[ruby-talk:03597] Re: Another small quiz

>From: Aleksi Niemel<[e-mail]>
>Reply-To: [e-mail]
>To: [e-mail] (ruby-talk ML)
>Subject: [ruby-talk:03595] RE:  Another small quiz
>Date: Wed, 21 Jun 2000 23:31:32 +0200
>
>Dat:
> > The following appeared in chapter 3 of the Ruby's User Guide:
> >
> > def fact(n)
> > 	if n == 0
> > 		1
> > 	else
> > 		n * fact(n-1)
> > 	end
> > end
> >
> > Is everything OK here?
>
>I can't see any problem here. We're calculating factorial in a normal
>imperative approach using recursion (which eats stack).
>
>Just don't call fact(-1) or with any negative argument (will eat you stack
>faster than you think, but *don't crash*, damn I like Ruby).
>
>Oh, and if you think why the code works, you can think it to be
>
>def fact(n)
>	if n == 0
>		return 1
>	else
>		return n * fact(n-1)
>	end
>	# puts n
>end
>
>The difference between the chapter 3 example and this code is that the
>latter will work even if you remove the comment character (but it won't
>print anything, if you happened to think so) whereas the chap 3 version
>won't.
>
>To amaze a little bit more (at least I was amazed with this), one can
>rewrite:
>
>def fact(n)
>	ret =
>		if n == 0
>			1
>		else
>			n * fact(n-1)
>		end
>	puts n
>	ret
>end
>
>Now we assign to ret the value of the whole if-else-end construct, which
>happens to be 1 or n*fact(n-1) depending of the if condition, then we print
>n and finally return the value of ret.
>
>If one doesn't say explicitly "return some_var" the return value of the
>function is the last evaluated value, that is the value of the last 
>executed
>code.
>
>	- Aleksi
>

Nothing is wrong.

When I watched fact with bigger arguments, the recursion gets slower and 
slower, so I wonder if recursion is fine in interpretative environment.

I cannot recall who advised to replace recursion with iteration wherever 
possible; even though it is claimed that recursion is natural but 
complicated recursion is not easy to grasp.

I prefer:
def fact(n)
	if n < 0
		raise "argument negative"
	end
	fact = 1
	for i in 2..n
		fact *= i
	end
	fact
end
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