如何在Python中的给定范围内循环变量

Question

is there an easy way in python to cycle a variable in a given range? For example: Given a range(), I want a variable goes like this: 0 1 2 3 2 1 0 1 2 3... till some conditions are satisfied.

Answer 1

You want to cycle the sequence 0, 1, ..., n, n-1, ..., 1 . You can easily build this sequence using 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)

Sample run:

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

You want itertools.cycle() , see here:

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

Answer 3

you need itertools.cycle :

demo:

>>> 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 is a good start. Else you can program it yourself:

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"""

Essentially, you just want to loop and constantly check the condition. And each time yeild the next item from the cycle. Now, admittedly there are better ways to check a condition than a function call, but this is just an example.

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)

output:

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