From: Robert Klemme <shortcutter@...>
Date: 2010-01-04T02:25:08+09:00
Subject: Re: Few clarifications on recursion

On 01/03/2010 05:52 PM, Phillip Gawlowski wrote:
> 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.

Actually that description is true for loops (aka iterations) as well. 
:-)  The important bit of recursion - and the part where recursion and 
iteration differ - in programming languages is, that it needs a 
_function (or method)_ which calls _itself_.  Most of the time it 
happens directly (as in this example) but it may as well happen 
indirectly (f calls g, g calls f) although I cannot think of a 
reasonable example right now.  (Most indirect recursions are probably 
programming errors which you notice when a SystemStackError pops up. :-))

Certain forms of recursion can actually be transformed into iterations 
(which some programming languages / runtimes actually do).  That way you 
usually get better performance.  See 
http://en.wikipedia.org/wiki/Tail_recursion

Kind regards

	robert

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