# [Pharo-project] Integer>>#factorial

Lorenzo Schiavina lorenzo at edor.it
Thu Jun 16 13:46:34 EDT 2011

```IMHO factorial is a good OO example because you have not the usual limitation of "traditional" languages and you can understand the OOP power.

Try, for example 1000 factorial in ST and in other languages.

Cheers

Lorenzo
----- Original Message -----
From: Hernan Wilkinson
To: Pharo-project at lists.gforge.inria.fr
Sent: Thursday, June 16, 2011 5:03 PM
Subject: Re: [Pharo-project] Integer>>#factorial

I agree with Alex, but I don't see why, alex, you say it is a good example for teaching OO (the smalltalk factorial implementation...)
For me is a good example of a recursive "function"
I think it would be a good OO example it it was using "object recursion" or the one that could be implemented in a prototyped language... why do you think it is a good oo example the smalltalk factorial implementation?

On Thu, Jun 16, 2011 at 9:52 AM, Alexandre Bergel <alexandre.bergel at me.com> wrote:

Hi Sven,

I am not sure it should replace the standard implementation. Integer>>factorial is essentially there for teaching purpose in my opinion. I am not sure how often factorial is used in practice. The most important for me is not it to be fast, but to be an excellent example of OOP.

Your implementation could be next to the standard one though.

Cheers,
Alexandre

On 16 Jun 2011, at 06:38, Sven Van Caekenberghe wrote:

> Hi,
>
>
> I had some old Lisp code hanging around, implementing an algorithm that I once found somewhere, and I ported it to Smalltalk. I am sure there are faster solutions, but this one is twice as fast as the original while still being easy to understand and it has a useful, safe helper method:
>
> Integer>>#svcFactorial
>
>       self = 0 ifTrue: [ ^ 1 ].
>       self > 0 ifTrue: [ ^ 1 productUpTo: self ].
>       self error: 'Not valid for negative integers'
>
> Integer>>#productUpTo: anInteger
>       "Answer the product of all integers from the receiver (non-inclusive) up to anInteger (inclusive)."
>
>       | difference split |
>       self assert: (self between: 0 and: anInteger).
>       "the idea is to multiply LargePositiveIntegers of approximately the same size"
>       difference := anInteger - self.
>       difference = 0 ifTrue: [ ^ 1 ].
>       difference = 1 ifTrue: [ ^ anInteger ].
>       difference = 2 ifTrue: [ ^ (anInteger - 1) * anInteger ].
>       difference = 3 ifTrue: [ ^ (anInteger - 2) * (anInteger - 1) * anInteger ].
>       difference = 4 ifTrue: [ ^ (anInteger - 3) * (anInteger - 2) * (anInteger - 1) * anInteger ].
>       split := (self + anInteger) bitShift: -1.
>       ^ (self productUpTo: split) * (split productUpTo: anInteger)
>
> Would it be a good idea to replace the current implementation with this one ?
>
> Sven
>
>

--
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Alexandre Bergel  http://www.bergel.eu
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Hernán Wilkinson
Agile Software Development, Teaching & Coaching
Mobile: +54 - 911 - 4470 - 7207
email: hernan.wilkinson at 10Pines.com
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