From: Doug Glidden <41mortimer@...>
Date: 2008-09-16T00:39:00+09:00
Subject: Re: Need someone who can help

Dot Baiki wrote:
> Hey! Amazing quick response! Outstanding. Sweet solutions! Will continue 
> reading my books I bought. Really cool. Thanks so much. This is what I 
> made now:
> 
> --- begin ---
> #!/usr/bin/env ruby
> 
> class AreaOfCircle
> 
>   PI = 3.14159265
> 
>   def initialize(radius)
>     @radius = radius
>     @area = nil
>   end
> 
>   def calc_area(radius)
>     @area = PI * @radius**2
>   end
> end
> 
> print 'Enter radius of circle: '
> radius = gets.to_f
> puts "You entered: #{radius}"
> 
> instance = AreaOfCircle.new(radius)
> screen_output = instance.calc_area(radius)
> puts "Result: #{screen_output}"
> --- end ---
> 
> You know, the cool thing is that I really soooooon will understand Ruby 
> and its beauty :-)
> 
> Baiki

Baiki,

I think you might end up getting surprised by what happens when you do 
this.  For instance, consider the results of the following code, given 
your existing AreaOfCircle class (of which I really dislike the name, 
but never mind that for now):

  circle = AreaOfCircle.new 5 # circle has @radius=5, @area=nil
  area = circle.calc_area 3   # assigns @area=78.540... (25*PI), i.e. 
the radius
                              # parameter is completely ignored

So get rid of the completely unnecessary parameter for calc_area. 
Furthermore, the user probably doesn't want to have to remember to 
manually call for the area to be calculated when he creates a circle, so 
just do that in your constructor.  Add accessors for @radius and @area 
and you're really cookin'.  It might look something like this:

  class MyCircle

    attr_accessor :radius, :area

    PI = 3.14159265

    def initialize(radius)
      @radius = radius
      calc_area
    end

    def calc_area
      @area = PI * @radius**2
    end
  end

Your program code could then look like this:

  print 'Enter radius of circle: '
  radius = gets.to_f
  puts "You entered: #{radius}"

  instance = MyCircle.new(radius)
  screen_output = instance.area
  puts "Result: #{screen_output}"

On the other hand, if all you're interested in is calculating the area 
and you don't actually need to store the information, use a mixin 
instead of a full-blown class:

  Module AreaOfCircle
    PI = 3.14159265

    def calc_area(radius)
      PI * radius**2
    end
  end

  # begin program code

  include AreaOfCircle

  print 'Enter radius of circle: '
  radius = gets.to_f
  puts "You entered: #{radius}"

  screen_output = calc_area radius
  puts "Result: #{screen_output}"

I know I threw a lot of stuff at you, so feel free to ask about anything 
you don't understand there.

Doug
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