[Date Index]
[Thread Index]
[Author Index]
Re: Pattern matching problem
*To*: mathgroup at smc.vnet.net
*Subject*: [mg88538] Re: Pattern matching problem
*From*: dh <dh at metrohm.ch>
*Date*: Thu, 8 May 2008 04:11:17 -0400 (EDT)
*References*: <fvs2kf$eo4$1@smc.vnet.net>
Hi Charlie,
use ReplaceAll (/.) instead of Count. ReplaceAll treats any piece at
most once. Here is an example, where "expression" is your expression. We
prevent any evaluation of the found multiplicity by my[..] and use Total
to add all arguments to my:
t=expression/. D[u[x],{x,i_}]^k_.:>my[i k]
Cases[t,my[x_]:>x,Infinity]//Total
hope this helps, Daniel
Charlie Brummitt wrote:
> Hi all,
> Here is my problem: Given a polynomial in the variables u[x,t] and its
> spatial derivatives (for example, the polynomial could be 1 + u +
> u_xx*u_xxx^2), count the number of spatial derivatives with multiplicity.
> That is, after writing the above polynomial as
>
>
> 1 + u + u_xx * u_xxx * u_xxx
>
> the output should be 2 + 3 + 3 (basically, you count the number of x's).
>
> I have tried implementing this using a pattern matching approach. Here is
> what I tried:
>
> f[equation_ ] := Sum[ k * j * Count[ equation , D[ u[x, t], {x, j} ] ^ k ,
> {0, \infinity} ], {j, 1, 50}, {k, 1, 50}]
>
> This fails to work on, for example, the input u_xx^2, because it outputs 6
> when it should output 4. This is because the u_xx is counted (+2 to the
> sum), and the u_xx^2 is counted (+4 to the sum). This is because the u_xx is
> nested inside the Power[ , 2] in its representation in Mathematica and so
> it gets counted too many times in my formula. I can't seem to figure out a
> way to use the "provided that" operator /; to make this formula work.
>
> I've also tried doing some replacement methods, but to no success.
>
> Thanks for any help you may be able to provide.
>
> -Charlie
>
>
Prev by Date:
**Re: Intersection of surfaces**
Next by Date:
**Re: Trouble computing conjugates**
Previous by thread:
**Re: Pattern matching problem**
Next by thread:
**Re: Pattern matching problem**
| |