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MathGroup Archive 1997

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Re: Replace expressions

  • To: mathgroup at smc.vnet.net
  • Subject: [mg7688] Re: Replace expressions
  • From: Stephen P Luttrell <luttrell at signal.dra.hmg.gb>
  • Date: Sun, 29 Jun 1997 22:17:23 -0400 (EDT)
  • Organization: Defence Research Agency
  • Sender: owner-wri-mathgroup at wolfram.com

Hans Steffani wrote:
> 
> hans.steffani at e-technik.tu-chemnitz.de (Hans Steffani) writes:
> 
> >I want to replace all expressions
> >Dot[a, xx]
> >Dot[xx, a]
> >for certain as by
> >Times[a,xx].
> 
> To say it in other words
> I have a list certainas, which contains a number of symbols.
> 
> This symbols are scalars and therefore I am allowed to substitute
> Dot[a,b] by Times[a,b] if a or b is in the list certainas.
> 
> How can this be done.
> 

Here is an example of how to solve this problem:

(1) Define a pair of assignments for simplifying expressions
of the form Dot[a_,xx] and Dot[xx,a_], provided that a is a member
of the list certainas (^:= is used rather than := in order to
associate the assignment with xx rather than Dot; this is a
matter of taste):

Dot[a_,xx]^:=a xx/;MemberQ[certainas,a];
Dot[xx,a_]^:=a xx/;MemberQ[certainas,a];

(2) Define an example list to work with:

certainas={a1,a2,a3};

(3) See whether the substitutions are made correctly in some specific
cases.

The following input:

Dot[a1,xx]

Dot[xx,a2]

Dot[b,xx]

Dot[xx,b]

produces the following output:

a1 xx

a2 xx

b.xx

xx.b

which is what was expected, given the chosen elements of certainas.

=====================================================================

Dr Stephen P Luttrell                  luttrell at signal.dra.hmg.gb
Adaptive Systems Theory                01684-894046 (phone)
Room EX21, Defence Research Agency     01684-894384 (fax)           
Malvern, Worcs, WR14 3PS, U.K.        
http://www.dra.hmg.gb/cis5pip/Welcome.html


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