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MathGroup Archive 2005

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Re: Re: how to find n in expression x^n using a pattern?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg58512] Re: [mg58487] Re: [mg58426] how to find n in expression x^n using a pattern?
  • From: Andrzej Kozlowski <akozlowski at gmail.com>
  • Date: Tue, 5 Jul 2005 01:57:51 -0400 (EDT)
  • References: <200507020806.EAA01589@smc.vnet.net> <200507040624.CAA05758@smc.vnet.net> <26A87A3E-2A81-483F-B880-FD4E30020592@gmail.com> <42C956F0.6090509@umbc.edu>
  • Sender: owner-wri-mathgroup at wolfram.com

On 5 Jul 2005, at 00:34, Pratik Desai wrote:

> Andrzej Kozlowski wrote:
>
>
>>
>> On 4 Jul 2005, at 15:24, Pratik Desai wrote:
>>
>>
>>> steve wrote:
>>>
>>>
>>>
>>>> I am learning patterns in Mathemtica, and I thought this will  
>>>> be  easy.
>>>>
>>>> suppose you are given a factored expression, such as x^n(x^2+2) 
>>>> (x^3 +4)
>>>> where 'n' above can be any number, including zero (i.e. there is no
>>>> (x) as a separate factor). I need to find 'n', which can be
>>>> 0,1,2,3,..etc..
>>>>
>>>> i.e I need to find the power of the factor x by itself if it  
>>>> exist in
>>>> the expression. not (x^n+anything), just (x^n) byitself.
>>>>
>>>> For example
>>>>
>>>> p= x (x^2+2)  ---> here n=1
>>>>
>>>> I tried this
>>>>
>>>> p /. x^n_(any_) -> n
>>>>
>>>> This did not work since x^1 does not match pattern x^n_.  I  
>>>> would  have
>>>> worked if I had x^2 (x^3+2) for example.
>>>>
>>>> I know that I need to use something like  x^_:1  to make it  
>>>> match x,
>>>> which
>>>> is really x^1, but I do not know how to use it in a replacement  
>>>> rule,
>>>> becuase
>>>> when I write
>>>>
>>>> p /. (x^n_:1)(any_) -> n
>>>>
>>>> it is an error.
>>>>
>>>> If I write
>>>>
>>>> p /. (x^_:1)(any_) -> _
>>>>
>>>> I do not get back anything.
>>>>
>>>> Next I tried the Cases command, and again had no luck.
>>>>
>>>> The main problem is that Mathematica makes a difference between
>>>> x, x^1, and x^2, etc... when it comes to matching with pattern x^n_
>>>>
>>>> any idea how to do this?
>>>> thanks,
>>>> steve
>>>>
>>>>
>>>>
>>>>
>>>>
>>> Why not do something like this , it sounds quite simplistic but   
>>> it  works :)
>>> Clear[p]
>>> p[x_] = x ^5(x^3 + 2x^2 + 3x - 13)
>>> sol2 = Solve[p[x] == 0, x]
>>> y = x /. sol2
>>> n = Count[y, 0]
>>>
>>> Best regards
>>>
>>> -- 
>>> Pratik Desai
>>> Graduate Student
>>> UMBC
>>> Department of Mechanical Engineering
>>> Phone: 410 455 8134
>>>
>>>
>>>
>>>
>>
>> In fact after seeing this posting I understood what the original   
>> poster wanted (probably). (He wrote that he was learning patterns  
>> so  I thought this was meant just to be an exercise in pattern  
>> matching).  However, using algebraic method, like the one above,  
>> could be  unnecessarily complicated if the second factor of the  
>> equation is a  high degree polynomial or something even more  
>> complicated. It is in  fact easier to use patterns. Compare:
>>
>> p[x_] = x^5*(x^32 + 2x^21 + 3x^17 - 13);
>>
>>
>> Timing[Cases[p[x], x^(n_.) -> n, {1}]]
>>
>>
>> {0.0005019999999973379*Second, {5}}
>>
>> (as I wrote in my first response, giving the correct level   
>> specification is important).
>>
>>
>> Timing[Count[x /. Solve[p[x] == 0, x], 0]]
>>
>>
>> {0.08699899999999872*Second, 5}
>>
>>
>>
>> Andrzej Kozlowski
>>
>> Chiba, Japan
>>
>>
>>
> Hi Andrzej
>
> I have had occasion to use patterns before and by far I have found  
> it the most difficult feature of mathematica  to work with and the  
> help on it  leaves a lot to be desired. I agree the poster seems to  
> be trying to learn patterns, I was just trying to point out, as you  
> have on your earlier reply,  that there are easier ways to do the  
> same job (i.e finding the highest common exponent in x). Again not  
> to put a finer point on it, it seems to me that the use of  
> patterning as you have shown seems only to work when the expression  
> is in its factored form (which I admit was the posters original  
> query)  whereas using Solve and count works irrespective of the  
> form of the expression. Or am I wrong?

You are of course right. Mathematica's pattern matching is (with some  
rudimentary exceptions) only  syntactic, which means that expressions  
are matched "literally" and not according to their "mathematical  
meaning" (that would be "semantic" pattern matching). A factored  
expression and an unfactored oen are quite different as far as  
pattern matching is concerned. Algebraic methods can do much more  
than pattern matching but they tend to be very slow and suitable  
mostly for "small" problems.  So both approaches have their strong  
and weak points and one or the other should be preferred depending on  
what one is trying to do. Obviously there is no panacea for all types  
of situations, which is why posters should try to state exactly what  
they are really trying to do.

Andrzej Kozlowski





>
> p[x_] = x^5*(x^32 + 2x^21 + 3x^17 - 13x^5);
> Timing[Cases[p[x], x^(n_.) -> n, {1}]]
> >>{0. Second, {5}}
>
> Timing[Count[x /. Solve[p[x] == 0, x], 0]]
> >>{0.01 Second, 10}
>
> Now if you use simplify
> p[x_] = x^5*(x^32 + 2x^21 + 3x^17 - 13x^5) // Simplify;
> Timing[Cases[p[x], x^(n_.) -> n, {1}]]
> {0. Second, {10}}
>
> Best regards
> Pratik
>
> -- 
> Pratik Desai
> Graduate Student
> UMBC
> Department of Mechanical Engineering
> Phone: 410 455 8134
>
>
>


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