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Using numpy to build an array of all combinations of two arrays

Cartesian product function by Stackoverflow user, pv.

See Answer here.

cartesian.py

import numpy as np

def cartesian(arrays, out=None):
    """
    Generate a cartesian product of input arrays.

    Parameters
    ----------
    arrays : list of array-like
        1-D arrays to form the cartesian product of.
    out : ndarray
        Array to place the cartesian product in.

    Returns
    -------
    out : ndarray
        2-D array of shape (M, len(arrays)) containing cartesian products
        formed of input arrays.

    Examples
    --------
    >>> cartesian(([1, 2, 3], [4, 5], [6, 7]))
    array([[1, 4, 6],
           [1, 4, 7],
           [1, 5, 6],
           [1, 5, 7],
           [2, 4, 6],
           [2, 4, 7],
           [2, 5, 6],
           [2, 5, 7],
           [3, 4, 6],
           [3, 4, 7],
           [3, 5, 6],
           [3, 5, 7]])

    """

    arrays = [np.asarray(x) for x in arrays]
    dtype = arrays[0].dtype

    n = np.prod([x.size for x in arrays])
    if out is None:
        out = np.zeros([n, len(arrays)], dtype=dtype)

    m = n / arrays[0].size
    out[:,0] = np.repeat(arrays[0], m)
    if arrays[1:]:
        cartesian(arrays[1:], out=out[0:m,1:])
        for j in xrange(1, arrays[0].size):
            out[j*m:(j+1)*m,1:] = out[0:m,1:]
    return out

👍 that's awesome.

A loop-based version that can be jitted with numba

@njit(cache=True)
def cartesian_jit(arrays):

    """
    Generate a cartesian product of input arrays.

    Parameters
    ----------
    arrays : list or tuple of arrays
        1-D arrays to form the cartesian product of.


    Returns
    -------
    out : ndarray
        2-D array of shape (M, len(arrays)) containing cartesian products
        formed of input arrays.

    Examples
    --------
    >>> cartesian(([1, 2, 3], [4, 5], [6, 7]))
    array([[1, 4, 6],
           [1, 4, 7],
           [1, 5, 6],
           [1, 5, 7],
           [2, 4, 6],
           [2, 4, 7],
           [2, 5, 6],
           [2, 5, 7],
           [3, 4, 6],
           [3, 4, 7],
           [3, 5, 6],
           [3, 5, 7]])

    """

    n = 1
    for x in arrays:
        n *= x.size
    out = np.zeros((n, len(arrays)))


    for i in range(len(arrays)):
        m = int(n / arrays[i].size)
        out[:n, i] = np.repeat(arrays[i], m)
        n //= arrays[i].size

    n = arrays[-1].size
    for k in range(len(arrays)-2, -1, -1):
        n *= arrays[k].size
        m = int(n / arrays[k].size)
        for j in range(1, arrays[k].size):
            out[j*m:(j+1)*m,k+1:] = out[0:m,k+1:]
    return out

It doesn't work,
python arrays don't have .size attribute.

@aero-108 Yes, numpy arrays do have a .size attribute...

a = np.array([1,2,3])

a.size
Out[2]: 3

A loop-based version that can be jitted with numba

@njit(cache=True)
def cartesian_jit(arrays):

    """
    Generate a cartesian product of input arrays.

    Parameters
    ----------
    arrays : list or tuple of arrays
        1-D arrays to form the cartesian product of.


    Returns
    -------
    out : ndarray
        2-D array of shape (M, len(arrays)) containing cartesian products
        formed of input arrays.

    Examples
    --------
    >>> cartesian(([1, 2, 3], [4, 5], [6, 7]))
    array([[1, 4, 6],
           [1, 4, 7],
           [1, 5, 6],
           [1, 5, 7],
           [2, 4, 6],
           [2, 4, 7],
           [2, 5, 6],
           [2, 5, 7],
           [3, 4, 6],
           [3, 4, 7],
           [3, 5, 6],
           [3, 5, 7]])

    """

    n = 1
    for x in arrays:
        n *= x.size
    out = np.zeros((n, len(arrays)))


    for i in range(len(arrays)):
        m = int(n / arrays[i].size)
        out[:n, i] = np.repeat(arrays[i], m)
        n //= arrays[i].size

    n = arrays[-1].size
    for k in range(len(arrays)-2, -1, -1):
        n *= arrays[k].size
        m = int(n / arrays[k].size)
        for j in range(1, arrays[k].size):
            out[j*m:(j+1)*m,k+1:] = out[0:m,k+1:]
    return out

Is it possible to make a generator function using Numba?