十四. Python基础(14)--递归
1 ● 递归(recursion)
概念: recursive functions—functions that call themselves either directly or indirectly in order to loop. 最大递归层数: the default maximum recursion depth in Python is 997. 限制条件: 一定有一个分支不调用自身 |
修改最大递归层数: import sys sys.setrecursionlimit(1500) COUNT = 0
def func(): global COUNT # 虽然函数内可以引用COUNT, 但是因为下面修改了COUNT, 所以要用global声明COUNT COUNT += 1 print(COUNT) func()
func() |
2 ● 递归的典型案例:
alex的年龄 阶乘 斐波那契数列 二分查找 三级菜单 |
3 ● alex的年龄
def age(n): if n == 4: return 40 return age(n+1) + 2
print(age(4)) # 40 |
4 ● 阶乘
def fact(n): if n == 1: return 1 return n * fact(n-1)
print(fact(4)) # 24 |
5 ● 斐波那契数列
def fib(n): if n ==1 or n == 2: return 1 return fib(n-1) + fib(n-2)
print(fib(6)) # 8 |
6 ● 二分查找
def search(num, l, start = None, end =None): start = start if start else 0 end = end if end else len(l)-1 mid = (end-start)//2 + start if start > end: return None elif l[mid] > num: return search(num, l, start, mid - 1) elif l[mid] < num: return search(num, l, mid + 1, end) elif l[mid] == num: return mid
l = [2,3,5,10,15,16,18,22,26,30,32,35,41,42,43,55,56,66,67,69,72,76,82,83,88] print(search(66,l)) # 17 |
7 ● 三级菜单
menu = { '北京': { '海淀': { '五道口': { 'soho': {}, '网易': {}, 'google': {} }, '中关村': { '爱奇艺': {}, '汽车之家': {}, 'youku': {}, }, '上地': { '百度': {}, }, }, '昌平': { '沙河': { '老男孩': {}, '北航': {}, }, '天通苑': {}, '回龙观': {}, }, '朝阳': {}, '东城': {}, }, '上海': { '闵行': { "人民广场": { '炸鸡店': {} } }, '闸北': { '火车战': { '携程': {} } }, '浦东': {}, }, '山东': {}, } |
def Three_Level_Menu(menu): while True: for k in menu:print(k) key = input('>>>') if key == 'q':return 'q' elif key == 'b': # 退出本轮循环(递归函数造就了一个循环体)break |
另一方法: pres_dic = menu # 当前词典 prev_dic = [] # 用来存储之前已经查询的词典
print("下面是可供查询的地点:") while True: for key in pres_dic.keys(): print(key) inp = input("请输入您要查询的地点, 输入b/B退回上一层, 输入q/Q退出查询:").strip() if inp not in pres_dic.keys() and inp.lower() != 'b' and inp.lower() != 'q': print("您输入的地点不存在, 请重新输入!") continue
if inp in pres_dic.keys(): prev_dic.append(pres_dic) pres_dic = pres_dic[inp] # 然后开始新一轮的循环
if inp.lower() == 'b': if prev_dic != []: # 等价于if prev_dic: pres_dic = prev_dic[-1] # 当前词典编程 之前已经查询的词典列表 的最后一个 del prev_dic[-1] # # 用来存储之前已经查询的词典列表去掉最后一个元素 else: print("没有更大级别的地点了!")
if inp.lower() == 'q': break |
8 ● 面试真题
# 提取各个fields对应的数据 data={"time":"2016-08-05T13:13:05", "some_id":"ID1234", "grp1":{ "fld1":1,"fld2":2}, "xxx2":{ "fld3":0,"fld5":0.4}, "fld6":11, "fld7":{"fld100": 0, "fld500": 0.4}, "fld46":8} #fields:由"|"连接的以"fld"开头的字符串,如:fld2|fld3|fld7|fld19 |
# 解法1: def select(data,fields): result = {} field_lst = fields.split('|') for key in data: if key in field_lst: result[key] = data[key] if type(data[key]) == dict: # 不要写成elif, 否则, "fld100"对应的数据无法提取出来 res = select(data[key],fields) result.update(res) return result |
# 解法2: def select(data,fields,result = {}): field_lst = fields.split('|') for key in data: if key in field_lst: result[key] = data[key] if type(data[key]) == dict: # 不要写成elif, 否则, "fld100"对应的数据无法提取出来 select(data[key], fields) return result
fields = 'fld2|fld3|fld7|fld19|fld100' ret = select(data,fields) print(ret) # {'fld2': 2, 'fld3': 0, 'fld7': {'fld100': 0, 'fld500': 0.4}, 'fld100': 0} |
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