追加到numpy数组 [英] Appending to numpy arrays

问题描述

我正在尝试构造一个numpy数组，然后向其附加整数和另一个数组. 我尝试这样做:

I'm trying to construct a numpy array, and then append integers and another array to it. I tried doing this:

xyz_list = frag_str.split()
nums = numpy.array([])
coords = numpy.array([])
for i in range(int(len(xyz_list)/4)):
    numpy.append(nums, xyz_list[i*4])
    numpy.append(coords, xyz_list[i*4+1:(i+1)*4])
print(atoms)
print(coords)

打印输出仅给出我的空数组.这是为什么? 另外，如何以允许我拥有如下2D数组的方式重写coords?

Printing out the output only gives my empty arrays. Why is that? In addition, how can I rewrite coords in a way that allows me to have 2D arrays like this: array[[0,0,0],[0,0,1],[0,0,-1]]?

推荐答案

numpy.append与python的list.append不同，它不会就地执行操作.因此，您需要将结果分配回一个变量，如下所示.

numpy.append, unlike python's list.append, does not perform operations in place. Therefore, you need to assign the result back to a variable, as below.

import numpy

xyz_list = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12]
nums = numpy.array([])
coords = numpy.array([])

for i in range(int(len(xyz_list)/4)):
    nums = numpy.append(nums, xyz_list[i*4])
    coords = numpy.append(coords, xyz_list[i*4+1:(i+1)*4])

print(nums)    # [ 1.  5.  9.]
print(coords)  # [  2.   3.   4.   6.   7.   8.  10.  11.  12.]

您可以按以下方式重塑coords:

You can reshape coords as follows:

coords = coords.reshape(3, 3)

# array([[  2.,   3.,   4.],
#        [  6.,   7.,   8.],
#        [ 10.,  11.,  12.]])

有关numpy.append行为的详细信息

More details on numpy.append behaviour

文档:

返回值:arr的副本，其值附加在axis上.注意 append不会就地发生:分配并填充了一个新数组.

Returns: A copy of arr with values appended to axis. Note that append does not occur in-place: a new array is allocated and filled.

如果您事先知道了numpy数组输出的形状，则可以有效地通过np.zeros(n)进行实例化，并在以后用结果填充它.

If you know the shape of your numpy array output beforehand, it is efficient to instantiate via np.zeros(n) and fill it with results later.

另一个选择:如果您的计算大量使用了在数组左侧 处插入元素，请考虑使用

Another option: if your calculations make heavy use of inserting elements to the left of an array, consider using collections.deque from the standard library.

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