[Date Index]
[Thread Index]
[Author Index]
1/Diagonal[A] gives different result compared to 1/Diagonal[Normal@A] when A is sparse
*To*: mathgroup at smc.vnet.net
*Subject*: [mg123270] 1/Diagonal[A] gives different result compared to 1/Diagonal[Normal@A] when A is sparse
*From*: "Nasser M. Abbasi" <nma at 12000.org>
*Date*: Wed, 30 Nov 2011 03:18:50 -0500 (EST)
*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com
I found this strange behavior, and I do not think it is correct.
This is version 8.04.
1/Diagonal[A] gives a divide by zero error, but 1/Diagonal[Normal@A] does
not. This is when A is sparse.
------------------------------
Clear["Global`*"]
makeMatrix[n_]:=Module[{numberOfUnknowns=n^2,r,A},
A=SparseArray[
{
Band[{1,1}]->4.0,
Band[{2,1}]->-1,
Band[{1,2}]->-1,
Band[{1,n+1}]->-1,
Band[{n+1,1}]->-1
},{numberOfUnknowns,numberOfUnknowns},0.
];
r=Range[n,n^2-n,n];
(A[[#,#+1]]=0.)&/@r;
(A[[#+1,#]]=0.)&/@r;
A
];
(A = makeMatrix[3])//MatrixForm
(Diagonal[A])//Normal
1/Diagonal[Normal@A] (* ===> OK *)
1/Diagonal[A] (* error *)
----------------------------------------
So, 1/Diagonal[Normal@A] gives
{0.25,0.25,0.25,0.25,0.25,0.25,0.25,0.25,0.25}
but 1/Diagonal[A] gives 1/0
In another system I use, both operations give
{0.25,0.25,0.25,0.25,0.25,0.25,0.25,0.25,0.25}
i.e. if the matrix is sparse or not, 1/Diagonal[A] should work
regardless. I think sparse matrices need to be more integrated into
all Mathemaitca matrix operations.
Or Am I missing something here?
Thanks,
--Nasser
Prev by Date:
**Ploting a transformation of a set**
Next by Date:
**Re: how to get string in sci. notation to a number?**
Previous by thread:
**Ploting a transformation of a set**
Next by thread:
**How to sort elements from a two-dimensional list**
| |