函数递归

函数递归

函数递归指的重复‘’直接‘’或‘’间接‘’调用函数本身，这是一种函数嵌套调用的表现形式

python中递归默认深度为998

获取递归深度 ：import sys

​ print(sys.getrecursionlimit())

设置递归深度：sys.setrecursionlimit(2000)

查看当前可承受递归深度

num = 1
def func():
    global num
    print('from func',num)
    num += 1
    func()
func()

单纯的递归调用无任何意义

递归

def f1():
	print('from f1')
    f1()
f1()  #进入死循环
——————————————————————————————————————————————————————————————————————————
#我们需要定一个规则来终止这个函数
def f1(num):
    if num < 3:
        f1(num + 1)  
        print(num)  #先调用再打印

f1(1)
>>>2
   1
 —————————————————————————————————————————————————————————————————————————  
def f1(num):
    if num < 3:
        print(num)
        f1(num + 1)  #先打印再调用
          
f1(1)
>>>1
   2
 #以上为直接调用，函数本身调用本身

间接调用:

两个函数之间相互调用，间接造成递归

def bar(num):
    if num > 0:
        print(num,'from bar')
        foo(num)
    
    
def foo(num):
    print(num,'from foo')
    bar(num-1)
    
bar(5)
print('*'*50)
foo(5)
>>>
5 from bar
5 from foo
4 from bar
4 from foo
3 from bar
3 from foo
2 from bar
2 from foo
1 from bar
1 from foo
**************************************************
5 from foo
4 from bar
4 from foo
3 from bar
3 from foo
2 from bar
2 from foo
1 from bar
1 from foo

递归若要有意义，必须遵循两个条件：

1、递推：一层一层递归调用下去，进入下一层递归的问题规模将会缩小，拿到一个更接近结果的结果

2、回溯：递归必须要有一个明确的结束条件，在满足该条件时，开始一层一层回溯

line = [1,2,3,4,5,6]
def howmanyin(lst):
    if lst[1:]:
        print(lst)
        print('me and the guys behind')
        return 1+ howmanyin(lst[1:])
    else:
        print('just me')
        return 1
    
print(howmanyin(line))
>>>
[1, 2, 3, 4, 5, 6]
me and the guys behind
[2, 3, 4, 5, 6]
me and the guys behind
[3, 4, 5, 6]
me and the guys behind
[4, 5, 6]
me and the guys behind
[5, 6]
me and the guys behind
just me
6