python - 从现有列表创建新数组

Question

我有一个给定的数组，我想从第一个值开始获取每个第三个值并将其 append 到一个新列表中。我可以获取我想要的值，但它返回为 1 x 28 数组。我希望它以 4 x 7 数组的形式返回。我如何告诉它当它到达第一行末尾时开始新行？

代码:

import numpy as np

list = [
    [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20],
    [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21],
    [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22],
    [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]
]
newlist = []
list = np.array(list)
for row in list:
    k = 0
    for value in row:
        if k % 3 == 0:
            newlist.append(value)
        else:
            pass
        k += 1
newlist = np.array(newlist)
print(newlist)

输出:

[ 1  4  7 10 13 16 19  2  5  8 11 14 17 20  3  6  9 12 15 18 21  4  7 10 13 16 19 22]

期望的输出:

[[ 1  4  7 10 13 16 19 ][ 2  5  8 11 14 17 20 ][ 3  6  9 12 15 18 21 ][ 4  7 10 13 16 19 22]]

Answer 1

您应该能够使用 l[:,::3] 直接在凹凸中对此进行索引:

import numpy as np

l = np.array([
    [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20],
    [2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21],
    [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22],
    [4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23]
])

l[:,::3]

结果

array([[ 1,  4,  7, 10, 13, 16, 19],
       [ 2,  5,  8, 11, 14, 17, 20],
       [ 3,  6,  9, 12, 15, 18, 21],
       [ 4,  7, 10, 13, 16, 19, 22]])

(另外，不要将变量命名为 list)