Re: [R] impossible to invert a spam-object, but possible when it's a matrix-object

[Uwe Ligges]

On 05.02.2013 13:29, Camarda, Carlo Giovanni wrote:
Dear R-users,

a question concerning sparse matrices in package "spam" (spam_0.29-2).

On one hand I have a spam object (n X n) from which I cannot compute the 
inverse. On the other hand, if I convert this object in a plain matrix, I can 
find the inverse without any problem.

Specifically I get the following error message:
   Error in chol.spam(a, ...) :
   Singularity problem when calculating the Cholesky factor.

Obviously I get similar behaviour when I compute determinants of these objects.

Please see below a toy-example which I created based on my actual problem.


Different algorithms are used to compute the inverse - and you are 
rather close to singularity: kappa(A) results in 59638727.

Best,
Uwe Ligges








Thanks in advance for any help,
GC


library(spam)
## creating a spam matrix A
ent <- c(2312.12324929972,-2000,1000,-2000,1000,-2000,
          5031.91011235955,-2000,-2000,1000,-1,1000,-2000,
          2049.8595505618,-2000,1000,-1,-2000,5036.89635854342,
          -2000,1000,-2000,1,-2000,-2000,8119.66292134831,-2000,
          -2000,-2000,1000,-2000,5058.83426966292,-2000,-1,1000,
          -2000,2051.85434173669,-2000,1000,1,1000,-2000,-2000,
          5043.87640449438,-2000,1,1000,-2000,1000,-2000,
          2110.68820224719,-1,1,0,-1,1,-1,1)
rr <- as.integer(c(1,6,12,18,24,29,35,41,47,52,55,57,59))
cc <- as.integer(c(1,2,3,4,7,1,2,3,5,8,10,1,2,3,6,9,11,1,
                    4,5,6,7,10,2,4,5,6,8,3,4,5,6,9,12,1,4,
                    7,8,9,11,2,5,7,8,9,12,3,6,7,8,9,2,4,10,3,7,6,8))
di <- as.integer(c(12,12))
A <-   new("spam",
            entries=ent,
            colindices=cc,
            rowpointers=rr,
            dimension=di)
## calculate the determinant
det(A)
## computes the inverse
solve(A)

## transform A in a plain matrix
A1 <- matrix(c(A), di)
## calculate the determinant
det(A1)
## computes the inverse
solve(A1)


version
                _
platform       i686-pc-linux-gnu
arch           i686
os             linux-gnu
system         i686, linux-gnu
status
major          2
minor          14.1
year           2011
month          12
day            22
svn rev        57956
language       R
version.string R version 2.14.1 (2011-12-22)

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