相关疑难解决方法(0)

在Python 2中,我可以执行以下操作:

import numpy as np    
f = lambda x: x**2
seq = map(f, xrange(5))
seq = np.array(seq)
print seq
# prints: [ 0  1  4  9 16]

在Python 3中,它不再起作用了:

import numpy as np    
f = lambda x: x**2
seq = map(f, range(5))
seq = np.array(seq)
print(seq)
# prints: <map object at 0x10341e310>

如何获得旧行为(将map结果转换为numpy数组)？

编辑:正如@jonrsharpe在他的回答中指出的那样,如果我先转换seq成一个列表,这可以修复:

seq = np.array(list(seq))

但我宁愿避免额外的电话list.

我喜欢使用np.fromiter,numpy因为它是一种构建np.array对象的资源惰性方式.但是,它似乎不支持多维数组,这些数组也非常有用.

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) …