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

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  • Subject: [mg50920] Re: [mg50842] Re: Telling Mathematica that a symbol is going to be a List?
  • From: Ichigaku Takigawa <1gac at>
  • Date: Tue, 28 Sep 2004 00:58:29 -0400 (EDT)
  • Organization: Hokkaido University
  • References: <ciqvop$in9$> <>
  • Sender: owner-wri-mathgroup at


Your problem seems to be in the "Part" function like "array[[i]]".
In mathematica, like a[[3]], there are immediately (and internally) 
evaluated expressions when it is called. The following note might
help you, especially "An Introduction to demystify scary things" part:

"Working with Unevaluated Expressions" by R. Villegas

It's my understanding that error messages in mathematica sometimes
will mean just a warning, so if your logic is correct, the result
will often work well though mathematica will display an message
(in this case, indeed you can just use "Off" function to suppress
the corresponding message.)

In your case, when evaluating "m", mathematica will think that the length
of "d" is undefined and just wonder if "d" object has the i-th element
required for an access to array elements. For example, simply by typing

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

you will get the same error message (this is a evaluating stage before
applying "/." rules when "m" called).

Moreover, you can also use the following expressions to do what you want
instead of what was previously mentioned in this list
(although I recommend that you define it as a function like
"m[d_List] := DiagonalMatrix[d]" and then use it as "m[{1,2,3}]".)

  m1 := {{d[[1]], 0, 0}, {0, d[[2]], 0}, {0, 0, d[[3]]}}
  Block[{d = {1,2,3}}, m1]

for local binding of variables, or something like

  m2 := {{d[1], 0, 0}, {0, d[2], 0}, {0, 0, d[3]}}
  m2 /. d -> {1, 2, 3} /. l_[i_] :> l[[i]]

(I know the second one seems to be a little bit tricky,
but this might show a example of "Part" evaluating problem)

At Thu, 23 Sep 2004 05:27:26 -0400 (EDT),
AES/newspost wrote:
> But the alternative fragment
>     m := {d[[1], 2 d[[2], 3d[[3]]}
>     s = m /. {d->{1,2,3}}
> which my mental model, augmented by a model of how "->" works, says 
> should be OK also.  Instead this fragment puts up error messages -- and 
> then, ignoring its own trouble messages, goes ahead and works OK anyway!!

    Ichigaku Takigawa

    Laboratory for Pattern Recognition and Machine Learning
    Research Group of Mathematical Information Science
    Division of Computer Science
    Graduate School of Information Science and Technology
    Hokkaido University
    Kita 14, Nishi 9, Kita-ku, Sapporo 060-0814, Japan
    tel:   +81-11-706-6854
    email: 1gac at

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