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