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MathGroup Archive 2000

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Re: Defining an UpValue for a specific List?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg23203] Re: [mg23153] Defining an UpValue for a specific List?
  • From: Bojan Bistrovic <bojanb at python.physics.odu.edu>
  • Date: Mon, 24 Apr 2000 01:12:09 -0400 (EDT)
  • Sender: owner-wri-mathgroup at wolfram.com

> Hello, 
> 
> I would like to make the following definition:
> 
> df[[a_, b_]] ^:= DFPart[df, a_, b_]
>  
> where df is a list of three components.
> 
> I get the folowing error:
> 
> UpSetDelayed::write: Tag List in {<<l>>}[[a_,b_]] is Protected.
> 
> Is there a possibility to redefine Part[] for some symbols?
> 
> Please email directly to
>         schnoerr at mailzone.com
> 
> Thank you very much!
> 
> Claudius
> 
Here's a different approach: define the funcion "MyPart" with properties
you like, and then use MakeExpression to reinterpret the sequence
df[[a,b]] as MyPart[df,a,b]:

In[1]:= MakeExpression[ RowBox[{aaa_,"[",RowBox[{"[",bbb_String,"]"}],"]"}],
    StandardForm]:=
  MakeExpression[ RowBox[{"MyPart","[",aaa,",",bbb,"]"}],StandardForm]

In[2]:= MakeExpression[ RowBox[{aaa_,"[",RowBox[{"[",
	RowBox[{bbb___}],"]"}],"]"}],
    StandardForm]:=
  MakeExpression[ RowBox[{"MyPart","[",aaa,",",bbb,"]"}],StandardForm]

It's not a general solution to the problem since it will only work if 
you're using Graphical Front End, but it might be enough.

Bojan

--
---------------------------------------------------------------------
Bojan Bistrovic,                                      bojanb at jlab.org
Old Dominion University, Norfolk VA & Jefferson Lab, Newport News, VA
---------------------------------------------------------------------


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