Python while 循环额外迭代？

Question

虽然代码按预期工作，但它重复的次数超出了需要，我不明白为什么。我包含了调试行，所以我可以看到过程的每一步，即使变量 'num' 在每次迭代结束时更新，一旦 num 的长度达到 0，它不会立即停止。我错过了什么？

这是我的输入代码：

def MathChallenge(num):
  count = 0

  #check for win
  while len(str(num)) > 1:
    # multiply function
    count +=1
    #print('count increased') #debug
    #print('input num') #debug
    #print num #debug
    num = list(str(num))
    #print('breaking down') #debug 
    #print (num) #debug
    #print ('starting multiply') #debug
    total = 1
    for i in num:
      total *= int(i)
    #  print(total) #debug
    num = int(total)
    #print ('ending multiply') #debug
    #print num #debug
    if len(str(num)) == 1:
      break
    MathChallenge(num)

  return count
# keep this function call here 
print MathChallenge(raw_input())

这是我的输出：

count increased
input num
9999
breaking down
['9', '9', '9', '9']
starting multiply
9
81
729
6561
ending multiply
6561
count increased
input num
6561
breaking down
['6', '5', '6', '1']
starting multiply
6
30
180
180
ending multiply
180
count increased
input num
180
breaking down
['1', '8', '0']
starting multiply
1
8
0
ending multiply
0
count increased
input num
180
breaking down
['1', '8', '0']
starting multiply
1
8
0
ending multiply
0
count increased
input num
6561
breaking down
['6', '5', '6', '1']
starting multiply
6
30
180
180
ending multiply
180
count increased
input num
180
breaking down
['1', '8', '0']
starting multiply
1
8
0
ending multiply
0
count increased
input num
180
breaking down
['1', '8', '0']
starting multiply
1
8
0
ending multiply
0
3

Answer 1

现在清楚了 - return 关键字 returns 来自 MathChallenge 的一次调用。你在做什么是在 while 循环中循环数字，每次迭代你运行 MathChallenge。每个新调用都会运行相同的循环。看看这个片段：

def foo(i):
    print(i, end=" ")
    for _ in range(i):
        foo(i - 1)
    return
      
print(foo(3)) # 3 2 1 0 1 0 2 1 0 1 0 2 1 0 1 0 hell lot numbers!

您应该决定采用迭代方法（只需删除 MathChallenge 的调用）还是递归方法：

import math


def MathChallenge(num, count):
    if num < 10:
        return count
    digits = (int(d) for d in str(num))
    # python3
    # return MathChallenge(math.prod(digits), count + 1)
    # python2
    return MathChallenge(reduce(lambda x, y: x * y, digits, 1), count + 1)



print(MathChallenge(9999, 0))

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