如何循環python中給定範圍內的變量

Question

python是否有一種簡單的方法可以在給定範圍內循環變量？ 例如： 給定一個範圍（），我想要一個變量是這樣的： 0 1 2 3 2 1 0 1 2 3 ...直到滿足一些條件。

Answer 1

你想cycle序列0, 1, ..., n, n-1, ..., 1。 您可以輕鬆地構建利用chain

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) 

樣品運行序列：

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

你想itertools.cycle()，在這裏看到：

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

Answer 3

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是一個良好的開端。否則，你可以自己編寫它：

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""" 

從本質上講，你只是想循環並不斷檢查車況。並且每次都會從循環中提取下一個項目。現在，無可否認有更好的方法來檢查一個條件而不是函數調用，但這僅僅是一個例子。

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) 

輸出：

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