From: "Carlo E. Prelz" <fluido@...>
Date: 2012-11-03T04:24:22+09:00
Subject: Re: Simple question

	Subject: Simple question
	Date: Sat 03 Nov 12 03:52:00AM +0900

Quoting August Y. (lists@ruby-forum.com):

> I have just picked up ruby and am trying to teach my self.  so this
> question is probably easy but its giving me fits.
> <code>
> class Test
>   def round
>     puts "the object is round"
>   end
>   def random
>     $radus=rand(10)+1          <---Global Variable
>   end
>   def coords
>     x=rand(1000)
>     y=rand(1000)
>     z=rand(1000)
>     lo=[x,y,z]
>     puts lo
>   end
>   def size
>     pi=Math::PI
>     area=pi*(radus^2)           <----error cant find radus?

If you want to refer to your global variable, you must always use the
dollar sign:

     area=pi*($radus^2)

If you just use 'radus', you refer to a different variable, which in
this case is not defined. The scope of variables whose first character
is a lower-case letter is limited to the method body.

>     volume = 4/3*pi*(radus^3)
>     puts volume
>     puts radus
>     puts area
>   end
>   def landmass
>     landmass=size.area*0.33         <--- why is this wrong?

size is a method. If you want the method to return the area, you must
rewrite it as

   def size
     pi=Math::PI
     area=pi*($radus^2)           <----error cant find radus?
     volume = 4/3*pi*($radus^3)
     puts volume
     puts $radus
     puts area

     return area
   end

and then you can write the line as

     landmass=size()*0.33

which means: assign to variable landmass the return value of methods
size, multiplied by 0.33.

>     puts landmass

this line just prints the value on screen, but you want to return the
value. You must add

      return landmass

If you do not specify an explicit return statement, methods return the
result of the last operation that has been executed. The #puts method
returns nil, so your two methods return nil.

Note that it is not the best thing to use a global variable. Since the
variable is accessed only within the instance methods of a class, it is
more appropriate to use a class instance variable (in all places,
write @radus instead of $radus). 

>   end
> 
>   t=Test.new
>   puts t.landmass
> end
> </code>

You have to understand better the concepts of classes, instances,
methods, There are dozens of different ways of doing things in Ruby,
but I would write your class by using class instance variables for
radus, area, volume, landmass.

Try again!

Carlo

-- 
  *         Se la Strada e la sua Virtu' non fossero state messe da parte,
* K * Carlo E. Prelz - fluido@fluido.as             che bisogno ci sarebbe
  *               di parlare tanto di amore e di rettitudine? (Chuang-Tzu)