NumPy and SciPy

As a lightweight computer algebra system, PICOS sits one level above numerics libraries such as NumPy and SciPy and acts in concert with them. Let’s define a variable and some data:

>>> import picos, numpy, scipy.sparse
>>> x = picos.RealVariable("x", 4)
>>> N = numpy.reshape(range(16), (4, 4))
>>> type(N)
<class 'numpy.ndarray'>
>>> S = scipy.sparse.spdiags(range(4), 0, 4, 4)
>>> type(S)
<class 'scipy.sparse._dia.dia_matrix'>

Taking input from NumPy or SciPy

PICOS also allows loading of NumPy and SciPy data on the fly, with one caveat to watch out for:

>>> x.T*N
<1×4 Real Linear Expression: xᵀ·[4×4]>
>>> N*x
<4×1 Real Linear Expression: [4×4]·x>
>>> x.T*S
<1×4 Real Linear Expression: xᵀ·[4×4]>
>>> S*x
Traceback (most recent call last):
picos.valuable.NotValued: Mutable x is not valued.

The last command fails as SciPy sparse matrices do not currently respect the __array_priority__ attribute, so that SciPy tries to load x as an array as opposed to conceding the operation to PICOS like NumPy does. You can fix this behavior as follows:

>>> picos.patch_scipy_array_priority()
>>> S*x
<4×1 Real Linear Expression: [4×4]·x>

Note that this monkey-patches SciPy, so that applications importing your code calling patch_scipy_array_priority will also see a patched version of SciPy.

Returning NumPy or SciPy data as output

PICOS uses CVXOPT as a numerics backend and thus outputs numeric values as CVXOPT (sparse) matrices or Python scalar types by default:

>>> x.value = range(4)
>>> x.value
<4x1 matrix, tc='d'>
>>> type(x.value)
<class 'cvxopt.base.matrix'>

However, all objects that can be valued, in particular expressions and problem instances, also offer properties to query that value as a NumPy type, namely np and np2d:

>>>  # Returns a NumPy scalar, 1D, or 2D array.
array([0., 1., 2., 3.])
>>> type(
<class 'numpy.ndarray'>
>>> x.np2d  # Always returns a 2D array.
>>> x.np2d.shape
(4, 1)

For SciPy, the sp property returns a sparse matrix whenever the data stored by PICOS internally is sparse and a NumPy 2D array otherwise:

>>> I = picos.I(3)
>>> print(I)
[ 1.00e+00     0         0    ]
[    0      1.00e+00     0    ]
[    0         0      1.00e+00]
>>> type(I.sp)
<class 'scipy.sparse._csc.csc_matrix'>
>>> J = picos.J(3, 3)
>>> print(J)
[ 1.00e+00  1.00e+00  1.00e+00]
[ 1.00e+00  1.00e+00  1.00e+00]
[ 1.00e+00  1.00e+00  1.00e+00]
>>> type(J.sp)
<class 'numpy.ndarray'>

A full list of methods for returning values in different formats can be found in the documentation of the Valuable base class.