在递归调用中交换列表元素 python

Question

我想用简单的函数交换列表中的随机元素。但它在递归调用中不起作用。

在第一次递归调用中，元素交换工作，但嵌套递归调用（或第一次递归调用中的嵌套递归调用）不起作用。

我不知道为什么只有第一个递归调用中的交换有效。

以下是结果。

谢谢大家。

def change(lst):
    if len(lst)>4:
        a, b = np.random.randint(0, len(lst)), np.random.randint(0, len(lst))
        print(lst)
        lst[a], lst[b] = lst[b], lst[a]
        print(lst)
        mid = int(len(lst)/2)
        change(lst[:mid])
        change(lst[mid:])
k = list(range(0, 20))
change(k)
print(k)

`

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
[0, 19, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 1]
[0, 19, 2, 3, 4, 5, 6, 7, 8, 9]
[3, 19, 2, 0, 4, 5, 6, 7, 8, 9]
[3, 19, 2, 0, 4]
[3, 0, 2, 19, 4]
[5, 6, 7, 8, 9]
[5, 6, 8, 7, 9]
[10, 11, 12, 13, 14, 15, 16, 17, 18, 1]
[10, 11, 12, 13, 14, 15, 16, 17, 18, 1]
[10, 11, 12, 13, 14]
[10, 14, 12, 13, 11]
[15, 16, 17, 18, 1]
[15, 16, 17, 18, 1]
[0, 19, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 1] <= result.

Answer 1

那是因为您通过 lst[:mid]、lst[mid:] 创建了原始列表的副本。一种解决方案是将相同的列表和（单独）要处理的范围传递给 change()。

Answer 2

问题是在你的递归调用中：

change(lst<b>[:mid]</b>)
change(lst<b>[mid:]</b>)

您使用 切片运算符。切片运算符构造一个 新列表，因此您的 更改是在新列表 上进行的，不会反映在原始列表中（因为它是复制).

您可以使用索引代替：

def change(lst<b>,frm=0,to=None</b>):
    <b>if to is None: # set the default to the end of the list
        to = len(lst)</b>
    if <b>to-frm</b> > 4:
        a, b = np.random.randint(<b>frm,to</b>), np.random.randint(<b>frm,to</b>)
        print(lst)
        lst[a], lst[b] = lst[b], lst[a]
        print(lst)
        mid = <b>(frm+to)//2</b>
        change(lst<b>,frm,mid</b>)
        change(lst<b>,mid,to</b>)

然后我们得到：

>>> k = list(range(0, 20))
>>> change(k)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 2, 3, 4, 12, 6, 7, 8, 9, 10, 11, 5, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 2, 3, 4, 12, 6, 7, 8, 9, 10, 11, 5, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 2, 3, 4, 12, 6, 7, 8, 9, 10, 11, 5, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 2, 3, 4, 12, 6, 7, 8, 9, 10, 11, 5, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 4, 3, 2, 12, 6, 7, 8, 9, 10, 11, 5, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 4, 3, 2, 12, 6, 7, 8, 9, 10, 11, 5, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 4, 3, 2, 7, 6, 12, 8, 9, 10, 11, 5, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 4, 3, 2, 7, 6, 12, 8, 9, 10, 11, 5, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 4, 3, 2, 7, 6, 12, 8, 9, 10, 11, 5, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 4, 3, 2, 7, 6, 12, 8, 9, 10, 11, 5, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 4, 3, 2, 7, 6, 12, 8, 9, 5, 11, 10, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 4, 3, 2, 7, 6, 12, 8, 9, 5, 11, 10, 13, 14, 15, 16, 17, 18, 19]
[0, 1, 4, 3, 2, 7, 6, 12, 8, 9, 5, 11, 10, 13, 14, 15, 16, 17, 18, 19]
>>> print(k)
[0, 1, 4, 3, 2, 7, 6, 12, 8, 9, 5, 11, 10, 13, 14, 15, 16, 17, 18, 19]

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