Re: Problem with evaluation of delayed rules

*To*: mathgroup at smc.vnet.net*Subject*: [mg21758] Re: Problem with evaluation of delayed rules*From*: "Allan Hayes" <hay at haystack.demon.co.uk>*Date*: Thu, 27 Jan 2000 22:56:39 -0500 (EST)*References*: <85mkef$1og@smc.vnet.net> <86bkpr$b3o@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Eckhard, More thoughts Investigating the Trott-Strzebonski method for In-Place Evaluation (see Robby Villegas: http://library.wolfram.com/conferences/devconf99/villegas/UnevaluatedExpress ions.nb) Lead me to If[x > 0, Difference[1, 2], Difference[3, 4]] /. Difference[x_, y_] :> Block[{}, symbols[[x]] - symbols[[y]] /; True] If[x > 0, a - b, c - d] And tracing the evaluation of this shows that the main work is done by the internal function RuleCondition. If[x > 0, Difference[1, 2], Difference[3, 4]] /. Difference[x_, y_] :> RuleCondition[symbols[[x]] - symbols[[y]], True] If[x > 0, a - b, c - d] So it seems that, as with Hartmut Wolf's solution, we need to wrap the right side of the rules appropriately. I'll get back on the evaluation steps later. Allan --------------------- Allan Hayes Mathematica Training and Consulting Leicester UK www.haystack.demon.co.uk hay at haystack.demon.co.uk Voice: +44 (0)116 271 4198 Fax: +44 (0)870 164 0565 "Eckhard Hennig" <hennig at itwm.uni-kl.de> wrote in message news:86bkpr$b3o at smc.vnet.net... > Thanks to everyone who responded to my message regarding evaluation of > delayed rules in held expressions. Unfortunately, none of the proposed > solutions fully meets my requirements, either because Condition 1 (no > specific solution for "If") was not taken into account or because it doesn't > work for my particular application. What I posted was just a simple example, > and the actual problem is indeed a bit more complicated: > > Let "equations" denote a list of mathematical expressions involving "If" > statements (or other control statements with HoldXXX attribute). In > "equations", I want to apply the following substitutions. All delayed rules > must be evaluated as soon as they apply because the variable "mnavar" (a > list of symbols) is local to a Block. > > equations /. > { Derivative[_][_][x_ /; x =!= $TimeVariable] -> 0, > Derivative[k_][Voltage[a_, b_, _]][$TimeVariable] > | Derivative[k_][Voltage[a_, b_, _]] :> > D[mnavar[[a]] - mnavar[[b]], {$TimeVariable, k}], > Voltage[a_, b_, _] :> mnavar[[a]] - mnavar[[b]], > Derivative[k_][Current[__, i_]][$TimeVariable] > | Derivative[k_][Current[__, i_]] :> > D[BranchCurrentIdentifier[i], {$TimeVariable, k}], > Current[__, i_] :> BranchCurrentIdentifier[i] } > > Several responses to my original message suggested using a "Cases" statement > to build up a list of evaluated replacement rules dynamically and then apply > this list to the expression. This solution has two disadvantages: it needs > more memory and it does not work for more than one delayed rule at a time, > thus requiring repeated application of the "Cases" approach for each of the > above rules. In my application, these drawbacks are not acceptable because > the "equations" may be very large, so that memory efficiency and performance > is an important issue. > > Any other solutions that don't involve specific solutions for the "If" > statement or degrade performance? > > Best regards, > > Eckhard > > ----------------------------------------------------------- > Dipl.-Ing. Eckhard Hennig mailto:hennig at itwm.uni-kl.de > Institut fuer Techno- und Wirtschaftsmathematik e.V. (ITWM) > Erwin-Schroedinger-Strasse, 67663 Kaiserslautern, Germany > Voice: +49-(0)631-205-3126 Fax: +49-(0)631-205-4139 > http://www.itwm.uni-kl.de/as/employees/hennig.html > > ITWM - Makers of Analog Insydes for Mathematica > http://www.itwm.uni-kl.de/as/products/ai > ----------------------------------------------------------- > > > > >