From: Ken Bloom <kbloom@...>
Date: 2008-05-03T04:40:06+09:00
Subject: Re: [QUIZ] Reverse Divisible Numbers (#161)

On Fri, 02 May 2008 11:44:21 -0500, Rob Biedenharn wrote:

> On May 2, 2008, at 12:10 PM, Ken Bloom wrote:
> 
>> On Fri, 02 May 2008 10:28:00 -0500, Matthew Moss wrote:
>>> Quiz #161
>>> Reverse Divisible Numbers
>>>
>>> This week's quiz is borrowed from Jon Galloway
>>> (http://tinyurl.com/5jvf37).
>>> Don't read the comments or solution there without trying this first!
>>>
>>> Your task is to write a bit of Ruby code that will find and report all
>>> integers that are divisible by their reverse. For example, 9801 is
>>> divisible by 1089.
>>>
>>> Your script should accept a single, optional argument to specify the
>>> upper
>>> limit of your search. If not provided, the default should be one
>>> million.
>>>
>>> There are two extra rules for finding these "reverse divisible"
>>> numbers:
>>>
>>> 1. Don't count palindromes; they are obviously reverse-divisible. 2.
>>> Don't count numbers ending with zero. Reversing such numbers forces
>>> you
>>>   to drop leading zeros on the divisor, and that's not as much fun.
>>
>> Here's what I've found:
>>
>> 1089 * 9 = 9801
>> 2178 * 4 = 8712
>> 10989 * 9 = 98901
>> 21978 * 4 = 87912
>> 109989 * 9 = 989901
>> 219978 * 4 = 879912
>>
>> real	0m55.234s
>> user	0m53.995s
>> sys	0m0.012s
>>
>> --Ken
>>
>> --
>> Ken (Chanoch) Bloom. PhD candidate. Linguistic Cognition Laboratory.
>> Department of Computer Science. Illinois Institute of Technology.
>> http://www.iit.edu/~kbloom1/
> 
> 2178 * 4 = 8712
> 1089 * 9 = 9801
> 21978 * 4 = 87912
> 10989 * 9 = 98901
> 219978 * 4 = 879912
> 109989 * 9 = 989901
> 
> real	0m13.113s
> user	0m12.411s
> sys	0m0.081s
> 
> While I find them in a slightly different order, I'm surprised by the 4x
> speedup.  I wonder if there's a better way to post comparisons that
> would negate machine differences and focus on algorithmic distinctions
> without posting code?

I did it in a *really* stupid way for my first pass. That's all.

--Ken

-- 
Ken (Chanoch) Bloom. PhD candidate. Linguistic Cognition Laboratory.
Department of Computer Science. Illinois Institute of Technology.
http://www.iit.edu/~kbloom1/