Re: Transformation rule exercise
- To: mathgroup at smc.vnet.net
- Subject: [mg4968] Re: [mg4898] Transformation rule exercise
- From: fransm at win.tue.nl (Frans Martens)
- Date: Thu, 17 Oct 1996 00:40:50 -0400
- Sender: owner-wri-mathgroup at wolfram.com
Robert Hall wrote Dear Abbey, This has baffled me for two days now. It's exercise 4 from p. 113 of Introduction to Programming with Mathematica, by Gaylord, Kamin & Wellin. The problem is to rewrite so that returns The authors provide the following information: "Hint: You need to maintain the lhs of the transformation rule unevaluated for purposes of pattern-matching and the rhs of the rule unevaluated until the rule is used." Unfortunately, both my solutions violate both clauses of the hint. g1[x_] := x /. Plus -> Times g2[x_] := x /. Plus[y___, z___] -> Times[y, z] I could replace Rule with RuleDelayed in either function (replace -> with :>) but it isn't necessary. The authors state that the lhs is evaluated before the rule is applied, and Trace[] bears this out. I'm looking for a solution to which the authors' hint applies, but for the life of me I can't find it. Am I overlooking something obvious? Baffled near Baltimore -- Bob Hall | "Know thyself? Absurd direction! rhall2 at gl.umbc.edu | Bubbles bear no introspection." -Khushhal Khan Khatak A solution is g[x_] := (x /. Literal[Plus[z___]] :> Times[z]) Frans Martens Eindhoven University of Technology The Netherlands