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MathGroup Archive 1995

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Re: Problems with high-dimensional lists

  • To: mathgroup at smc.vnet.net
  • Subject: [mg2526] Re: Problems with high-dimensional lists
  • From: withoff (David Withoff)
  • Date: Thu, 16 Nov 1995 01:45:46 -0500
  • Organization: Wolfram Research, Inc.

In article <489hmv$gm7 at ralph.vnet.net> Sascha at sunmail.lrz-muenchen.de,  
Unzicker at lrz.uni-muenchen.de writes:
> Let me give an example:
> 
> a=Table[0,{3},{4},{5},{6}];
> 
> Dimensions[a]
> 
> gives you 
> Out[86]= {3, 4, 5, 6},
> of course.
> 
> But if you try to assign a new value to a sublist
> a[[1,1]]=Table[x,{6},{5}];
> Mathematica forgets that a still should have the Dimension {3,4,5,6}:
> 
> In[88]:= Dimensions[a]
> Out[88]= {3, 4}
> In[89]:= Transpose[a,{4,1,2,3}];
> 
> Transpose::tperm: 
>    Permutation {4, 1, 2, 3} is longer than the dimensions {3, 4} of the  
array.
> 
> is now impossible, although 
> 
> In[90]:= a[[3,4,5,6]]
> Out[90]= 0
> returns still the correct value. Why does Mathematica make this  
difference?
> If you print a, there is no difference.
> 
> How can you return to the desired Dimension?
> 
> a=Array[a,{3,4,5,6}];
> 
> is not possible because a[[1,1,1,1]] gives a mess.
> 
> Thanks in advance,
> 
> Sascha Unzicker

Did you perhaps mean to enter

    a[[1,1]]=Table[x,{5},{6}];

rather than

    a[[1,1]]=Table[x,{6},{5}];

???  The latter destroys the rectangular structure of the original
tensor because it replaces a sublist of dimensions {5, 6} with one
of dimensions {6, 5}.  I am not aware of anything unusual in this
part of Mathematica, but if you could explain what you expected this
to do, we could perhaps take another look at it.

Dave Withoff
Wolfram Research


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