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Re: how to find n in expression x^n using a pattern?
*To*: mathgroup at smc.vnet.net
*Subject*: [mg58462] Re: [mg58426] how to find n in expression x^n using a pattern?
*From*: "David Park" <djmp at earthlink.net>
*Date*: Sun, 3 Jul 2005 03:57:17 -0400 (EDT)
*Sender*: owner-wri-mathgroup at wolfram.com
Steve,
You will probably get a better answer, but I wouldn't use a pattern. I would
use CoefficientList and the Position of the first nonzero entry. You don't
really have to Factor.
expr1 = x^5(x^2 + 2)(x^3 + 4);
expr2 = (x^2 + 2)(x^3 + 4);
minexponent[poly_, var_:x] :=
First@First@Position[CoefficientList[poly, var], _?(# != 0 &), {1}, 1] - 1
minexponent[expr1]
5
minexponent[expr2]
0
David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/
From: steve [mailto:nma124 at hotmail.com]
To: mathgroup at smc.vnet.net
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
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