GNU Octave

Time for action – instantiating a sparse matrix

1. In Octave, we can define a matrix to be sparse using the sparse function. The simplest way to instantiate a sparse matrix with, say 5 rows and 6 columns, is:

   octave:1 > A=sparse(5,6)
   
   A=
   
    Compressed Column Sparse (rows=5, cols=6, nnz=0, [0%])
   

2. To assign non-zero values to elements in the matrix, you simply use normal assignments, for example:

   octave:2> A(1,2) = 42; A(3,:)=3;
   
   octave:3> A
   
   A =
   
    Compressed Column Sparse (rows=5, cols=6, nnz=7, [23%])
   
      (3,1) ->1
   
      (1,2) -> 42
   
      (3,2) -> 1
   
      (3,3) -> 1
   
      (3,4) -> 1
   
      (3,5) -> 1
   
      (3,6) -> 1
   

3. It is possible to extract the full matrix (and include all the zeros) from sparse matrix by using the full function:

   octave:4> B=full(A)
   
   B =
   
      0   42    0    0    0    0
   
      0    0    0    0    0    0
   
    3 3 ...