13、表达式、生成式、递归函数、匿名函数

一、三元表达式

def max(x,y):
	if x>y:
		return x
	else:
		returny	

这个比较大小的代码，if判断可以用一行代码搞定。

res=x if x >y else y
#条件算一元
#条件成立返回的值算一元
#条件不成立返回的值算一元

二、列表生成式

案例1

l=[]
for i in range (10):
	l.append(i)
print(l)

列表生成式

l=[i for i in range (10)]
print(l)

案例2

l=[]
for i in range (10):
	if i > 5:
		l.append(i)
print(l)

l = [i for i in range (10) if i > 5]
print (l)

案例3

names = ['lxx','hxx','wxx','lili']
l = []
for name in names:
	l.append(name + '_sb')
print(l)

l = [name + '_sb' for name in names]
print (l)

案例4

names = ['egon','lxx_sb','hxx_sb','wxx_sb']
res = [name for name in names if name.endswith('sb')]
print(res)

字典生成式

res=  {i:i**2 for i in range (5)}
items = [('k1',111),('k2',222),('k3',333)]
print(dict(items))
print({k:v for k,v in items})

集合生成式

res = {i for i in range (5)}
print(res)

生成器表达式

res = (i for i in range (3))
print(res)
#造老母鸡
print(next(res))
print(next(res))
print(next(res))

with open ('a.txt',mode='rt',encond='utf-8')as f:
	res= f.read()
	print(len(res))
	
	res = 0
	for line in f:
		res+= len(line)
	print(res)

res = sum((len(line) for line in f))
print(res)

#精简
res = sum(len(line) for line in f)
print(res)

三、函数的递归调用

​ 在调用一个函数的内部又调用自己，所以递归调用的本质就是一个循环的过程

def func():
	print('func')
	func()
func()

import sys
sys.getrecursionlimit()# 查看多少层
print
sys.setrecursionlimit()#设置多少层

​ 大前提：递归调用一定要在某一层结束

​ 递归的两个阶段

​ 1、回溯：向下一层一层挖井

​ 2、递推：向上一层一层返回

四、二分法

例：
nums = [-3,1,3,7,13,23,37,43,57,63,77,91,103]
find_num = 64

def find(nums,find_num):
    print(nums)
    if len(nums) == 0:#如果列表的长度为0，则输出没有这个值
        print("not exists")
        return#结束后续操作
    mid_index = len(nums) // 2#中间值=列表的长度//2
    if find_num > nums[mid_index]:
        # in the right
        find(nums[mid_index+1:],find_num)
    elif find_num < nums[mid_index]:
        # in the left
        find(nums[:mid_index], find_num)
    else:
        print('you got it')


find(nums,find_num)

五、匿名函数

​ 匿名函数：没有名字的函数

​ 特点：临时用一次

​

def f(x,y):
	return x+y
	
lambda x,y:x+y
#使用：(lambda x,y:x+y)()

salaries = {
    "egon":3000,
    "tom":1000000,
    "zxx":1000
}

print(max([11,22,33]))
print(max(salaries.values()))
print(max(salaries))

def func(k):
    return salaries[k]


print(max(salaries,key=func))
print(max(salaries,key=lambda k:salaries[k]))
print(min(salaries,key=lambda k:salaries[k]))
print(sorted(salaries,key=lambda k:salaries[k]))
print(sorted(salaries,key=lambda k:salaries[k],reverse=True))

补充：

map
names = ["lxx",'hxx',"wxx",'lili']
                    规则
       "lxx_sb","hxx_sb"

l = (name + "_sb" for name in names)
res = map(lambda name:name + "_sb",names)
print(list(res))

filter
names = ["lxx_sb",'egon',"wxx_sb",'lili_sb']
print([name for name in names if name.endswith('sb')])

res = filter(lambda name:name.endswith('sb'),names)
print(list(res))

reduce
from functools import reduce

res = reduce(lambda x,y:x+y,[1,2,3])
res = reduce(lambda x,y:x+y,["aa","bb","cc"])
print(res)


res = pow(10,2,3)  # 10 ** 2 % 3
print(res)