Myujth

Consider the following code.

class A
 attr_reader :n

 def initialize
 @n = 1
 end

 def set_local_n
 n = 10
 end

 def using_field_reader_locally
 n + 100
 end

 def using_field_reader_to_set_local_x
 x = n + 1000
 end

 def using_field_reader_to_set_local_n
 n = n + 1000 # This line raises a NoMethodError. Why?
 end
end

Why would the commented line produce an error? Why Ruby doesn't allow setting a local variable, n in this case, using an expression containing a field reader with the same name as the local variable? Why would all other instance methods run without any problem but using_field_reader_to_set_local_n produce an error.

Please note that I am NOT intending to define n locally in using_field_reader_to_set_local_n with an expression involving the locally defined n in set_local_n. I am intending to define n locally in using_field_reader_to_set_local_n with an expressing involving the field reader of @n. The definition of set_local_n is for showing that n can be defined locally while a field reader named n exists.

If you look closely, the NoMethodError isn't about there being no method n, but method + for NilClass.

In this statement:

n = n + 1000

Ruby has to decide what n is. As you are assigning something to it (n =) it's not a method. If you wanted to call the n= method, you had to specify an explicit receiver (self.n =).

So it has to be a variable. In this case local variable.

Now to make sense of something like:

n = n

Ruby ends up assigning nil to n. This has to do with the internals having to put a placeholder for a variable name that is being assigned.

So when you do:

n = n + 1000

What happens is:

1. Create a new local variable n with placeholder value (nil).


2. Attempt to assign to it whatever is on the right.


3. Calculate what is on the right - nil + 1000.


4. 
   nil doesn't have a + method so an error is raised.

To do what you wanted to do, you have to explicitly reference the instance variable for its value:

n = @n + 1000

Or explicitly call the attribute reader:

n = self.n + 1000