circulation 循环

if

judgement 必须是一个布尔类型

if judgement:
    operation

if judgement:
    operation
else:
    operation

if judgement:
    operation
elif judgement:
    operation
else
    operation

真值判断

对象/常量	True or False
""	False
"string"	True，空字符串为False
0	False，其他数字为True
>=1	True
\<=-1	True
() 空元组	False
[] 空列表	False
	False
None	False
bool	True为True False为False
object对象	None为False，其他为True

In [13]: if "":
    ...:     print('yes')
    ...: else:
    ...:     print('no')
    ...:
no

In [4]: if True:
   ...:     print('Hello Python')
   ...:     
Hello Python

In [5]: if 2 > 1: 
   ...:     print('2 is greater than 1')
   ...:     
2 is greater than 1

In [6]: if 1 > 2:
   ...:     print('1 is greater than 2')
   ...: else:
   ...:     print('1 is not greater than 2')
   ...:     
1 is not greater than 2

In [7]: if 1 > 2:
   ...:     print('1 is greater than 2')
   ...: elif 2 > 1:
   ...:     print('1 is not greater than 2')
   ...: else:
   ...:     print('1 is equal to 2')
   ...:     
1 is not greater than 2

while

judgement 为True 进入循环体

循环正常执行结束，执行else子句

使用break 终止，else子句不会执行

while judgement:
    operation
else:
    operation

In [8]: n = 1

In [9]: while n <= 10:
   ...:     print(n)
   ...:     n += 1
   ...:     
1
2
3
4
5
6
7
8
9
10

for

for i in range(10):
    print(i)
>>>
0
1
2
3
4
5
6
7
8
9

In [72]: items = {'a': 1, 'b':2, 'c': 3}                                                                                                                                     

In [73]: for i in items: 
    ...:     print(i) 
    ...:                                                                                                                                                                     
a
b
c

for... else

完成循环，就执行else

guess_num = 34

for i in range(1):
    my_num = int(input("my_num: "))
    if my_num == guess_num:
        print('Yes')
        break
    else:
        print("No")
else:
    print('This is else from for circulation.')
>>>
my_num: 1
No
This is else from for circulation.

破坏循环，跳过else

guess_num = 34

for i in range(1):
    my_num = int(input("my_num: "))
    if my_num == guess_num:
        print('Yes')
        break
    else:
        print("No")
else:
    print('This is else from for circulation.')
>>>
my_num: 34
Yes

for vs while

下面二者等价

i = 0
while i < 1000000:
    i += 1

for i in range(0, 1000000):
    pass

要知道，range()函数是直接由C语言写的，调用它速度非常快。而while循环中的“i += 1”这个操作，得通
过Python的解释器间接调用底层的C语言;并且这个简单的操作，又涉及到了对象的创建和删除(因为i是整
型，是immutable，i += 1相当于i = new int(i + 1))。所以，显然，for循环的效率更胜一筹。

break

结束整个循环, 以后的循环都不会被执行

for i in range(5):
    if i == 2:
        break
    print(i)
>>>
0
1

循环嵌套中，只会影响语句所在的那一层循环

In [17]: for i in range(0, 3):
    ...:     print(i)
    ...:     for j in range(10, 16):
    ...:         if j == 11:
    ...:             break
    ...:         print(j)
    ...:
0
10
1
10
2
10

continue

跳出本次循环，进入下一次循环

for i in range(5):
    if i == 2:
        continue
    print(i)
>>>
0
1
3
4

循环嵌套中，只会影响语句所在的那一层循环

In [18]: for i in range(0, 3):
    ...:     print(i)
    ...:     for j in range(10, 16):
    ...:         if j == 11:
    ...:             continue
    ...:         print(j)
    ...:
0
10
12
13
14
15
1
10
12
13
14
15
2
10
12
13
14
15