在Python中协调np.fromiter和多维数组 [英] Reconcile np.fromiter and multidimensional arrays in Python

问题描述

我喜欢使用numpy中的np.fromiter，因为它是构建np.array对象的资源延迟方式.但是，似乎它不支持多维数组，多维数组也非常有用.

I love using np.fromiter from numpy because it is a resource-lazy way to build np.array objects. However, it seems like it doesn't support multidimensional arrays, which are quite useful as well.

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

在保持这种基于生成器的惰性构造的同时，如何使a成为多维数组?

How can I get a to be a multidimensional array while keeping such a lazy construction based on generators?

推荐答案

np.fromiter 仅支持构造一维数组，因此，它期望一个可迭代的对象将产生单个值，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 来解包"输出生成器表达式转换为单个一维值序列:

By itself, np.fromiter only supports constructing 1D arrays, and as such, it expects an iterable that will yield individual values rather than tuples/lists/sequences etc. One way to work around this limitation would be to use itertools.chain.from_iterable to lazily 'unpack' the output of your generator expression into a single 1D sequence of values:

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)

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