如何进行从k到给定数然后回到零的无限循环？

Question

我想要类似的东西（如果从2开始）：

2, 3, ..., n-1, n, n-1, ..., 3, 2, 1, 0, 1, 2, 3, ... (forever)

有什么比这更简单的吗？：

def print_numbers_forth_and_back_forever(first_number, upper_limit):
    for i in range(first_number, upper_limit):
        print(i)
    while True:
        # back from n to 0
        for i in reversed(range(0, upper_limit+1)):
            print(i)
        # 1 to n-1
        for i in range(1, upper_limit):
            print(i)


print_numbers_forth_and_back_forever(4, 10)

Answer 1

以下使用itertools的将提供应满足要求的生成器-只需更改range(a, b)的参数即可更改输出：

from itertools import cycle, chain

r = range(10)
r_reversed = reversed(r[1:-1])
gen = cycle(chain(r, r_reversed))

输出：

>>> from itertools import islice
>>> list(islice(gen, 20))
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, 1]

要以任意整数开始，请对start使用islice自变量，如下所示：

>>> list(islice(gen, 4, 20))
[4, 5, 6, 7, 8, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, 1]

Answer 2

与您使用生成器相同：

# you can add start argument to replace 0
def gen_infinite_bounce(first, end) :
  yield from range(first,end)
  while True:
    yield from range(end, 0, -1)
    yield from range(0, end)

first = 2 # first Value
end = 5 # end bounce
# you can create a variable start to replace 0
for i in gen_infinite_bounce(first, end):
  print(i)

您也可以使用itertools：

first = 2 # first Value
end = 5 # end bounce
# you can create a variable start to replace 0
from itertools, import chain, cycle
for i in chain(
  range(start, end), cycle(
    chain(
      range(end, 0, -1),
      range(0, end)
    )
  )
):
  print(i)

Answer 3

您可以将itertools与range()对象一起使用，以有效地创建所需的输出：

from itertools import cycle, chain, islice

def cycle_fwd_back(first, last):
    yield from range(first, last)
    yield from cycle(chain(reversed(range(last + 1)), range(1, last)))


# First 30
print(list(islice(cycle_fwd_back(4, 10), 30)))

输出：

[4, 5, 6, 7, 8, 9, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 9, 8, 7]

Answer 4

您可以使用下面的代码

def print_numbers_forth_and_back_forever(first_number, upper_limit):
inc = 
i = first_number
while True:
    if i == upper_limit:
        inc = -1
    elif i == 0:
        inc = 1
    print(i,end = ' ')
    i += inc