Question

在python中有一种简单的方法可以在给定范围内循环变量吗？例如：给定range（），我想要一个变量如下： 0 1 2 3 2 1 0 1 2 3 ...直到满足某些条件。

Answer 1

您希望cycle序列0, 1, ..., n, n-1, ..., 1。您可以使用chain

轻松构建此序列

from itertools import chain, cycle

def make_base_sequence(n):
    base = range(n+1)                   # 0, ..., n
    rev_base = reversed(range(1, n))    # n-1, ..., 1
    return chain(base, rev_base)        # 0, ..., n, n-1, ..., 1

for x in cycle(make_base_sequence(5)):
    print(x)

示例运行：

In [2]: from itertools import chain, cycle
   ...: 
   ...: def make_base_sequence(n):
   ...:     base = range(n+1)
   ...:     rev_base = reversed(range(1, n))
   ...:     return chain(base, rev_base)
   ...: 
   ...: for i, x in enumerate(cycle(make_base_sequence(5))):
   ...:     print(x, end=' ')
   ...:     if i > 20:
   ...:         break
   ...:     
0 1 2 3 4 5 4 3 2 1 0 1 2 3 4 5 4 3 2 1 0 1

Answer 2

您想要itertools.cycle()，请参阅此处：

https://docs.python.org/2/library/itertools.html#itertools.cycle

Answer 3

您需要itertools.cycle：

演示：

>>> import itertools
>>> count = 0
>>> for x in itertools.cycle(range(3)): 
...     if count == 10:
...         break
...     print x,
...     count += 1
... 
0 1 2 0 1 2 0 1 2 0

Answer 4

itertools.cycle是一个好的开始。否则你可以自己编程：

cycle = [0,1,2,3,2,1]
i = 0
while some_condition:
    value = cycle[i]
    i = (i+1) % len(cycle)
    #do stuff

Answer 5

import itertools

def f(cycle_range, condition_func):
    sequence = range(cycle_range) + range(cycle_range)[-2:0:-1]
    cycle_generator = itertools.cycle(sequence)
    while not condition_func():
        yield next(cycle_generator)

def condition_func():
    """Checks some condition"""

基本上，你只想循环并不断检查条件。并且每次从周期开始下一个项目。现在，诚然，有更好的方法来检查条件而不是函数调用，但这只是一个例子。

Answer 6

import time

def cycle(range_):
    num = -1
    current = 0
    a=time.time()
    while 1:
        print current
        if current in (0, range_):
            num*=-1
        current += num
        if time.time() - a > 0.002:
            break

cycle(3)

输出：

0 1 2 3 2 1 0 1 2 3 2 1 0 1 2 3 2 1 0 1 2 3 2 1 0 1 2 3 2

如何在python中循环给定范围内的变量

6 个答案: