主页 / coding / 问题 / 78245928
Accepted
kesarling He-Him
Asked: 2024-03-30 04:58:57 +0800 CST

如何修改列表理解中的外部变量？

  • 772

Python代码：

i: int = 1
table: list = [[i, bit, None, None] for bit in h]

预期行为：i每次迭代需要增加 1。

伪代码：

i: int = 1
table: list = [[i++, bit, None, None] for bit in h]
python

3 个回答

  1. Best Answer

    列表推导式不应用于外部副作用。在这种情况下，请使用enumerate而不是外部变量：

    h = '10010010'  # Undefined in OP example.  Just something to enumerate.
table = [[i, bit, None, None ] for i, bit in enumerate(h, start=1)]
for row in table:
    print(row)

    输出：

    [1, '1', None, None]
[2, '0', None, None]
[3, '0', None, None]
[4, '1', None, None]
[5, '0', None, None]
[6, '0', None, None]
[7, '1', None, None]
[8, '0', None, None]
    • 6

  2. 作为一个思想实验，这就是你问题的字面答案：

    >>> i=0
>>> [[i:=i+1, bit, None, None] for bit in h]
[[1, '1', None, None], [2, '0', None, None], [3, '0', None, None], [4, '1', None, None], [5, '0', None, None], [6, '0', None, None], [7, '1', None, None], [8, '0', None, None]]

    enumerate你绝对不应该这样做，而是按照接受的答案去做

    • 4

  3. 获得结果的更多方法：

    [[i, bit, None, None]
 for i in [0]
 for bit in h
 for i in [i+1]]

[a[:]
 for a in [[None] * 4]
 for a[:2] in enumerate(h, start=1)]

[[*tpl]
 for nones in [repeat(None)]
 for tpl in zip(count(1), h, nones, nones)]

[*map(list, zip(count(1), h, nones := repeat(None), nones))]

    基准测试，当前无法运行:-(

    import timeit

setup = '''
from itertools import count, repeat
h = '10010010'
'''

codes = [
    'lst = [[i, bit, None, None ] for i, bit in enumerate(h, start=1)]',
    'i=0; lst = [[i:=i+1, bit, None, None] for bit in h]',
    'lst = [[i, bit, None, None] for i in [0] for bit in h for i in [i+1]]',
    'lst = [a[:] for a in [[None] * 4] for a[:2] in enumerate(h, start=1)]',
    'lst = [[*tpl] for nones in [repeat(None)] for tpl in zip(count(1), h, nones, nones)]',
    'lst = [*map(list, zip(count(1), h, nones := repeat(None), nones))]',
]

exec(setup)
for code in codes:
    exec(code)
    print(lst)
    del lst

setup = '''
from itertools import count, repeat
h = '10010010' * 1000
'''

for code in codes:
    t = min(timeit.repeat(code, setup, number=10, repeat=25)) / 10
    print(f'{t * 1e3:4.2f} ms ', code)

    在线尝试这个！

    • 0

相关问题