NumPy matrix to SciPy sparse matrix: What is the safest way to add a scalar?

Question

First off, I\'m no mathmatician. I admit that. Yet I still need to understand how ScyPy\'s sparse matrices work arithmetically in order to switch from a dense NumPy matrix t

Answer 1

Admittedly sparse matrices aren't really in my wheelhouse, but ISTM the best way forward depends on the matrix type. If you're DOK:

>>> S = dok_matrix((5,5))
>>> S[2,3] = 10; S[4,1] = 20
>>> S.todense()
matrix([[  0.,   0.,   0.,   0.,   0.],
        [  0.,   0.,   0.,   0.,   0.],
        [  0.,   0.,   0.,  10.,   0.],
        [  0.,   0.,   0.,   0.,   0.],
        [  0.,  20.,   0.,   0.,   0.]])

Then you could update:

>>> S.update(zip(S.keys(), np.array(S.values()) + 99))
>>> S
<5x5 sparse matrix of type ''
    with 2 stored elements in Dictionary Of Keys format>
>>> S.todense()
matrix([[   0.,    0.,    0.,    0.,    0.],
        [   0.,    0.,    0.,    0.,    0.],
        [   0.,    0.,    0.,  109.,    0.],
        [   0.,    0.,    0.,    0.,    0.],
        [   0.,  119.,    0.,    0.,    0.]])

Not particularly performant, but is O(nonzero).

OTOH, if you have something like COO, CSC, or CSR, you can modify the data attribute directly:

>>> C = S.tocoo()
>>> C
<5x5 sparse matrix of type ''
    with 2 stored elements in COOrdinate format>
>>> C.data
array([ 119.,  109.])
>>> C.data += 1000
>>> C
<5x5 sparse matrix of type ''
    with 2 stored elements in COOrdinate format>
>>> C.todense()
matrix([[    0.,     0.,     0.,     0.,     0.],
        [    0.,     0.,     0.,     0.,     0.],
        [    0.,     0.,     0.,  1109.,     0.],
        [    0.,     0.,     0.,     0.,     0.],
        [    0.,  1119.,     0.,     0.,     0.]])

Note that you're probably going to want to add an additional

>>> C.eliminate_zeros()

to handle the possibility that you've added a negative number and so there's now a 0 which is actually being recorded. By itself, that should work fine, but the next time you did the C.data += some_number trick, it would add somenumber to that zero you introduced.