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

[Camarda, Carlo Giovanni]

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.

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)

----------
This mail has been sent through the MPI for Demographic Research.  Should you receive a mail that is apparently from a MPI user without this text displayed, then the address has most likely been faked. If you are uncertain about the validity of this message, please check the mail header or ask your system administrator for assistance.