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
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