Setter doesn't work with -=, +=, etc? [duplicate]

Answer 1

UPDATE: It's not clear whether you want an instance variable or a class variable for 'var'. The example below is for an instance variable.

I would use attr_reader, or attr_accessor if you want to do this external to the class.

class Test
   attr_reader :var

   def initialize
      @var=2.4
   end

   def test
     puts @var
     @var -= 1
     puts @var
     @var = @var - 1
     puts @var
   end
end

t=Test.new
t.test
puts t.var

$ /tmp/t.rb
2.4
1.4
0.3999999999999999
0.3999999999999999

Answer 2

Write as

def test
    puts var
    self.var -= 1
    puts var
    self.var = var - 1
    puts var
end

If you don't use self, then Ruby will treat those var as a local variables, rather then setter method calls.

Just remember, in Ruby method call can never be made without receiver(implicit/explicit). Now if you write var = 1, Ruby will treat as local variable assignment. But if you write self.var, Ruby will parse it as a method call, and will try to call your setter method, if you defined. Also remember self.var = 1 is a syntactic sugar of self.var=(1).

There is more interesting discussion about the same, worth to read Private setters can be called by self, why not getters?

There is a recent bug was found regarding the private setters. Here is the Bug ticket, and it is now fixed too.

Answer 3

Because var is nil. you need to assign some value to it first

def test
    puts var
    var=0 
    var -= 1 # <<< crash: undefined method for nil class
    puts var
    var=0
    var = var - 1 # <<< crash: undefined method for nil class
    puts var
  end