RE: possible bug in Mathematica?
- To: mathgroup at smc.vnet.net
- Subject: [mg25980] RE: [mg25967] possible bug in Mathematica?
- From: "David Park" <djmp at earthlink.net>
- Date: Wed, 15 Nov 2000 02:09:35 -0500 (EST)
- Sender: owner-wri-mathgroup at wolfram.com
Peter,
The Mathematica documentation is a bit misleading under Part. You have to
put both indices within one set of brackets so that you have one part and
not a part of a part.
matrix = Table[0, {2}, {3}]
{{0, 0, 0}, {0, 0, 0}}
matrix[[1,1]] = 11;
matrix[[1,2]] = 12;
matrix
{{11, 12, 0}, {0, 0, 0}}
You could use this method to set a number of elements in one row at once.
The parentheses around the Set statement are necessary.
MapThread[(matrix[[2,#1]] = #2) & ,
{{1, 2, 3}, {5, 6, 7}}];
matrix
{{11, 12, 0}, {5, 6, 7}}
David Park
djmp at earthlink.net
http://home.earthlink.net/~djmp/
> -----Original Message-----
> From: Peter Joseph [mailto:joseph at oasis.rad.upenn.edu]
To: mathgroup at smc.vnet.net
>
> I am running Mathematica version 4.0 under Windows 98 2nd edition.
>
> I only recently discovered your newsgroup on technical questions about
> Mathematica. I am not sure if you consider the following problem to be a
> question or a bug report. I have already submitted this to Wolfram as a
> bug report.
>
>
> (* The problem is how to redefine or recalculate the elments of a list
> with more than one dimension, such as a matrix
> First, demonstrate that there is no such problem with a simple list *)
>
> In[8]:= vector = Table[0, {2}]
> Out[8]= {0, 0}
>
> (* next redefine the values in the vector *)
>
> In[9]:=vector[[1]] = 1
> Out[9]=1
>
> In[10]:=vector[[2]] = 2
> Out[10]=2
>
> In[11]:=vector
> Out[11]={1, 2}
>
> (* That was successful, now try exactly the same technique with a matrix
> *)
>
> In[12]:=matrix = Table[0, {2}, {3}]
> Out[12]={{0, 0, 0}, {0, 0, 0}}
>
> In[13]:=matrix[[1]][[1]] = 11
> Set::"setps": "\!\(matrix \[LeftDoubleBracket] 1 \[RightDoubleBracket]\)
> in \
> assignment of part is not a symbol."
> Out[13]=11
>
> In[14]:=matrix
> Out[14]={{0, 0, 0}, {0, 0, 0}}
>
> (* evidently, Mathematica interprets the meaning of vector[[1]] very
> differently than matrix[[1]][[1]]
> It considers the vector elements to be variables that can be redefined,
> while it considers the matrix elements constants that are
> protected. I tried using Unprotect in various forms, but that did not
> work.
>
> So the question is, how does one do computations on lists which are nested
> more than one level deep?? *)
>
> Peter M. Joseph, Ph.D.
> Professor of Radiologic Physics in Radiology
> Hospital of the University of Pennsylvania
> Philadelphia, PA, 19104-4283
> Telephone 215-662-6679
> email joseph at rad.upenn.edu
>