从现有列表创建新数组答案

【问题标题】：creating a new array from an existing list从现有列表创建新数组
【发布时间】：2020-05-10 08:59:03
【问题描述】：

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附加到一个新列表中。我可以取我想要的值，但它以1 x 28 数组的形式返回。我希望它以4 x 7 数组的形式返回。我怎么知道当它到达第一行的末尾时开始新的一行？

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]]

【问题讨论】：

标签： python arrays list numpy append

【解决方案1】：

你应该可以用l[:,::3]直接索引这个：

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）

【讨论】：

【解决方案2】：

尝试使用带有列表理解的::3 切片：

l = [
    [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]
]
print(np.array([i[::3] for i in l]))

要了解有关切片的更多信息，请查看此处：

Understanding slice notation

【讨论】：