【Python】-3.Python中的流程控制

所有编程语言在编写时都要遵循语言结构和流程控制，它们控制了整个程序运行的步骤。流程控制包括顺序控制、条件控制和循环控制。所谓控制顺序，就是按照正常的代码执行顺序，从上到下、从文件头到文件尾依次指定每条语句。

1. if判断

1.1 if语句

1.1.1 if语法

语法：

if 表达式：
语句1（4个空格）
语句2

x = True
if x:
    print("It's True") # It's True

1.1.2 条件是数字

x = 18
if x:
    print("x is: ", x) # x is:  18

# 只有数字为0时是False
x = 0
if x:
    print("x is not zero")

1.1.3 条件是字符串

x = "Hello"
if x:
    print("x is: ", x) # x is:  Hello

# 只有字符串为空时是False
x = ""
if x:
    print("x is: ", x)

1.1.4 条件是列表、元组、字典

列表、元组、字典类型的数据，当他们不包含任何元素时，条件测试结果是 “False”；含有任意元素时，条件测试结果是 “True”

x1 = []
x2 = [1, 2]
if x1:
    print("x1")
if x2:
    print("x2") # x2

x3 = ()
x4 = (1, 2)
if x3:
    print("x3")
if x4:
    print("x4") # x4

x5 = {}
x6 = {"Hello": "World"}
if x5:
    print("x5")
if x6:
    print("x6") # x6

1.2 else语句

x = 0
if x:
    print("x is not zero")
else:
    print("x is zero")

# x is zero

1.3 elif语句

有的时候可能需要多个条件，这时可以使用 elif

x = 89
if x > 90:
    print("优")
elif x > 80:
    print("良")
elif x > 60:
    print("及格")
# 良

x = 49
if x > 90:
    print("优")
elif x > 80:
    print("良")
elif x > 60:
    print("及格")
else:
    print("不及格")
# 不及格

2. 循环

Python中主要有两种循环结构：while循环和for循环

2.1 while循环

语法：

while 语法：
while 表达式:
语句1
语句2

x = 1
while x <= 10:
    print(x)
    x += 1
# 1
# 2
# 3
# ...
# 10

2.2 for循环

2.2.1 语法

for 语法：
for 变量 in 序列:
语句1
语句2

for x in (1, 2, 3, 4, 5, 6, 7, 8, 9, 10):
    print(x)
# 1
# 2
# 3
# ...
# 10

2.2.2 range

rang 函数作为迭代条件

for x in range(10):
    print(x)
# 1
# 2
# 3
# ...
# 9

rang 定义开始和结尾：不包含结尾

for x in range(1, 10):
    print(x)
# 1
# 2
# 3
# ...
# 9

range 定义步长

for x in range(1, 10, 2):
    print(x)
# 1
# 3
# 5
# 7
# 9

2.3 break和continue

break 的作用是立即退出循环

for i in range(10):
    if i > 5:
        break
    print(i)
# 0
# 1
# 2
# 3
# 4
# 5

continue 作用是跳过当前循环体，执行下次循环

for i in range(10):
    if i == 5:
        continue
    print(i)
# 0
# 1
# 2
# 3
# 4
# 6
# 7
# 8
# 9

3. pass语句

在Python中的pass语句是空语句，其作用是保持结构的完整性。pass不做任何操作，一般用作占位语句

for i in range(10):
    if i == 3:
        pass
    else:
        print(i)

# 0
# 1
# 2
# 3
# 4
# 6
# 7
# 8
# 9        

x = 35
if x == 35:
    # TODO
    pass
else:
    print("x 不是 35")

4. 循环中的else

在Python中，不只if可以和else组合，while和for也可以和else组合出现

4.1 while+else

count = 0
while count < 5:
    print(count, " is less than 5")
    count = count + 1
else:
    print(count, " is not less than 5")

# 0  is less than 5
# 1  is less than 5
# 2  is less than 5
# 3  is less than 5
# 4  is less than 5
# 5  is not less than 5

4.2 while+else+break

如果中途break退出while循环，不会执行else后的代码块

count = 0
while count < 5:
    print(count, " is less than 5")
    if count == 3:
        break
    count = count + 1
else:
    print(count, "is not less than 5")

# 0  is less than 5
# 1  is less than 5
# 2  is less than 5
# 3  is less than 5

4.3 for+else

for count in range(5):
    print(count, " in for segment")
else:
    print(count, " in else segment")

# 0  in for segment
# 1  in for segment
# 2  in for segment
# 3  in for segment
# 4  in for segment
# 4  in else segment

4.4 for+else+break

如果中途break退出for循环，不会执行else后的代码块

for count in range(5):
    print(count, " in for segment")
    if count == 3:
        break
else:
    print(count, " in else segment")

# 0  in for segment
# 1  in for segment
# 2  in for segment
# 3  in for segment