Re: Variables[] in general expressions
- To: mathgroup at smc.vnet.net
- Subject: [mg20041] Re: Variables[] in general expressions
- From: "Allan Hayes" <hay at haystack.demon.co.uk>
- Date: Sat, 25 Sep 1999 18:46:06 -0400
- References: <7shptg$1r2@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
Ken Rice <kmr1004 at hermes.cam.ac.uk> wrote in message news:7shptg$1r2 at smc.vnet.net... > Hi there, > > Is there any way of extracting a list of variables from a general > expression, not just a polynomial? I want something which will do, > e.g. > > In AllVariables[y + Exp[x]] > Out {y,x} > > Apologies if I'm just being dim. > > Ken Rice > _____________________________________________________ > Kenneth Rice > Churchill College > Cambridge > CB3 0DS, UK > E-mail: kmr1004 at cam.ac.uk > > Ken, What is a variable? For mathematics, taking it to be a symbol that is not a head and is not a system constant, and allowing for some other exclusions, we might use AllVariables[expr_, exclusions_List:{}] := Complement[Cases[expr, _Symbol, {-1}, Heads -> True], Cases[expr, h_Symbol[___] -> h, Infinity, Heads -> True], {Catalan, Degree, E, EulerGamma, Glaisher, GoldenRatio, Khinchin, Pi}, exclusions ]This gives AllVariables[y + Exp[x]] {x, y} AllVariables[y + Exp[x], {x}] {y} Note that Exp[a] is converted to E^a on evaluation. Exp[a] // InputForm E^a However we might also want to allow for programming constructs like Function[x,Sin[x]], With[{x=a} Sin[x]] -- 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