From: Chris White <cwprogram@...>
Date: 2011-08-02T04:50:58+09:00
Subject: Re: newbie question


> class Factorial
> def fact(n)
> if n == 0
>   1
> else
>   n * fact(n-1)
> end
> end
> end
> 
> my_fact = Factorial.new
> puts "enter a positive integer: "
> my_fact = gets.to_i
> puts "factorial: #{fact(n)}"
> 
> I get this error:
> 
> marcc$ ruby factorial7.rb
> enter a positive integer: 4
> factorial7.rb:14:in `<main>': undefined local variable or method `n' for
> main:Object (NameError)
> 

There's two issues here. The first is that you renamed `n` to `my_fact`, so it becomes undeclared as the error mentions. The other is that since fact is now part of the Factorial class, you need to change how fact() is called:

puts "factorial: #{my_fact.fact(n)}"

More interesting stuff to help you along the learning path. Right now in order to access your fact function, you have to create a new instance of the class. Instead you can utilize what is known as a singleton method. So the class can be rewritten as such:

class Factorial
  def Factorial.fact(n)
    if n == 0
      1
    else
      n * fact(n-1)
    end
  end
end

puts Factorial.fact(10)

Now we don't have to create a new instance just to call the method, and the method still remains encapsulated in the Factorial class for organizational purposes. Next up is to make the code a bit shorter:

class Factorial
  def Factorial.fact(n)
    (n == 0) ? 1 : n * fact(n-1)
  end
end

puts Factorial.fact(10)

This uses the ternary operator ( http://invisibleblocks.wordpress.com/2007/06/11/rubys-other-ternary-operator/ ) which has the format:

(condition to test) ? if condition is true run this : if condition is false run this

Finally for something even more advanced, you can use inject and kill the need for a recursive call all together:

class Factorial
  def Factorial.fact(n)
    (1..n).inject(1, :*)
  end
end

puts Factorial.fact(10)

More information on inject can be found here: http://www.ruby-doc.org/core/classes/Enumerable.html#M001494

I'd recommend reading it over and write some code that uses it. It is a very powerful tool!

Regards,
Chris White
Twitter: http://www.twitter.com/cwgem