Dictionary of Keys Format (DOK)

• subclass of Python dict

  • keys are (row, column) index tuples (no duplicate entries allowed)

  • values are corresponding non-zero values

• efficient for constructing sparse arrays incrementally

• constructor accepts:

  • dense array/matrix

  • sparse array/matrix

  • shape tuple (create empty array)

• efficient O(1) access to individual elements

• flexible slicing, changing sparsity structure is efficient

• can be efficiently converted to a coo_array once constructed

• slow arithmetic (for loops with dict.items())

• use:

  • when sparsity pattern is not known apriori or changes

Examples

• create a DOK array element by element:

  >>> mtx = sp.sparse.dok_array((5, 5), dtype=np.float64)
  >>> mtx
  <5x5 sparse array of type '<... 'numpy.float64'>'
          with 0 stored elements in Dictionary Of Keys format>
  >>> for ir in range(5):
  ...     for ic in range(5):
  ...         mtx[ir, ic] = 1.0 * (ir != ic)
  >>> mtx
  <5x5 sparse array of type '<... 'numpy.float64'>'
          with 20 stored elements in Dictionary Of Keys format>
  >>> mtx.toarray()
  array([[0.,  1.,  1.,  1.,  1.],
         [1.,  0.,  1.,  1.,  1.],
         [1.,  1.,  0.,  1.,  1.],
         [1.,  1.,  1.,  0.,  1.],
         [1.,  1.,  1.,  1.,  0.]])
  

• slicing and indexing:

  >>> mtx[1, 1]
  0.0
  >>> mtx[[1], 1:3]
  <1x2 sparse array of type '<... 'numpy.float64'>'
        with 1 stored elements in Dictionary Of Keys format>
  >>> mtx[[1], 1:3].toarray()
  array([[0.,  1.]])
  >>> mtx[[2, 1], 1:3].toarray()
  array([[1.,  0.],
         [0.,  1.]])