Re: Newbie Question About An Error Message

*To*: mathgroup at smc.vnet.net*Subject*: [mg55451] Re: Newbie Question About An Error Message*From*: dh <dh at metrohm.ch>*Date*: Thu, 24 Mar 2005 03:41:50 -0500 (EST)*References*: <d1ri0i$79c$1@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Hi Adam, in abs[x_] := Sqrt[x[[1]]^2+x[[2]]^2+x[[3]]^2] you request that x is a vector (List) by using its components x[[i]]. A saver way to do this is by defining abs[x_VectorQ]. Later on you feed abs[] with a List {x-xi} that has only one component, namley x-xi. Therefore Mathematica complains that there are no other cmponents. Presumably you wanted to use () and NOT {} Sincerely, Daniel Adam Hartshorne wrote: > 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 >