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MathGroup Archive 2004

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Re: Telling Mathematica that a symbol is going to be a List?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg50822] Re: Telling Mathematica that a symbol is going to be a List?
  • From: Paul Abbott <paul at physics.uwa.edu.au>
  • Date: Wed, 22 Sep 2004 04:52:00 -0400 (EDT)
  • Organization: The University of Western Australia
  • References: <ciqvop$in9$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

In article <ciqvop$in9$1 at smc.vnet.net>,
 AES/newspost <siegman at stanford.edu> wrote:

> I want to define a function containing parameters that are going to be 
> elements in a list, e.g. something like
> 
>   m := { { d[[1]], 0, 0}, {0, d[[2]], 0}, {0, 0, d[[3]]}
> 
> but not define the list of values of  d  until later (and not put the 
> parameters in as explicit arguments of  m  just to keep the appearance 
> less cluttered and typing easier).
> 
> If I then give an input line
> 
>     m /. {d -> {d1, d2, d3} } // MatrixForm
> 
> I get "Part::partd" errors -- but the matrix then displays correctly.
> 
> I can of course just Off[] the errors -- but is there a better way.

Since m is a _function_ containing parameters that are going to be 
elements in a list, why not write

  m[d_] := {{d[[1]], 0, 0}, {0, d[[2]], 0}, {0, 0, d[[3]]}}

and then use

  m[{d1, d2, d3}]

For this particular example,

  m[d_] := DiagonalMatrix[d]

is simpler.

Cheers,
Paul

-- 
Paul Abbott                                   Phone: +61 8 6488 2734
School of Physics, M013                         Fax: +61 8 6488 1014
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