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MathGroup Archive 2005

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Re: Types in Mathematica, a practical example

  • To: mathgroup at smc.vnet.net
  • Subject: [mg62946] Re: [mg62800] Types in Mathematica, a practical example
  • From: Sseziwa Mukasa <mukasa at jeol.com>
  • Date: Fri, 9 Dec 2005 05:10:27 -0500 (EST)
  • References: <000f01c5fbf8$e3979330$996f1081@fy.chalmers.se>
  • Sender: owner-wri-mathgroup at wolfram.com

On Dec 8, 2005, at 8:11 AM, Ingolf Dahl wrote:

> Thanks for all answers.
> In my previous submission I asked for a way to define/declare list  
> with
> undefined elements.

[deleted]

>
> What I would prefer is a new Mathematica word: "Undefined". Then I  
> could
> define a list x as
>
> x = {1,Undefined,2};
>
> x[[1]] should evaluate to 1 as usual, but for this word "Undefined"  
> the list
> part x[[2]] should evaluate to x[[2]], precisely as for an undefined
> unindexed symbol.
> x[[4]] should give the usual error message:
>
> "Part::partw: Part 4 of {1, Undefined, 2} does not exist."
>
> x[[3]] =. and Clear[x[[3]]] should redefine x as  
> {1,Undefined,Undefined},

You can already do this:

In[111]:=
myclear[x_[[i_]]]:=x[[i]]=undefined
SetAttributes[myclear,HoldFirst]
x={a,b,c};
myclear[x[[2]]]
x

Out[114]=
undefined

Out[115]=
{a,undefined,c}


> and x = Undefined should be interpreted as x =.

This would be problematic because x=Undefined assigns a value to x,  
x=. also assigns x a value but it's a different value, anything that  
depended on that behavior would break.

In the sense of a general name for an undefined term, some languages  
offer a value called bottom, that would be an interesting addition to  
Mathematica but as far as I can tell it's not a common feature of  
LISP like languages.

Regards,

Ssezi



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