Question

x是一个数组。
请注意下面的两个比较。正如预期的那样，第一个应该产生“真实”。第二个抛出错误。这是怎么回事？

为什么nil的负面比较在第二次测试中不会产生错误？

ruby-1.9.2-p136 :079 > x[2]['comments']['data'][0]['from']['name'] != nil
 => true 

x[2]['comments']['data'][1]['from']['name'] != nil
NoMethodError: You have a nil object when you didn't expect it!
You might have expected an instance of Array.
The error occurred while evaluating nil.[]
    from (irb):78
    from /Users/justinz/.rvm/gems/ruby-1.9.2-p136/gems/railties-3.0.3/lib/rails/commands/console.rb:44:in `start'
    from /Users/justinz/.rvm/gems/ruby-1.9.2-p136/gems/railties-3.0.3/lib/rails/commands/console.rb:8:in `start'
    from /Users/justinz/.rvm/gems/ruby-1.9.2-p136/gems/railties-3.0.3/lib/rails/commands.rb:23:in `<top (required)>'
    from script/rails:6:in `require'
    from script/rails:6:in `<main>'

Answer 1

我猜是因为值x[2]['comments']['data'][1]是零。

您可能希望使用此辅助方法：

def nullsafe_index(a, keys)
    keys.each{|key|
        return nil if a.nil?
        a = a[key]
    }
    return a
end

像这样使用：

nullsafe_index(x, [2, 'comments', 'data', 0, 'from', 'name']).nil?

Answer 2

x [2] ['comments'] ['data'] [1]是一个空哈希，所以当你在它上面调用['from']时，结果是nil，这意味着调用['name']在nil的结果上，产生错误。以下是如何重现它：

x = {}
x['from'] #=> nil
x['from']['name'] #=> NoMethodError

您可以将您的请求视为函数调用的集合：

x[2]['comments']['data'][1]['from']['name']
# is equivalent to:
x.[](2).[]('comments').[]('data').[](1).[]('from').[]('name')

如果这些函数调用中的任何一个返回nil，则不能在没有错误的情况下对其进行另一个[]函数调用。

Answer 3

看起来x[2]['comments']['data']没有第二个元素。这相当于调用nil['from']，这也会引发异常。

Answer 4

x[2]['comments']['data'][1] == nil？

尝试评估表达的每一部分：

x[2]
x[2]['comments']
x[2]['comments']['data']
x[2]['comments']['data'][1]
x[2]['comments']['data'][1]['from']
x[2]['comments']['data'][1]['from']['name']

nil的ruby测试会抛出错误

4 个答案: