day4_python之三元表达式、列表推导式、生成器表达式

一、三元表达式

name=input('姓名>>: ')
res='SB' if name == 'alex' else 'NB'
print(res)

二、列表解析　

l = []
for i in range(10):
    if i >= 5:
        l.append('egg%s' % i)

print(l)


####列表解析
l = ['egg%s' % i for i in range(10) if i >= 5]
print(l)

三、生成器表达式

g = ('egg%s' % i for i in range(1000))   ###用在数据量比较大的场景下
print(g)
print(next(g))
print(next(g))
print(next(g))

##############################################


###把a文件里的内容最长的一行打印出来
with open('a.txt', encoding='utf-8') as f:
    # res=max((len(line) for line in f))  ###[len(line) for line in f]
    res = max(len(line) for line in f)####(len(line) for line in f) ，统计每一行的长度
    print(res)


print(max([1, 2, 3, 4, 5, 6]))###求列表中最大值



#####此方法适用大文件
with open('a.txt', encoding='utf-8') as f:
    g = (len(line) for line in f)
    print(max(g))

四、应用

res = sum(i for i in range(3))    #sum求和
print(res)

'''
1、求总共花了多少钱
db.txt文件内容：

apple 10 3
tesla 100000 1
mac 3000 2
lenovo 30000 3
chicken 10 3

'''

####笨拙的方法
with open('db.txt', encoding='utf-8') as f:
    l = []
    for line in f:
        goods = line.split()##取出每一行按空格做切分
        price = float(goods[1])###取出商品的价钱
        count = int(goods[2])####取出商品的个数
        cost = price * count####价钱乘以个数
        l.append(cost)#加入到列表中

    print(sum(l))  # 196060.0



####利用生成器表达式
with open('db.txt', encoding='utf-8') as f:
    l = (float(line.split()[1]) * int(line.split()[2]) for line in f)
    print(sum(l))




###把db文件里的商品取出来放到一个列表里去，最终实现[{'name': 'apple', 'price': 333, 'count': 3}, ]
with open('db.txt', encoding='utf-8') as f:
    info = [{'name': line.split()[0],
             'price': float(line.split()[1]),
             'count': int(line.split()[2])} for line in f if float(line.split()[1]) >= 30000]
    print(info)