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MathGroup Archive 2005

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Re: Newbie Question About An Error Message

  • To: mathgroup at smc.vnet.net
  • Subject: [mg55456] Re: [mg55443] Newbie Question About An Error Message
  • From: Sseziwa Mukasa <mukasa at jeol.com>
  • Date: Thu, 24 Mar 2005 03:41:55 -0500 (EST)
  • References: <200503231034.FAA04057@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

On Mar 23, 2005, at 5:34 AM, 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]

To make answering your question easier I'm going to interrupt here and 
comment on this function.  You have written a function abs which takes 
one argument that should have at least three parts.

> Ok then I try
>
> D[(abs[{x-xi}])^2,x]

You have called your function abs with the argument {x-xi}, assuming x 
and xi have not otherwise been defined this is a list with only one 
part.

> 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.

I think these messages should make sense now.

> Any help will be much appreciated,

I hope this helped.

If you redefine your abs function as abs[x_]:=Sqrt[Plus@@#^2&/@x] then 
you will no longer be making assumptions about the length of x and your 
D[___] expression will work as you expected.

Regards,

Ssezi


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