Question

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?

Answer 1

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.

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  

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" 

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 🙂

Happy coding.