将可变大小的子列表的嵌套列表展平为SciPy数组

Question

如何使用numpy/scipy来展平具有不同大小的子列表的嵌套列表？速度非常重要,列表很大.

 lst = [[1, 2, 3, 4],[2, 3],[1, 2, 3, 4, 5],[4, 1, 2]]

有什么比这更快？

 vec = sp.array(list(*chain(lst)))

Answer 1

怎么样np.fromiter:

In [49]: %timeit np.hstack(lst*1000)
10 loops, best of 3: 25.2 ms per loop

In [50]: %timeit np.array(list(chain.from_iterable(lst*1000)))
1000 loops, best of 3: 1.81 ms per loop

In [52]: %timeit np.fromiter(chain.from_iterable(lst*1000), dtype='int')
1000 loops, best of 3: 1 ms per loop

Answer 2

你可以尝试numpy.hstack

>>> lst = [[1, 2, 3, 4],[2, 3],[1, 2, 3, 4, 5],[4, 1, 2]]
>>> np.hstack(lst)
array([1, 2, 3, 4, 2, 3, 1, 2, 3, 4, 5, 4, 1, 2])

Answer 3

从迭代器创建numpy数组的最快方法是使用numpy.fromiter:

>>> %timeit numpy.fromiter(itertools.chain.from_iterable(lst), numpy.int64)
100000 loops, best of 3: 3.76 us per loop
>>> %timeit numpy.array(list(itertools.chain.from_iterable(lst)))
100000 loops, best of 3: 14.5 us per loop
>>> %timeit numpy.hstack(lst)
10000 loops, best of 3: 57.7 us per loop

正如您所看到的,这比转换到列表要快,而且速度要快得多hstack.