ruby about attr_accessor, instance variables, local varibles - ruby-on-rails

Question

I'm so confused about that..
like this
class Box
attr_accessor :item ,:item2
def initialize(item2)
#item = []
#item2 = item2
end
def add(product)
item << product
end
def empty?
item.empty?
end
def increment(n=1)
item2 +=1
end
end
cart =Box.new(123)
cart.add(1)
puts cart.empty? #false
puts cart.item #1
in the 'add' and 'empty?' methods
I use local variable 'item' right?
why I can get the value from #items ??
and I try this
cart.item2 = 345
puts cart.item2 #345
puts cart.increment #'increment': undefined method `+' for nil:NilClass (NoMethodError)
now I can't get the value?
please fix my brain thx

Answer 1

First, read this answer, which is the most-upvoted Ruby post in StackOverflow history. It will help you understand attr_accessor and its cousins attr_reader and attr_writer.
Besides that, your code has many problems.
First, you should not name an Array with a singular variable name like item. Use a plural items to make its purpose clear.
Second, the name item2 is not good. For your attribute, use something descriptive like counter, and for the variable passed as an argument to initialize it, let's use something descriptive like initial_count.
Third, your increment method takes an optional argument but then ignores it. Wouldn't it be surprising if someone called box.increment(2) and the attribute was incremented by only 1? The intent of this method is to use counter += n instead of counter += 1.
Fourth, to set counter from within the class, we need to use self. So instead of counter += n, we have to do self.counter += n.
Finally, consider whether you want the attributes to be readable and writable from an outside source, or whether you want to reserve write privileges to the object itself. Because you have methods to add things to items and to increment counter, you probably want to conceal write privileges. I would use attr_reader publicly and attr_writer privately.
Incorporating these suggestions, here's the resulting code:
class Box
attr_reader :counter, :items
def initialize(initial_count)
#counter = initial_count
#items = []
end
def add(product)
items << product
end
def empty?
items.empty?
end
def increment(n = 1)
self.counter += n
end
private
attr_writer :counter, :items
end
Now you can do this, all of which makes sense, more or less:
>> cart = Box.new(123)
>> cart.increment(2)
>> cart.counter
#> 125
>> cart.add('A product')
>> cart.add('Another product')
>> cart.items
#> ["A product", "Another product"]
But if you try to set counter or items directly, you'll get an error:
>> cart.counter = 1
#> NoMethodError: private method `counter=' called for #<Box:0x007fc13e17dc50>