From: Phillip Gawlowski <pg@...>
Date: 2010-01-04T01:52:23+09:00
Subject: Re: Few clarifications on recursion

On 03.01.2010 17:16, Panagiotis Atmatzidis wrote:

> Although it introduces many interesting concepts to me (i. e. I did not knew that after the "if" statement a single _1_ with no quotes could be used) I fail to see how the num changes value. The way I read the code here:
>
>    num * factorial(num-1)
>
> is 30*29 for "puts factorial(30)" not 30*29*28... which is how it works. I've tested the code works fine.
>
> The book although, in my opinion is excellent (so far) for starters, does not explain - or it does but I did not understand it nevertheless - how this piece of code works.

In a nutshell, recursion means performing the same action until a 
condition is met.

In your case, factorial does "num * (num - 1)" until num is 1 (or 
smaller than one). It then gives you the result.

You can test this with:

def factorial num
   if num < 0
    puts "You can't take the factoria of a negative number"
   end

   puts "num is: #{num}" # this prints our the value of num before each
		 	# recursion

   if num <= 1
      1
   else
     num * factorial(num-1)
   end

end

puts factorial 30

I hope that clears up your confusing about recursion? (If that *was* 
where you were confused..)

--
Phillip Gawlowski