### 问题4

def falling(n, k):
"""Compute the falling factorial of n to depth k.

>>> falling(6, 3)  # 6 * 5 * 4
120
>>> falling(4, 3)  # 4 * 3 * 2
24
>>> falling(4, 1)  # 4
4
>>> falling(4, 0)
1
"""
total, stop = 1, n-k
while n > stop:
total, n = total*n, n-1
return total

### 问题5

def sum_digits(y):
"""Sum all the digits of y.

>>> sum_digits(10) # 1 + 0 = 1
1
>>> sum_digits(4224) # 4 + 2 + 2 + 4 = 12
12
>>> sum_digits(1234567890)
45
>>> a = sum_digits(123) # make sure that you are using return rather than print
>>> a
6
"""
total = 0
while y > 0:
total, y = total + y % 10, y // 10
return total

### 问题7

def double_eights(n):
"""Return true if n has two eights in a row.
>>> double_eights(8)
False
>>> double_eights(88)
True
>>> double_eights(2882)
True
>>> double_eights(880088)
True
>>> double_eights(12345)
False
>>> double_eights(80808080)
False
"""
prev_eight = False
while n > 0:
last_digit = n % 10
if last_digit == 8 and prev_eight:
return True
elif last_digit == 8:
prev_eight = True
else:
prev_eight = False
n = n // 10
return False