convert itertools array into numpy array

Question

I\'m creating this array:

A=itertools.combinations(range(6),2)

and I have to manipulate this array with numpy, like:

A.resh         


        

Answer 1

An alternative way to get every pairwise combination of N elements is to generate the indices of the upper triangle of an (N, N) matrix using np.triu_indices(N, k=1), e.g.:

np.vstack(np.triu_indices(6, k=1)).T

For small arrays, itertools.combinations is going to win, but for large N the triu_indices trick can be substantially quicker:

In [1]: %timeit np.fromiter(itertools.chain.from_iterable(itertools.combinations(range(6), 2)), np.int)
The slowest run took 10.46 times longer than the fastest. This could mean that an intermediate result is being cached 
100000 loops, best of 3: 4.04 µs per loop

In [2]: %timeit np.array(np.triu_indices(6, 1)).T
The slowest run took 10.97 times longer than the fastest. This could mean that an intermediate result is being cached 
10000 loops, best of 3: 22.3 µs per loop

In [3]: %timeit np.fromiter(itertools.chain.from_iterable(itertools.combinations(range(1000), 2)), np.int)
10 loops, best of 3: 69.7 ms per loop

In [4]: %timeit np.array(np.triu_indices(1000, 1)).T
100 loops, best of 3: 10.6 ms per loop