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Re: Newbie Question About An Error Message


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
> 


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