三元表达式，列表推倒式，生成器表达式

一. 三元表达式

def max_(a, b):
    return a if a > b else b

print(max_(3,4))

#result
4

二. 列表推导式

#列表推导式形式：
[表达式 for 变量 in 可迭代对象] 或者  [表达式 for 变量 in 可迭代对象 if 条件]
#字典推导式形式：
{k:v for循环 if判断}

l = [1,2,3,4,5,6,7,8,9,10]

print([x**2 for x in l])

print([x**2 for x in l if x>5])

print(dict([(x,x*2) for x in l]))


print([ (x, y) for x in range(10) if x % 2 if x > 3 for y in range(10) if y > 7 if y != 8 ])

a = [2,5,9]
b = [4,1,-6]

print([a[i] + b[i] for i in range(len(a))])

print("\n".join(("\t".join(["%sx%s=%s"%(x,y,x*y) for y in range(1, x+1)]) for x in range(1,10))))#一行代码打印99乘法表

def mu(i):
    return i*2

print([mu(i) for i in l])

dic = {"jj": "林俊杰", "jay": "周杰伦", "zs": "赵四", "ln":"刘能"}   #字典推导式

d1 = {v:k for k,v in dic.items()}

print(d1)

#结果
[1, 4, 9, 16, 25, 36, 49, 64, 81, 100]
[36, 49, 64, 81, 100]
{1: 2, 2: 4, 3: 6, 4: 8, 5: 10, 6: 12, 7: 14, 8: 16, 9: 18, 10: 20}
[(5, 9), (7, 9), (9, 9)]
[6, 6, 3]
1x1=1
2x1=2    2x2=4
3x1=3    3x2=6    3x3=9
4x1=4    4x2=8    4x3=12    4x4=16
5x1=5    5x2=10    5x3=15    5x4=20    5x5=25
6x1=6    6x2=12    6x3=18    6x4=24    6x5=30    6x6=36
7x1=7    7x2=14    7x3=21    7x4=28    7x5=35    7x6=42    7x7=49
8x1=8    8x2=16    8x3=24    8x4=32    8x5=40    8x6=48    8x7=56    8x8=64
9x1=9    9x2=18    9x3=27    9x4=36    9x5=45    9x6=54    9x7=63    9x8=72    9x9=81
[2, 4, 6, 8, 10, 12, 14, 16, 18, 20]
{'林俊杰': 'jj', '周杰伦': 'jay', '赵四': 'zs', '刘能': 'ln'}

三. 生成器表达式

#列表推导式把[] 换成（）就是生成器表达式。

l = [x*x for x in range(10)]

g = (x*x for x in range(10))

print(l)
print(g)

for i in g:
    print(i)

#result
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
<generator object <genexpr> at 0x101f92f48>
0
1
4
9
16
25
36
49
64
81

#生成器表达式的优点，惰性计算，一次取一个值，节省内存。

 四. 声明式编程练习

# 四 声明式编程练习题
#
# 1、将names=['egon','alex_sb','wupeiqi','yuanhao']中的名字全部变大写
# names=['egon','alex_sb','wupeiqi','yuanhao']
# new_names = [x.upper() for x in names]
# print(new_names)

# 2、将names=['egon','alex_sb','wupeiqi','yuanhao']中以sb结尾的名字过滤掉，然后保存剩下的名字长度
# names=['egon','alex_sb','wupeiqi','yuanhao']
# new_names2 = [x for x in names if not x.endswith("sb") ]
# print(new_names2)

# 3、求文件a.txt中最长的行的长度（长度按字符个数算，需要使用max函数）
# with open("a.txt", "r", encoding="utf-8") as f:
#     print(max((len(i) for i in f)))

# 4、求文件a.txt中总共包含的字符个数？思考为何在第一次之后的n次sum求和得到的结果为0？（需要使用sum函数）
# with open("a.txt", "r", encoding="utf-8") as f:
#     print(sum((len(i) for i in f)))   #文件句柄中的数据读完了


# 5、思考题
#
# with open('a.txt') as f:
#     g=(len(line) for line in f)
# print(sum(g)) #为何报错？

#因为生成器是惰性取值的，当print执行的时候f文件句柄已经关闭

# 6、文件shopping.txt内容如下
#
# mac,20000,3
# lenovo,3000,10
# tesla,1000000,10
# chicken,200,1
# 求总共花了多少钱？
#
# 打印出所有商品的信息，格式为[{'name':'xxx','price':333,'count':3},...]
# with open("shopping.txt", "r", encoding="utf-8") as f:
    # print([{"name":x.split(",")[0],"price":x.split(",")[1],"count":x.split(",")[2].strip()} for x in f])


# 求单价大于10000的商品信息,格式同上

# with open("shopping.txt", "r", encoding="utf-8") as f:
#     print([{"name":x.split(",")[0],"price":x.split(",")[1],"count":x.split(",")[2].strip()} for x in f if int(x.split(",")[1]) > 10000])