Re: Why nil

On 2018-07-04 08:06, dade wrote:

> Hello, let's look at an example 
> 
> class A 
> attr_accessor :a 
> 
> def initialize(a) 
> @a = a 
> end 
> 
> def method1(val) 
> a = val 
> end 
> 
> def method2(val) 
> a = a + val 
> end 
> end 
> 
> a = A.new(1) 
> a.method1(2) #=> 3 
> a.method2(2) #=> undefined method `+' for nil:NilClass 
> 
> Why the second a is nil?

NOTE: I am not a programmer; I just dabble.  But, unless I miss my mark,
the "a" being used in the initialization isn't available elsewhere in
the class.  So, when you say  
a = a + val 
you are, in essence, saying 
undefinedvariable = undefinedvariable + val 
Which is a non-starter. 
Let's check: 
[1] pry(main)> class Foo
[1] pry(main)* def initialize(a)
[1] pry(main)* @a = a
[1] pry(main)* end 
[1] pry(main)* def method1()
[1] pry(main)* puts a
[1] pry(main)* end 
[1] pry(main)* end 
=> :method1
[2] pry(main)> bar = foo.new(5)
NameError: undefined local variable or method `foo' for main:Object
from (pry):9:in `__pry__'
[3] pry(main)> bar = Foo.new(5)
=> #<Foo:0x000055916a3fc220 @a=5>
[4] pry(main)> bar.method1
NameError: undefined local variable or method `a' for
#<Foo:0x000055916a3fc220 @a=5>
Did you mean? @a
from (pry):6:in `method1' 

The fact that (if I had used attr_accessor) ":a" can be accessible as
foo.a is simply a nicety of attr_accessor; it doesn't reflect what's
going on in the class, itself. 

I'm sure, if I'm mistaken, someone will show me the error of my ways, 

-Ken 

> dade. 
> 
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