[Pysparse-users] PysparseSpDiagsMatrix usage from spdiags in matlab

[zipeppe <zi...@gm...>]

Dear all,

I don't know it this is the right place to ask, but unfortunately the
official Python-forum doesn't help.

I am quite new to Python and I am unable to correctly translate the
following MATLAB script in Python code, specially because of the
*spdiags*proprietary function:

function bd=BLC(b,A,B,s)
   L=length(b);
   Bs=-B/s;  As=-A/s;
   bd=ones(L,1)*median(b);bd0=b;nm=norm(bd-bd0);nm0=RealMax;
   M0=-ones(L,1)/As;
   e=ones(L,1);
   D0=spdiags([1*e -4*e 6*e -4*e 1*e],-2:2,L,L);
   D0(1,1)=2; D0(L,L)=2;
   D0(2,1)=-4; D0(1,2)=-4; D0(L,L-1)=-4; D0(L-1,L)=-4;
   D0(2,2)=10;D0(L-1,L-1)=10;
   I=0;
   while nm>10 & I<30
       %& nm<nm0;
       I=I+1;
       M=M0;D=D0;bd0=bd;nm0=nm;
       for i=1:L
           if bd(i)>b(i)
               M(i)=M(i)+2*Bs*b(i)/As;
               D(i,i)=D(i,i)+2*Bs/As;
           end
       end
       bd=D\M;
       nm=norm(bd0-bd);
   end

The algorithm receives *b* as (Nx1 data vector), *A* (1x1), *B* (1x1) and *s
* (1x1) as local parameters, and creates *bd* (Nx1data vector) as output.

It looks like the PySparse package is the best promising, despite that I am
not able to use *PysparseSpDiagsMatrix* function in any way... I got only
errors like:

*TypeError: unsubscriptable object  *
- or -
*IndexError: Not as many row indices as values*

Please, help me to understand what is going on in MATLAB and how to
translate this operation using the PySparse module:

Let have a B matrix as follow:

B =
   1    6   11
   2    7   12
   3    8   13
   4    9   14
   5   10   15

The spdiags MATLAB function works as follows:

A = full (spdiags (B, [-2 0 2], 5, 5))

 Matrix B                      Matrix A

1    6   11                 6    0   13    0    0
2    7   12                 0    7    0   14    0
3    8   13  == spdiags =>  1    0    8    0   15
4    9   14                 0    2    0    9    0
5   10   15                 0    0    3    0   10

A(3,1), A(4,2), and A(5,3) are taken from the upper part of B(:,1)

A(1,3), A(2,4), and A(3,5) are taken from the lower part of B(:,3).
How can I implement the same operation in python using *
PysparseSpDiagsMatrix* ?



Thanks for your help,
ZPP


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