python - 为什么我的回避功能甚至可以工作?语法混乱
问题描述
我编写了两个递归函数,它们的工作方式几乎相同。我试图让递归正确,然后我不小心偶然发现了答案,但语法让我感到困惑:
def fac(N):
"""
Factorial of N.
"""
################# This makes no goddamn sense to me #################
if N == 1:
return N
####### because N == 1, yet 'return N' is the correct result ########
elif N == 0:
return 1
return N*fac(N-1)
N == 1 如何作为退出条件为真,但也存储 fac(N) 的结果?与一个函数相同prod()
,它与sum()
.
def prod(List):
"""
Product of all numbers in a list.
"""
if len(List) == 1:
return List[-1]
return List[-1]*prod(List[:-1])
我不明白最终结果是如何存储在List[-1]
. python解释器是否return arg*func(arg)
以特殊方式理解?
解决方案
考虑计算 所需的循环fac(4)
:
1: fac(4) -> 4 * fac(3) # It then has to calculate fac(3)
2: fac(3) -> 3 * fac(2) # It then has to calculate fac(2)
3: fac(2) -> 2 * fac(1) # It then has to calculate fac(1)
4: fac(1) -> 1 # Finally we've returned a value - now back up through the loops
3: fac(2) -> 2 * fac(1) == 2 * 1 == 2
2: fac(3) -> 3 * fac(2) == 3 * 2 == 6
1: fac(4) -> 4 * fac(3) == 4 * 6 == 24
第二部分实际上是同样的事情——向下递归直到你得到一个值,然后一直插入到你原来的请求。