ","method":"trust"}]'

table = JSON.parse(data)

p table
# [{"type"=>"local", "db"=>"all", "user"=>"postgres", "addr"=>nil, "method"=>"ident"}, {"type"=>"local", "db"=>"all", "user"=>"all", "addr"=>nil, "method"=>"ident"}, {"type"=>"host", "db"=>"all", "user"=>"all", "addr"=>"0.0.0.0/0", "method"=>"md5"}, {"type"=>"host", "db"=>"all", "user"=>"all", "addr"=>"::1/128", "method"=>"md5"}, {"type"=>"local", "db"=>"communicator", "user"=>"communicator", "addr"=>" ", "method"=>"trust"}]

table.map!{|auth| 
  new_auth = {}
  auth.each{|k, v| new_auth[k.to_sym] = v}
  new_auth
}

p table
# [{:type=>"local", :db=>"all", :user=>"postgres", :addr=>nil, :method=>"ident"}, {:type=>"local", :db=>"all", :user=>"all", :addr=>nil, :method=>"ident"}, {:type=>"host", :db=>"all", :user=>"all", :addr=>"0.0.0.0/0", :method=>"md5"}, {:type=>"host", :db=>"all", :user=>"all", :addr=>"::1/128", :method=>"md5"}, {:type=>"local", :db=>"communicator", :user=>"communicator", :addr=>" ", :method=>"trust"}]

Answer 2

问题在于，{"user":"postgres"} 在 JSON 中的意思是：

{"user" => "postgres"}

在 Ruby 中，而在 Ruby 中则意味着：

{:user => "postgres"}

由于您将其作为 JSON 格式，因此您需要按以下方式访问该值：

auth["user"]

而不是：

auth[:user]

这是nil，因为哈希缺少密钥:user。

Answer 3

因为'key' 是String 而:key 是Symbol - 这在Ruby 中是两个不同的东西。

这可能有点令人困惑，因为:'key' 或'key': 也将是Symbol 要使其正常工作，只需使用Symbol 访问Hash 字段，例如：

if (auth[:user])

要将 String indexed Hash 转换为 Symbol indexed Hash，请参考这个问题：

Best way to convert strings to symbols in hash