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

> 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,
>