week4-python之迭代器

迭代：是一个重复的过程，每一次重复，都是基于上一次的结果而来

#单纯的重复
while True: 
    print('hello')

l=['a','b','c','d']
count=0
while count < len(l):
    print(l[count])
    count+=1

dic={'name':'egon','sex':'m',"age":18} #上述按照索引的取值方式，不适于没有索引的数据类型

　　

迭代器：可迭代对象iterable：凡是对象下有__iter__方法：对象.__iter__，该对象就是可迭代对象

s='hello'
l=['a','b','c','d']
t=('a','b','c','d')
dic={'name':'egon','sex':'m',"age":18}
set1={1,2,3}
f=open('db.txt')

s.__iter__()
l.__iter__()
t.__iter__()
dic.__iter__()
set1.__iter__()
f.__iter__()

　　

迭代器对象：可迭代对象执行内置的__iter__方法，得到的结果就是迭代器对象

dic={'name':'egon','sex':'m',"age":18}

i=dic.__iter__()
# print(i) #iterator迭代器

# i.__next__() #next(i)
print(next(i))
print(next(i))
print(next(i))
print(next(i)) #StopIteration

l=['a','b','c','d']

i=l.__iter__()
print(next(i))
print(next(i))
print(next(i))
print(next(i))
print(next(i)) #StopIteration

#不依赖于索引的取值方式

l=['a','b','c','d']
dic={'name':'egon','sex':'m',"age":18}
iter_l=iter(l)
iter_dic=iter(dic)
while True:
    try:
        # print(next(iter_l))
        k=next(iter_dic)
        print(k,dic[k])
    except StopIteration:
        break

什么是迭代器对象：

1.有__iter__,执行得到仍然是迭代本身

2.有__next__

迭代器对象的优点：

1. 提供了一种统一的（不依赖于索引的）迭代方式

2.迭代器本身，比起其他数据类型更省内存

　　

l=['a','b','c','d']
i=iter(l)

dic={'a':1,'b':2}
x=dic.keys()
print(x)
i=x.__iter__()

with open('a.txt') as f:
    # print(next(f))
    # print(next(f))
    # print(next(f))
    f.read()

　　

迭代器对象的缺点

1.一次性，只能往后走，不能回退，不如索引取值灵活

2.无法预知什么时候取值结束，即无法预知长度

l=['a','b','c','d']
i=iter(l)
print(next(i))
print(next(i))
print(next(i))

for循环原理

l=['a','b','c','d']
for item in l: #iter_l=l.__iter__()
    print(item)

for item in {1,2,3,4}:
    print(item)

with open('a.txt') as f:
     for line in f: #i=f.__iter__()
         print(line)
    print(f is f.__iter__())

判断可迭代对象与迭代器对象(了解)

　

from collections import Iterable,Iterator
s='hello'
l=['a','b','c','d']
t=('a','b','c','d')
dic={'name':'egon','sex':'m',"age":18}
set1={1,2,3}
f=open('a.txt')

print(isinstance(s,Iterable))
print(isinstance(l,Iterable))
print(isinstance(t,Iterable))
print(isinstance(dic,Iterable))
print(isinstance(set1,Iterable))
print(isinstance(f,Iterable))

print(isinstance(s,Iterator))
print(isinstance(l,Iterator))
print(isinstance(t,Iterator))
print(isinstance(dic,Iterator))
print(isinstance(set1,Iterator))
print(isinstance(f,Iterator))