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

• To: mathgroup at smc.vnet.net
• Subject: [mg50838] Re: Telling Mathematica that a symbol is going to be a List?
• From: p-valko at tamu.edu (Peter Valko)
• Date: Thu, 23 Sep 2004 05:27:19 -0400 (EDT)
• References: <ciqvop\$in9\$1@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

```The error is happening, because an element of the list is referred to
before the list turns out to be a list.

To change the order of happenings, I would suggest:

m[d_]:= { {d[[1]], 0, 0}, {0, d[[2]], 0}, {0, 0, d[[3]]}  }
m[d/. d -> {d1, d2, d3} ]  // MatrixForm

Then the symbol d turns out to be a list of {d1,d2,d3} before it is
used in the evaluation of m and there will be no error message.

Peter

AES/newspost <siegman at stanford.edu> wrote in message news:<ciqvop\$in9\$1 at smc.vnet.net>...
> 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.

```

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