How do you use global variables or constant values in Ruby

Question

I have a program that looks like    offset   Point new 100  200    def draw point    pointNew    offset   point    drawAbsolute point  end  draw Point new 3  4      the use of  offset seems a bit weird   In C  if I define something outside of any function  it is a global variable automatically  Why in Ruby does it have to be  offset but cannot be offset and still be global   If it is offset  then it is a local   But local to where  because it feels very much global   Are there better ways to write the code above   The use of  offset may seem a bit ugly at first     Update   I can put this offset inside a class definition  but what if two or several classes need to use this constant  In this case do I still need to define an  offset

User · Accepted Answer

One thing you need to realize is in Ruby everything is an object. Given that, if you don't define your methods within Module or Class, Ruby will put it within the Object class. So, your code will be local to the Object scope.

A typical approach on Object Oriented Programming is encapsulate all logic within a class:

class Point
  attr_accessor :x, :y

  # If we don't specify coordinates, we start at 0.
  def initialize(x = 0, y = 0)
    # Notice that `@` indicates instance variables.
    @x = x
    @y = y
  end

  # Here we override the `+' operator.
  def +(point)
    Point.new(self.x + point.x, self.y + point.y)
  end

  # Here we draw the point.
  def draw(offset = nil)
    if offset.nil?
      new_point = self
    else
      new_point = self + offset 
    end
    new_point.draw_absolute
  end

  def draw_absolute
    puts "x: #{self.x}, y: #{self.y}"
  end
end

first_point = Point.new(100, 200)
second_point = Point.new(3, 4)

second_point.draw(first_point)

Hope this clarifies a bit.

User · Answer

One of the reasons why the global variable needs a prefix  called a  sigil   is because in Ruby  unlike in C  you don t have to declare your variables before assigning to them  The sigil is used as a way to be explicit about the scope of the variable    Without a specific prefix for globals  given a statement pointNew   offset   point inside your draw method then offset refers to a local variable inside the method  and results in a NameError in this case   The same for   used to refer to instance variables and    for class variables   In other languages that use explicit declarations such as C  Java etc  the placement of the declaration is used to control the scope

User · Answer

Variable scope in Ruby is controlled by sigils to some degree  Variables starting with   are global  variables with   are instance variables     means class variables  and names starting with a capital letter are constants  All other variables are locals  When you open a class or method  that s a new scope  and locals available in the previous scope aren t available   I generally prefer to avoid creating global variables  There are two techniques that generally achieve the same purpose that I consider cleaner    Create a constant in a module  So in this case  you would put all the classes that need the offset in the module Foo and create a constant Offset  so then all the classes could access Foo  Offset  Define a method to access the value  You can define the method globally  but again  I think it s better to encapsulate it in a module or class  This way the data is available where you need it and you can even alter it if you need to  but the structure of your program and the ownership of the data will be clearer  This is more in line with OO design principles

User · Answer

I think it s local to the file you declared offset  Consider every file to be a method itself   Maybe put the whole thing into a class and then make offset a class variable with   offset   Point new 100  200

[ruby] How do you use global variables or constant values in Ruby?

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