Re: possible bug in Mathematica?
- To: mathgroup at smc.vnet.net
- Subject: [mg25982] Re: possible bug in Mathematica?
- From: adam_smith at my-deja.com
- Date: Wed, 15 Nov 2000 02:09:36 -0500 (EST)
- References: <8ur0ag$qsq@smc.vnet.net>
- Sender: owner-wri-mathgroup at wolfram.com
This is not a bug. You have the syntax wrong. For a matrix that
is "m" rows by "n" columns you specify the element with:
matrix[[row,column]
I have included an example. Note that this format extends to more
that "2-dimensional" matrices.
Adam Smith
In[1]:=
vector = Table[i, {i, 1, 3}]
Out[1]=
{1, 2, 3}
In[2]:=
matrix = Table[i*j, {i, 1, 2}, {j, 1, 3}]
Out[2]=
{{1, 2, 3}, {2, 4, 6}}
In[3]:=
matrix[[1, 3]]
Out[3]=
3
In[4]:=
matrix[[2, 2]]
Out[4]=
4
In article <8ur0ag$qsq at smc.vnet.net>,
Peter Joseph <joseph at oasis.rad.upenn.edu> wrote:
>
> 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
>
>
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