在Python中协调np.fromiter和多维数组
我喜欢使用在Python中协调np.fromiter和多维数组,python,arrays,numpy,multidimensional-array,lazy-evaluation,Python,Arrays,Numpy,Multidimensional Array,Lazy Evaluation,我喜欢使用np.fromiterfromnumpy,因为这是一种构建np.array对象的资源惰性方法。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
np.fromiter
fromnumpy
,因为这是一种构建np.array
对象的资源惰性方法。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
如何使a
成为多维数组,同时保持基于生成器的惰性构造?本身只支持构造1D数组,因此,它需要一个可生成单个值的iterable,而不是元组/列表/序列等。解决此限制的一种方法是使用惰性方式将生成器表达式的输出“解包”为单个1D值序列:
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)