迭代器:

1, 什麼是迭代器
器: 工具
迭代: 是一個重複的過程, 但是每次重複都是基於上一次的結果而來的

name = ["egon", "lqz", "tank"]

count = 1
while count < len(names):
   print(nmes[count])
   count += 1

迭代器: 就是一種不依賴索引的取值工具

2, 為何要有迭代器
特性:
1, 是一種通用的迭代取值方案
2, 惰性計算, 節省記憶體

3, 如何用迭代器

dic = {"name": "egon", 'age': 18, 'gender': "male"}

dic_iterator = dic.__iter__()
res1 = dic_iterator.__next__()
print(res1)
res2 = dic_iterator.__next__()
print(res2)
res = dic_iterator.__next__()
print(res)

dic_iterator.__next__()  # StopIteration  丟擲異常

while True:
   try:
      res = dic_iterator.__next__()
      print(res)
   except StopIteration:  # 解決丟擲異常
      break
 

可迭代物件與迭代器物件
內建有__iter__方法的型別稱之為: 可迭代物件 / 型別
字典
集合
檔案物件(也是迭代器物件)
字串
列表
元祖
迭代器物件: 內建有__iter_, __next___方法

dic = {"name": "egon", 'age': 18, 'gender': "male"}

dic_iterator = dic.__iter__()
dic_iterator.__next__()

for工作迴圈原理

dic = {"name": "egon", 'age': 18, 'gender': "male"}

dic_iterator = dic.__iter__()
while True:
   try:
      res = dic_iterator.__next__()
      print(res)
   except StopIteration:
      break

for k in dic:
   print(k)
 

步驟1
dic_iterator = dic.__iter__()
步驟2
k = dic_iterator.__next__(), 執行迴圈體程式碼
步驟3
迴圈往復，直到丟擲異常，for迴圈會幫我們捕捉異常結束迴圈

dic = {"name": "egon", 'age': 18, 'gender': "male"}
dic_iterator = dic.__iter__()
for k in dic_iterator:
   print(k)

print(dic_iterator)

四：基於同一迭代器的重複取值，效果如何？？？？
示例1：

dic = {"name": "egon", 'age': 18, 'gender': "male"}
dic_iterator = dic.__iter__()

while True:
   try:
      res = dic_iterator.__next__()
      print(res)
   except StopIteration:
      break

while True:
   try:
      res = dic_iterator.__next__()
      print(res)
   except StopIteration:
      break

示例2：

dic = {"name": "egon", 'age': 18, 'gender': "male"}
for k in dic:  # dic.__iter__()
   print(k)

for k in dic:  # dic.__iter__()
   print(k)

自定義迭代器來實現惰性計算, 從而達到節省記憶體的效果

生成器

什麼是生成器
但凡是函式內出現了yield關鍵字, 呼叫函式將不會執行函式體程式碼, 會得到一個返回值,
該返回值就是我們自定義的迭代器, 稱之為生成器

def func():
   print("hello")
   yield 1
   print("hello")
   yield 2
   print("hello")
   yield 3


g = func()
print(g)  # 生成器本身就是一個迭代器
res = next(g)
print(res)

res = next(g)
print(res)

res = next(g)
print(res)

yield與return

相同點: 都可以用來返回值
不同點:
return只能返回一次值, 函式就立刻結束了
yield能返回多次值, yield可以掛起函式

案例

def func():
   res = 0
   while True:
      res += 1
      yield res


g = func()
for i in g:
   print(i)

總結迭代器的優缺點
優點:
1, 是一種通用的迭代取值方案
2, 惰性計算, 節省記憶體

缺點:
1, 取值不如索引, key的取值方式靈活
2, 取值是一次性的, 只能往後取, 不能預估值得個數

案例:

def my_range(start, stop, step=1):
   while start < stop:
      yield start
      start += stop


for i in my_range(1, 5, 2):
   print(i)

生成式:

列表生成式

l = [i ** 2 for i in range(5) if i > 2]
print(l)
names = ['lqz_sb', 'yj_sb', 'jason_sb', 'egon']
l = [name for name in names if name.endswith("sb")]
print(l)

集合生成式

res = {i for i in range(3)}

print(res)

字典生成式

res = {f"k{i}": i ** 2 for i in range(5)}

print(res)

生成器表示式

res(i for i in range(5))
print(res, type(res))
nums = (i for i in range(200000))
res = sum(nums)
print(res)

with open("a.txt", mode='rt', encoding='utf-8')as f:
  # data = f.read()
  # print(len(data))

  # res = 0
  # for line in f:
  #    res += len(line)
  # res = sum(len(line) for line in f)

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