Basics | Python flow control statements

This issue brings you the flow control statements that we often use when writing programs: Sequential execution statements, selection execution statements, and loop execution statements , the corresponding structures are: Sequential structure, selection structure (branch structure), loop structure,Hope to help you Helps.

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1. Sequential structure

The sequential structure has no keywords. The default execution structure of the computer program is the sequential structure, that is, from top to bottom, from left to right, sequential execution.

##2. Select structure

2.1 Single branch

Statement format:

if 判断条件:
    代码块

If the code block of a single branch statement has only one statement, you can write the if statement and the code on the same line:

if 判断条件: 一句代码

Example:

"""
登录密码
"""

user = 'Python 当打之年'
password = '123456'

# 用法1
if user == 'Python 当打之年':
    print('用户名正确！')

# 用法2
if password == '123456': print('密码正确！')

# 用法3
if user == 'Python 当打之年':
    if password == '123456':
        print('用户名和密码正确！')

# 用法4
if user == 'Python 当打之年' and password == '123456':
    print('用户名和密码正确！')

##2.2 Dual Branch

语句格式：

if 判断条件:
    代码块1
else:
    代码块2

如果判断条件为真，则执行代码块1，否则执行代码块2

示例：

user = 'Python 当打之年'
password = '123456'

if user == 'Python 当打之年':
    print('用户名正确！')
else:
    print('用户名错误！')
if password == '123456':
    print('密码正确！')
else:
    print('密码错误！')

2.3 多分支

语句格式：

if 判断条件1:
    代码块1
elif 判断条件2:
    代码块2
...
elif 判断条件n:
    代码块n
else:
    代码块n+1

依次遍历每个判断条件，如果判断条件为真，则执行相应代码块，否则执行最后一个代码块，所有代码块中有且仅有一个代码块会被执行。

示例：

"""
成绩等级
"""
score = float(input('请输入成绩: '))
if score >= 90:
    grade = 'A'
elif score >= 80:
    grade = 'B'
elif score >= 70:
    grade = 'C'
elif score >= 60:
    grade = 'D'
else:
    grade = 'E'
print('学生成绩等级为: ', grade)

3. 循环结构

3.1 for循环

语句格式：

for 迭代变量 in 可迭代对象:
    循环体语句

依次遍历迭代对象，执行循环体语句

示例：

for i in range(10):
    print('i = ', i)

# 100以内偶数和
sum = 0
for i in range(2, 101, 2):
    sum += i
print('sum = ', sum)

关于 range (前闭后开):

range(100): Generate an integer ranging from 0 to 100. Note that 100 cannot be obtained.

range(1, 100): Generate an integer ranging from 1 to 100.

range(1, 100, 2): Generate odd numbers from 1 to 100, 2 is the step size.

range(100, 0, -2): Generate an even number from 100 to 1, -2 is the step size.

3.2 while循环

语句格式：

while 判断条件：
    代码块

当判断条件为真时，执行代码块，直到判断条件为假时退出。

示例：

# 用法1
i = 0
while i < 10:
    print(&#39;i = &#39;, i)
    i += 1

# 用法2
i = 0
while True:
    if i < 10:
        print(&#39;i = &#39;, i)
        i += 1
    else:
        break

注意：代码块内一定要有退出条件，否则会出现死循环

3.3 break、continue、pass

break: 在代码块执行过程中终止循环，并且跳出整个循环

continue: Terminate the current loop during the execution of the code block, jump out of the loop, and execute the next loop.

pass: is an empty statement to maintain the integrity of the program structure.

Example:

"""
输出 0-10 之间大于 2 的奇数
"""
n = 10
while n > 0:
    n -= 1
    if n == 2:
        break
    if n % 2 == 0:
        continue
    else:
        pass
        print('执行pass语句')
        print(n)
        
# 执行pass语句
# 9
# 执行pass语句
# 7
# 执行pass语句
# 5
# 执行pass语句
# 3

3.4 嵌套循环结构

语句格式：

# while 嵌套
while 条件表达式1:
   while 条件表达式2：
   循环体2
   循环体1

# for 嵌套
for 迭代变量1 in 迭代对象1:
   for 迭代变量2 in 迭代对象2:
   循环体2
   循环体1

也可以在循环体内嵌入其他的循环体，如在while循环中嵌入for循环， 在for循环中嵌入while循环。

示例：

"""
九九乘法表
"""
for i in range(1, 10):
    for j in range(1, i + 1):
        print(f'{i}*{j}={i * j}', end='\t')
    print()
    
# 1*1=1
# 2*1=2 2*2=4
# 3*1=3 3*2=6 3*3=9
# 4*1=4 4*2=8 4*3=12 4*4=16
# 5*1=5 5*2=10 5*3=15 5*4=20 5*5=25
# 6*1=6 6*2=12 6*3=18 6*4=24 6*5=30 6*6=36
# 7*1=7 7*2=14 7*3=21 7*4=28 7*5=35 7*6=42 7*7=49
# 8*1=8 8*2=16 8*3=24 8*4=32 8*5=40 8*6=48 8*7=56 8*8=64
# 9*1=9 9*2=18 9*3=27 9*4=36 9*5=45 9*6=54 9*7=63 9*8=72 9*9=81

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