列表生成器 和 生成器表达式

声明式编程风格
列表生成器
将一系列代码封装到一行，以列表的形式输出结果

#用一个列表生成多个值
l = []
for i in range(100):
    l.append('%s'%i)
print(l)

l = ['egg%s'%i for i in range(100) if i > 50]
print(l)

#读取文件的绝对路径
import os
g = os.walk('F:\egon')
file_path = []
for i in g:
    for j in i[-1]:
        file_path.append('%s\%s'%(i[0],j))
print(file_path)

l=['%s\%s'%(i[0],j) for i in g for j in i[-1]]
print(l)

#添加条件
l = [1,2,3,4]
s = 'hello'
l1 = []
for num in l:
    for s1 in s:
        t = (num ,s1)
        l1.append(t)
print(l1)

l2 = [(num,s1) for num in l for s1 in s if num > 2]
print(l2)


#生成器表达式
# #有时候程序的运行结果一个列表装不下，内存不够用，所以讲运行结果返回为一个生成器，一次返回一个结果
import os
g = os.walk('F:\')
l=('%s\%s'%(i[0],j) for i in g for j in i[-1])
print(next(l))
print(type(l))   #l为生成器

#求文件中商品总价
money= []
with open('a.txt') as f:
    for line in f:
        goods = line.split()       #每一行变成列表
        money.append(float(goods[1])*float(goods[2]))
print(sum(money))

with open('a.txt') as f:
    l=(float(line.split()[1])*float(line.split()[2]) for line in f)
    print(sum(l))    #sum 可以对可迭代对象求和    内置了iter next函数，将可迭代对象变为迭代器再执行next函数

#求文件中的每行变为一个字典放入一个列表中
res = []
with open('a.txt') as f:
    for line in f:
        l = line.split()
        d ={}
        d['name'] = l[0]
        d['price'] = l[1]
        d['count'] = l[2]
        res.append(d)
print(res)



with open('a.txt') as f:
    res = (line.split() for line in f)       #生成器表达式返回了一个  可以返回字符串内容的 生成器
    dic_g = ({'name':i[0],'price':i[1],'count':i[2]} for i in res )   #返回字典内容的生成器
    apple_dic = next(dic_g)
    print(apple_dic['count'])

with open('a.txt') as f:
    res = (line.split() for line in f)
    dic_g = ({'name':i[0],'price':i[1],'count':i[2]} for i in res if float(i[1]) > 10000)
    for j in dic_g:
        print(j)



应用

linux=['钢弹','小壁虎','小虎比','alex','wupeiqi','yuanhao']
python=['dragon','钢弹','zhejiangF4','小虎比']
#问题一：得出既报名linux又报名python的学生列表
l1 = []
for i in linux:
    for j in python:
        if i == j:
            l1.append(i)
print(l1)

l1 = [ i for i in linux for j in python if i == j]
print(l1)
#问题二：得出只报名linux，而没有报名python的学生列表
l2 = []
for i in linux:
    if i not in python:
        l2.append(i)
print(l2)
l2 = [i for i in linux if i not in python]
#问题三：得出只报名python，而没有报名linux的学生列表
l3 = []
for i in python:
    if i not in linux:
        l3.append(i)
print(l3)

l3 = [i for i in python if i not in linux]


shares={
   'IBM':36.6,
   'lenovo':27.3,
   'huawei':40.3,
   'oldboy':3.2,
   'ocean':20.1
   }

   #问题一：得出股票价格大于30的股票名字列表

l = [i for i in shares if float(shares[i]) > 30]
print(l)
   #问题二：求出所有股票的总价格
l =sum([shares[i] for i in shares])
print(l)

l=[10,2,3,4,5,6,7]
   #得到一个新列表l1，新列表中每个元素是l中对应每个元素值的平方
l = [i**2 for i in l]
print(l)
   #过滤出l1中大于40的值，然后求和
l = sum([i for i in l if i >40])
print(l)