Numpy quirk: Apply function to all pairs of two 1D arrays, to get one 2D array

Question

Let\'s say I have 2 one-dimensional (1D) numpy arrays, a and b, with lengths n1 and n2 respectively. I also have a functi

Answer 1

May I suggest, if your use-case is more limited to products, that you use the outer-product?

e.g.:

import numpy

a = array([0, 1, 2])
b = array([0, 1, 2, 3])

numpy.outer(a,b)

returns

array([[0, 0, 0, 0],
       [0, 1, 2, 3],
       [0, 2, 4, 6]])

You can then apply other transformations:

numpy.outer(a,b) + 1

returns

array([[1, 1, 1, 1],
       [1, 2, 3, 4],
       [1, 3, 5, 7]])

This is much faster:

>>> import timeit
>>> timeit.timeit('numpy.array([[i*j for i in a] for j in b])', 'import numpy; a=numpy.arange(3); b=numpy.arange(4)')
31.79583477973938

>>> timeit.timeit('numpy.outer(a,b)', 'import numpy; a=numpy.arange(3); b=numpy.arange(4)')
9.351550102233887
>>> timeit.timeit('numpy.outer(a,b)+1', 'import numpy; a=numpy.arange(3); b=numpy.arange(4)')
12.308301210403442