The following code prints nothing, I was expecting welcomeBack. Please explain.
class Hello
@instanceHello = "welcomeBack"
def printHello
print(@instanceHello)
end
Hello.new().printHello();
end
I just started to lean ruby, so please forgive if the question looks dumb.
If you could memorize only one thing about defining methods and setting variables, it's this: always ask yourself, what is self at this point?
class Hello
# This ivar belongs to Hello class object, not instance of Hello.
# `self` is Hello class object.
@instanceHello = "welcomeBack"
def printHello
# Hello#@instanceHello hasn't been assigned a value. It's nil at this point.
# `self` is an instance of Hello class
print @instanceHello
end
def self.printSelfHello
# Now this is Hello.@instanceHello. This is the var that you assigned value to.
# `self` is Hello class object
print @instanceHello
end
end
Hello.new.printHello # >>
Hello.printSelfHello # >> welcomeBack
If you want to set a default value for an ivar, do it in the constructor:
class Hello
def initialize
@instanceHello = "welcomeBack"
end
def printHello
print @instanceHello
end
end
Hello.new.printHello # >> welcomeBack
In Ruby, instance variables are defined and used in instance methods. So you need to put your assignment in the initialize method:
class Hello
def initialize
@instanceHello = "welcomeBack"
end
def printHello
print(@instanceHello)
end
end
Hello.new.printHello();
Also, note that I moved the printHello call outside of the class definition. This is because the class isn't actually defined until after it is closed the first time; that is, after the last end.
class Hello
@instanceHello = "welcomeBack"
def printHello
puts self.class.instance_variable_get(:@instanceHello)
end
Hello.new.printHello; #=> welcomeBack
end
This is not good programming, but just to illustrate that a class (which is an instance of the class Class) can also have instance variables, like any other instance. They are called "class instance variables" and are preferred to class variables. The following example illustrates how a counter could be defined at the class level :
class Hello
@counter = 0
class << self # access methods for class instance variables must be
# defined in the singleton class
attr_accessor :counter
end
def printHello
self.class.counter += 1
puts "Hello #{self.class.counter}"
end
end
Hello.new.printHello; #=> Hello 1
Hello.new.printHello; #=> Hello 2
p Hello.singleton_methods #=> ["counter=", "counter"]
change @instanceHello to self.class.instance_variable_get(:@instanceHello)
@instanceHello is a class instance variant
the code is this:
class Hello
@instanceHello = "welcomeBack"
@@classHello = "greeting!"
def printHello
print(self.class.instance_variable_get(:@instanceHello))
print(self.class.class_variable_get(:@@classHello))
end
end
Hello.new().printHello();
