Re: Newbie Question About An Error Message

*To*: mathgroup at smc.vnet.net*Subject*: [mg55454] Re: [mg55443] Newbie Question About An Error Message*From*: Bob Hanlon <hanlonr at cox.net>*Date*: Thu, 24 Mar 2005 03:41:53 -0500 (EST)*Reply-to*: hanlonr at cox.net*Sender*: owner-wri-mathgroup at wolfram.com

Your definition requires that x be a list of dimension 3. If it is not a list, then the parts don't exist and an error occurs. A more general definition is abs[x_]:=Sqrt[x.x]; abs[{x1,x2,x3}] Sqrt[x1^2 + x2^2 + x3^2] abs[{x1,x2}] Sqrt[x1^2 + x2^2] abs[{x1}] Sqrt[x1^2] abs[x] Sqrt[x . x] D[abs[{x-xi}]^2,x] 2 (x-xi) Bob Hanlon > > From: Adam Hartshorne <oracle3001 at yahoo.com> To: mathgroup at smc.vnet.net > Date: 2005/03/23 Wed AM 05:34:54 EST > To: mathgroup at smc.vnet.net > Subject: [mg55454] [mg55443] Newbie Question About An Error Message > > Hi All, > > I am a real newbie to mathematica and have just encountered the > following error message and I have no idea why and what it means. > > I have written a function to calculate the abs of a vector as follows > > abs[x_] := Sqrt[x[[1]]^2+x[[2]]^2+x[[3]]^2] > > then i was doing the following fine until a few moments ago and > mathematica starting complaining. I am to my knowledge not asking > anything different to what I asked it before, but I don't know enough > about mathematica to know much more. > > Ok then I try > > D[(abs[{x-xi}])^2,x] > > Which is know the answer to is 2(x-xi), and mathematica has given me the > answer. However now it is saying, maybe I have changed my input and no > am typing something wrong. > > Part::partw: Part 2 of {x - xi} does not exist. > > Part::partw: Part 3 of {x - xi} does not exist. > > Any help will be much appreciated, > > Adam > >