如果尚未定义，则设置 Ruby 变量 [英] Set Ruby variable if it is not already defined

问题描述

在 Ruby 中，如果变量尚未定义，如何将变量设置为某个值，如果已定义，如何保留当前值?

In Ruby, how do you set a variable to a certain value if it is not already defined, and leave the current value if it is already defined?

推荐答案

While x ||= value 是一种表达如果 x 包含假值，包括 nil(隐含在这个结构如果 x 未定义，因为它出现在赋值的左侧)，为 x 赋值"，它就是这样做的.

While x ||= value is a way to say "if x contains a falsey value, including nil (which is implicit in this construct if x is not defined because it appears on the left hand side of the assignment), assign value to x", it does just that.

它大致等同于以下内容.(但是，x ||= value 不会像这个代码那样抛出 NameError可能，它总是会为 x 赋值 因为这段代码没有 - 关键是要看到 x ||= value 对于 x 中的 any 假值(包括默认值")的作用相同nil 值):

It is roughly equivalent to the following. (However, x ||= value will not throw a NameError like this code may and it will always assign a value to x as this code does not -- the point is to see x ||= value works the same for any falsey value in x, including the "default" nil value):

if !x
  x = value
end  

要查看变量是否真正没有被赋值，请使用defined?方法:

To see if the variable has truly not been assigned a value, use the defined? method:

>> defined? z
=> nil                                                                  
>> z = nil                                                              
=> nil                                                                  
>> defined? z                                                           
=> "local-variable"                                                     
>> defined? @z                                                          
=> nil                                                                  
>> @z = nil                                                             
=> nil                                                                  
>> defined? @z                                                          
=> "instance-variable" 

但是，几乎在所有情况下，使用 defined? 是代码异味. 小心电源.做明智的事情:在尝试使用变量之前先给它们值:)

However, in almost every case, using defined? is code smell. Be careful with power. Do the sensible thing: give variables values before trying to use them :)

快乐编码.

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