python - 在Python中协调np.fromiter和多维数组

我喜欢使用np.fromiterfromnumpy的方法，因为它是构建np.array对象的一种资源懒惰的方法。然而，它似乎不支持多维数组，这也是非常有用的。

import numpy as np

def fun(i):
    """ A function returning 4 values of the same type.
    """
    return tuple(4*i + j for j in range(4))

# Trying to create a 2-dimensional array from it:
a = np.fromiter((fun(i) for i in range(5)), '4i', 5) # fails

# This function only seems to work for 1D array, trying then:
a = np.fromiter((fun(i) for i in range(5)),
        [('', 'i'), ('', 'i'), ('', 'i'), ('', 'i')], 5) # painful

# .. `a` now looks like a 2D array but it is not:
a.transpose() # doesn't work as expected
a[0, 1] # too many indices (of course)
a[:, 1] # don't even think about it

我如何才能让a成为一个多维数组，同时保持这种基于生成器的惰性构造？

最佳答案

本身，np.fromiter只支持构造一维数组，因此，它期望一个iterable，它将生成单个值而不是元组/列表/序列等。解决此限制的一种方法是使用itertools.chain.from_iterable将生成器表达式的输出延迟“解包”为单个一维数值顺序：

import numpy as np
from itertools import chain

def fun(i):
    return tuple(4*i + j for j in range(4))

a = np.fromiter(chain.from_iterable(fun(i) for i in range(5)), 'i', 5 * 4)
a.shape = 5, 4

print(repr(a))
# array([[ 0,  1,  2,  3],
#        [ 4,  5,  6,  7],
#        [ 8,  9, 10, 11],
#        [12, 13, 14, 15],
#        [16, 17, 18, 19]], dtype=int32)

关于python - 在Python中协调np.fromiter和多维数组，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/34018470/