flow control statement

1️⃣Introduction

Computers have three organizational structures when executing code:

• Sequential structure: Programs are executed sequentially from top to bottom.
• Selection structure: select and execute different code blocks according to conditions.
• Loop structure: Repeatedly execute a certain block of code.

2️⃣Conditional judgment

If a program is followed step by step from beginning to end without turning points in the middle, it will not be able to complete too much work. Program design often requires a turning point, which is called flow control in programming.

⚜️ Relational Operators

Relational operators are often needed for comparisons in selection constructs and loop constructs.

The relational operators in Python are as follows ⬇️:

The result returned by the operation is of boolean type. Returns True for True and False for False.

⚜️ Logical operators

• and: Both conditions are met. (and)
• or: satisfy condition 1 or satisfy condition 2. (or)
• not: The opposite of the conditional result. (No)

andThe following is a diagrammatic description of logical operators ⬇️:

Cleverly remember the formula:one false is false

orThe following is a diagrammatic description of logical operators ⬇️:

Cleverly remember the formula:true is true

The following is a textual description of the logical operators not⬇️:

If the expression is true (True), the result of the not expression is false (False); if the expression is false (False), the result of the not expression is true (True).

⚜️if statement

The basic syntax of an if statement is as follows ⬇️:

if (conditional judgment):
​ code block

If the result of the conditional expression is true, the statement is executed, and if it is false, it is skipped.
Case ⬇️

age = 18
if age == 18:		# 判断18是否等于变量age中的值
    print('您已经成年了。')

If the code block has only one statement it can be simplified:

if (conditional judgment): code block

Case ⬇️

age = 18
if age >= 18: print('您已经成年了。')

⚜️if...else statement

A more common situation in programming is to execute a block of code when a condition is judged to be true (True); to execute another block of code when the condition is false (False). if... elseThis is where sentences are needed .

The basic syntax of the if...else statement is as follows ⬇️:

if (conditional judgment):

​ Code block 1

else:

​ Code block 2

Case ⬇️

age = 18
if age >= 18:				# 判断条件，如果满足执行代码块1不满足执行代码块2
    print('您已经成年了。')
else:
    print('您还未成年。')

⚜️if...elif...else statement

When the program requires multiple conditional judgments, relying solely on if or if...else cannot meet our needs. This is the if...elif...elsestatement we can use.

The basic syntax of the if... elif...else statement is as follows ⬇️:

if (conditional judgment):

​ Code block 1

elf:

​ Code block 2

…

else:

​ code block n

Case ⬇️

score = 93
if score >= 90:				
   print('您的成绩等级为A')
elif score >= 80:
   print('您的成绩等级为B')
elif score >= 60:
   print('您的成绩等级为C')
else:
   print('您的成绩等级为D')

⚜️match…case

Python 3.10 adds the conditional judgment match...caseof . The function is similar to the switch in the Java language.

Python3.10 installation package link: https://pan.baidu.com/s/1S3o10uIyhjmIkVtDc_ae0g?pwd=h1ur
Extraction code: h1ur:

The basic syntax of the match...case statement is as follows ⬇️:

match (conditional judgment):

​ case value 1:

​ Code block 1

​ case value 2:

​ Code block 2

​…

​        case _:

​ code block n

_match any result with a single underscore

Case ⬇️

score = 90
match score//10:
        case 10,9:
            print('您的成绩等级为A')
        case 8:
            print('您的成绩等级为B')
        case 6,7:
            print('您的成绩等级为C')
        case _:
            print('您的成绩等级为D')

3️⃣Loop structure

⚜️while

The four steps of the while loop are: initial value, loop condition, loop body, iteration

The basic syntax of the while loop is as follows ⬇️:

while (conditional judgment) :

​ code block

Case ⬇️

num = 1				# 1.赋初始值
sum = 0
while num <= 100:	# 循环条件
    sum += num		# 循环体
    num += 1		# 迭代更新，改变条件变量
print('1~100之间的累加和为：',sum)

⚜️while...else statement

When the condition is judged to be true (True), the code block in the loop is executed; when the condition is false (Flase), the code block in the else is executed.

The basic syntax of while...else is as follows ⬇️:

while (conditional judgment) :

​ Code block 1

else:

​ Code block 2

Case ⬇️

num = 1
sum = 0
while num <= 100:
    sum += num
    num += 1
else:
	print('1~100之间的累加和为：',sum)

⚜️for

The basic syntax of a for loop is as follows ⬇️:

for (loop variable) in (traversal object):

​ code block

The traversal object can be: string, file, list, range function, etc.

Case ⬇️

for i in range(1,11,2):
    print(i)	# 打印1~10之间的奇数

⚜️for...else statement

When the condition is judged to be true (True), the code block in the loop is executed; when the condition is false (Flase), the code block in the else is executed.

The basic syntax of for...else is as follows ⬇️:

for loop variable in (traversal object):

​ Code block 1

else:

​ Code block 2

Case ⬇️

for i in range(1,11,2):
    print(i)	# 打印1~10之间的奇数
else:
    print('程序结束')

4️⃣ Exit the loop

⚜️continue

If you want to skip the loop when certain conditions occur, you can use it at this time continue, and continue generally needs to be used with if.

The basic syntax of continue is as follows ⬇️:

for (loop variable) in (traversal object):

​ code block

​ if (conditional judgment): # If the condition is true, exit this loop

​               continue

Case ⬇️

for i in range(1,11):
    if i % 2 == 0:		# 如果i是偶数跳过
    	continue		
    print(i)	# 打印1~10之间的奇数

⚜️break

If you want to exit the entire loop when certain conditions occur, you can use it at this time break, and break also needs to be used in conjunction with if.

The basic syntax of break is as follows ⬇️:

for (loop variable) in (traversal object):

​ code block

​ if (conditional judgment): # If the condition is true, exit the loop

​                break

Case ⬇️

for i in range(1,11,2):
    print(i)	# 打印1~10之间的奇数
    if i == 7:		# 当i等于7时提前退出循环
        break

Note: In the case of for...else or while...else, use break, the statement after else will not be executed.