Constants & Variable in Ruby

main.rb

# Date Created: 20 June 2021
# Ruby Version 3.0.0p

# initiated constant
FRUIT = 'Apple' # => Apple
# can be modified
FRUIT << 's'  # => Apples
# and can also give totally different value, but with a warning raised by ruby
FRUIT = 'Orange'  # =>  warning: already initialized constant FRUIT
                  # =>  Orange


# we can freeze strings so that it cant be modified later
FRUIT.freeze
# FRUIT << 's'  # =>  FrozenError, cannot be modified

# but we can change its value with warning massage
FRUIT = 'Banana'  # =>  warning: already initialized constant FRUIT
                  # =>  Banana


# classes, modules etc are all constants, hence the first letter is capital
class On_constants
  # constants cannot be defined inside the methods
  Student = 4
  # varaible
  student = 0

  def const
    # calling constant
    puts Student
  end

  def var
    #puts student   # => NameError, as 'student' var is not founc in this method
  end
end

# accessing constant through class
p On_constants::Student   # => 4

# constant available to any methods of class
On_constants.new.const  # => 4

# accesssing variable student inside a method is not possible
# On_constants.new.var  # => # NameError