Ruby accessing values of an object

Question

Say I have:

class Cell 
  def initialize(x_var, y_var)
    x = x_var
    y = y_var
  end
end

How would I be able to access the variables of the object cell?

cell1 = Cell.new(0, 0)

x = cell1.x
y = cell1.y

Is this correct?

Answer 1

There are a few things for you to learn:

1- Variable's scope

class Cell 
  def initialize(x_var, y_var)
    x = x_var
    y = y_var
  end

  def print_x
    puts x
  end
end

Cell.new(1,1).print_x

You get the following:

NameError: undefined local variable or method `x' for #<Cell:0x007ff901d9f448>

In the code above, the variable x gets "lives" between def initialize and end. This is the scope of x. If you try to access x outside that scope, you'd get a NameError telling you it's not defined.

2- Now, if you want a variable that lives as long as the object does, you can use instance variables. Instance variables start with an @ and live as long as the object does.

class Cell
  def initialize(x_var, y_var)
    @x = x_var
    @y = y_var
  end

  def x
    return @x
  end

  def x=(val)
    @x = val
  end

  def y
    return @y
  end

  def y=(val)
    @y = val
  end
end

c = Cell.new(1,2)
puts c.x

Gives the following:

1
=> nil

3- You had to write too much code in my example above to achieve very little. This is where @aruprakshit's attr_reader comes handy:

attr_reader :x generates the method:

def x
  return @x
end

4- If you want to read and write to @x, i.e., generate both the x and x= methods, attr_accessor does that for you:

class Cell
  attr_accessor :x, :y

  def initialize(x_var, y_var)
    @x = x_var
    @y = y_var
  end
end

c = Cell.new(1,2)
puts c.x
c.x = 3
puts c.x

Gives the following:

1
=> nil
=> 3
3
=> nil

Answer 2

How would I be able to access the variables of the object cell?

You need to look into attr_reader for this purpose.

class Cell 
  attr_reader :x,:y
  def initialize(x_var, y_var)
    @x = x_var
    @y = y_var
  end
end
cell1 = Cell.new(0, 0)

cell1.x # => 0
cell1.y # => 0

Read this Why use Ruby's attr_accessor, attr_reader and attr_writer? to know about attr_reader.