title: SciPy storing a sparse matrix with default value different from 0
body: <p>I don't know much about the internal representation of a sparse matrix in SciPy
  But from my very basic understandings the zeros are compressed, while non-zero references are saved along with their position inside the matrix.</p>
  
  <p>What I need is to replace the compressed number (zero) with another one.</p>
  
  <p>My poblem arise from the fact that I start from a normal sparse matrix and then I subtract a constant <code>c</code> from it, thus I come with a matrix whose most common number is <code>-c</code>.</p>
  
  <p>In conclusion is it possible to store this new matrix as a sparse one? I need a SciPy based solution, but all the suggestions are accepted.</p>