在Ruby中定义[方括号]方法如何工作?

问题描述：

我正在经历 Programming Ruby-实用的程序员指南，偶然发现了以下这段代码:

I am going through Programming Ruby - a pragmatic programmers guide and have stumbled on this piece of code:

class SongList
  def [](key)
    if key.kind_of?(Integer)
      return @songs[key]
    else
      for i in 0...@songs.length
        return @songs[i] if key == @songs[i].name
      end
    end
    return nil
  end
end

我不明白定义[]方法是如何工作的?

I do not understand how defining [ ] method works?

为什么键位于[]之外，但是调用该方法时却位于[]内?

Why is the key outside the [ ], but when the method is called, it is inside [ ]?

键可以不带括号吗?

我意识到有更好的方法来编写此代码，并且知道如何编写自己的方法，但是此[]方法让我感到困惑.非常感谢您的帮助.

I realize there are far better ways to write this, and know how to write my own method that works, but this [ ] method just baffles me... Any help is greatly appreciated, thanks

答

Ruby中的方法与许多语言不同，可以包含一些特殊字符.其中之一是数组查找语法.

Methods in ruby, unlike many languages can contain some special characters. One of which is the array lookup syntax.

如果您要实现自己的哈希类，则在其中检索哈希中的项目时，您想将其反转，则可以执行以下操作:

If you were to implement your own hash class where when retrieving an item in your hash, you wanted to reverse it, you could do the following:

class SillyHash < Hash

  def [](key)
    super.reverse
  end

end

您可以通过以下方法调用哈希来证明这一点:

You can prove this by calling a hash with the following:

a = {:foo => "bar"}
 => {:foo=>"bar"} 
a.[](:foo)
 => "bar" 
a.send(:[], :foo)
 => "bar"

因此def []定义了执行操作时使用的方法.my_array["key"]您可能对其他方法感到陌生的是:

So the def [] defined the method that is used when you do my_array["key"] Other methods that may look strange to you are:

class SillyHash < Hash

  def [](key)
    super.reverse
  end

  def []=(key, value)
    #do something
  end

  def some_value=(value)
    #do something
  end

  def is_valid?(value)
    #some boolean expression
  end

end

只需澄清一下，[]方法的定义与数组或哈希无关.以以下(人为)示例为例:

Just to clarify, the definition of a [] method is unrelated to arrays or hashes. Take the following (contrived) example:

class B
  def []
    "foo"
  end
end

 B.new[]
 => "foo"

在Ruby中定义[方括号]方法如何工作?

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