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MathGroup Archive 2008

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SparseArray

  • To: mathgroup at smc.vnet.net
  • Subject: [mg92310] SparseArray
  • From: Uli Wuerfel <uli.wuerfel at fmf.uni-freiburg.de>
  • Date: Sat, 27 Sep 2008 06:51:00 -0400 (EDT)

Hello,

I want to create a SparseArray with the elements being zero except for the diagonal elements and their direct neighbours. For those, I would like to assign them a value from another list, depending on the index i.

The code is like this:

anz=150;
kg=10^-3;
dx=10^-5;
delx={};
For[i=1,i<=anz,
delx=Append[delx,{i,dx}];
i++];

matr=SparseArray[{1,1}->-2*kg*delx[[1]][[2]],{i_,i_}/;1<i<anz->-2*kg*(delx[[i]][[2]]+delx[[i-1]][[2]]),{1,2}->kg*delx[[1][[2]],{i_,j_}/;i-j==1&&i>1->kg*delx[[i]][[2]],{i_,j_}/;j-i==1->kg*delx[[i-1]][[2]],{anz,anz}->-kg*(delx[[anz]][[2]]+delx[[anz-1][[2]])},{anz,anz}];

Here delx is a very simple list, but later I would like to do the same with a list with varying elements.
The reason is that I would like to have a non-constant
grid spacing in order to solve a differential equation with the SparseArray and using LinearSolve.
This works well as long as I assign constants to the SparseArray-item. If I try to assign values from a list to the SparseArray-item, it does not work.

Can you help?

Regards,
Uli


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