Question

执行以下代码后x的值是多少？

x = []
for i in range(3):
    x = [x + x]

A.[[[[]]]].
B.[[[],[]]].
C.[[[[],[]],[[],[]]]].
D.[[],[],[],[],[],[]]

答案是 c ，有人可以解释为什么会这样吗？我理解2/3迭代，但不理解它是如何从第1到第2，因为它为什么没有成为[[],[]]

Answer 1

我使用额外的空格来表达清晰。

首次运行：

x + x = [] + [] = []
空列表+空列表是另一个空列表

所以[x + x] = [ [] ]
注意！ [[]]不是空列表，它是包含空列表的列表

第二次运行：

x + x = [[]] + [[]] = [[], []]
所以[x + x] = [ [[], []] ]

第三次运行：

x + x = [[[], []]] + [[[], []]] = [[[[], []]], [[[], []]]]

所以[x + x] = [ [[[[], []]], [[[], []]]] ]

Answer 2

x = []
for i in range(3):
    print('\ni =', i)
    print('x =' , x)
    print('x + x =', x + x)
    print('[x + x] =', [x + x])
    x = [x + x]

<强>输出：

i = 0
x = []
x + x = []  # Here is why. If you extend [] by [], you get []. 
[x + x] = [[]]  # And we wrap the result.

i = 1
x = [[]]
x + x = [[], []]
[x + x] = [[[], []]]

i = 2
x = [[[], []]]
x + x = [[[], []], [[], []]]
[x + x] = [[[[], []], [[], []]]]

Answer 3

让我们逐步完成代码，是吗？：

x = []：创建一个名为x
for i in range(3)：重复以下代码三次：
- x = [x + x]将x的值重新分配给x的当前值，加上x的当前值并将其包装在列表中。

基本上，每次迭代发生的事情是您的列表嵌套在一个级别，列表中的列表当前成为该嵌套列表的子列表。

用外行人的话来说，最简单的解释是 - 在for循环的每次迭代中，变量x的值被重新赋值为两个嵌套的值，其中还可以包含两个嵌套列表，单一列表。

以下是在for - 循环的每次迭代中显示x值的说明：

>>> # x is empty
>>> x = []
>>> 
>>> # 1st iteration
>>> x = [x + x]
>>> # x
>>> x
[[]] # your list is nested by one level
>>> 
>>> # 2nd iteration
>>> x = [x + x]
>>> # x
>>> x
[[[], []]] # your list is nested by three levels
>>> # 3rd iteration
>>> x = [x + x]
>>> # x
>>> x
[[[[], []], [[], []]]] # your list is nested by four levels
>>>

如果上面的插图过于拥挤，请选择较短的插图：

First iteration
Value of x: [[]] 

Second iteration
Value of x: [[[], []]] 

Third iteration
Value of x: [[[[], []], [[], []]]]

我还发现从嵌套列表中创建一个类似树的结构，有助于更好地理解发生的事情：

[ # 1st level
    [ # second level
        [ # third level
            [], # fourth level
            [] 
        ], 
        [
            [], 
            [] # fourth level
        ] # third level
    ] # second level
] # 1st level

嵌套括号空循环解释

3 个答案: