The code we saw earlier is executed in sequence , That is to say, the first step is to execute the first step 1 statement , And then there's the 2 strip 、 The first 3 strip …… Until the last sentence , This is called sequential structure .
But in many cases , Sequence structured code is not enough , For example, a program is limited to adults only , Children are not old enough , No permission to use . At this point, the program needs to make a judgment , See if the user is an adult , And give tips .
stay [Python] in , have access to if else Statement to judge the condition , Then execute different code according to different results , This is called a selective structure or branching structure .
Python Medium if else Statements can be broken down into three forms , Namely if sentence 、if else Statement and if elif else sentence , Their syntax and execution flow are shown in table 1 Shown .
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Of the above three forms , The second and third forms are interlinked , If elif The block does not appear , It becomes the second form . in addition ,elif and else Can't be used alone , It has to be with if Come together , And pair it right .
An explanation of grammatical forms ：
:, Don't forget it .Once an expression holds ,Python It will execute the corresponding code block after it ; If all expressions don't hold , Then perform else The following code block ; without else part , Then do nothing . Last , If you don't have a lot of time , And want to quickly python Improve , The most important thing is not afraid of hardship , I suggest you can put up a letter （ Homophone ）：276 3177 065 , That's really good , A lot of people are making rapid progress , I need you not to be afraid of hardship ！ You can take a look at it ~, Including a copy compiled by Xiaobian 2022 Abreast of the times Python Information and 0 Introduction to Basics , Welcome to junior and advanced students . In not busy time I will give you the solution
The simplest form of execution is the first form —— only one if part . If the expression holds （ really ）, Just execute the following code block ; If the expression doesn't hold （ false ）, Just do nothing .
For the second form , If the expression holds , Is executed if The code block that follows 1; If the expression doesn't hold , Is executed else The code block that follows 2.
For the third form ,Python It will judge whether the expression is true from top to bottom , Once you come across a valid expression , Just execute the following block of statements ; here , The rest of the code is no longer executed , Whether the following expression holds or not . If all the expressions don't hold , Is executed else The following code block .
In total , No matter how many branches there are , Can only execute one branch , Or none of them , Multiple branches cannot be performed at the same time .
【 example 1】 Use the first selection structure to determine whether the user meets the criteria ：
1. age = int( input(" Please enter your age ：") )
3. if age < 18 :
4. print(" You're underage , It is recommended to use the software with family members ！")
5. print(" If you have the consent of your parents , Please ignore the above tips .")
7. # The statement does not belong to if Code block for
8. print(" The software is in use ...")
Running results 1：
Please enter your age ：16
You're underage , It is recommended to use the software with family members ！
If you have the consent of your parents , Please ignore the above tips .
The software is in use …
Running results 2：
Please enter your age ：24
The software is in use …
As you can see from the results , If the entered age is less than 18, Is executed if Statement block after ; If the entered age is greater than or equal to 18, No execution if Statement block after . The sentence block here is two of the four spaces indented print() sentence .
【 example 2】 Improve the code above , Exit the program when you don't meet your age ：
1. import sys
3. age = int( input(" Please enter your age ：") )
5. if age < 18 :
6. print(" Warning ： You're underage , You can't use the software ！")
7. print(" Minors should study hard , Go to a good University , Serve the motherland .")
8. sys.exit()
9. else:
10. print(" You are an adult , You can use the software .")
11. print(" Precious time , Please don't waste too much time on the software .")
13. print(" The software is in use ...")
Running results 1：
Please enter your age ：16
Warning ： You're underage , You can't use the software ！
Minors should study hard , Go to a good University , Serve the motherland .
Running results 2：
Please enter your age ：20
You are an adult , You can use the software .
Precious time , Please don't waste too much time on the software .
The software is in use …
sys Modular exit() Function is used to exit the program .
【 example 3】 Judge whether a person's figure is reasonable ：
1. height = float(input(" Enter the height （ rice ）："))
2. weight = float(input(" Enter the weight （ kg ）："))
3. bmi = weight / (height * height) # Calculation BMI Index
5. if bmi<18.5:
6. print("BMI Index is ："+str(bmi))
7. print(" Underweight ")
8. elif bmi>=18.5 and bmi<24.9:
9. print("BMI Index is ："+str(bmi))
10. print(" normal range , Pay attention to keep ")
11. elif bmi>=24.9 and bmi<29.9:
12. print("BMI Index is ："+str(bmi))
13. print(" Overweight ")
14. else:
15. print("BMI Index is ："+str(bmi))
16. print(" obesity ")
Running results ：
Enter the height （ rice ）：1.7
Enter the weight （ kg ）：70
BMI Index is ：24.221453287197235
normal range , Pay attention to keep
It's important to note that ,Python Is a very unique programming language , It identifies code blocks by indenting , Lines of code with the same indentation belong to the same block of code , So you can't just indent , This can easily lead to grammatical errors . Last , If you don't have a lot of time , And want to quickly python Improve , The most important thing is not afraid of hardship , I suggest you can put up a letter （ Homophone ）：276 3177 065 , That's really good , A lot of people are making rapid progress , I need you not to be afraid of hardship ！ You can take a look at it ~, Including a copy compiled by Xiaobian 2022 Abreast of the times Python Information and 0 Introduction to Basics , Welcome to junior and advanced students . In not busy time I will give you the solution
In other languages （ Such as C Language 、[C++]、[Java] etc. ）, The selection structure also includes switch sentence , Multiple choices can also be achieved , But in Python There is no switch sentence , So when it comes to multiple choices , Only use if else Branch statement .
It said ,if and elif hinder “ expression ” It's very free form , As long as the expression has a result , Whatever the type of result ,Python It can be judged that it is “ really ” still “ false ”.
Boolean type （bool） There are only two values , Namely True and False,Python Will be able to True treat as “ really ”, hold False treat as “ false ”.
For numbers ,Python Will be able to 0 and 0.0 treat as “ false ”, Think of other values as “ really ”.
For other types , When the object is empty or is None when ,Python They will be treated as “ false ”, Other situations are treated as true . such as , The following expressions are not true ：
“” # An empty string
[ ] # An empty list
( ) # An empty tuple
{ } # An empty dictionary
None # Null value
【 example 】if elif Judge various types of expressions ：
1. b = False
2. if b:
3. print('b yes True')
4. else:
5. print('b yes False')
7. n = 0
8. if n:
9. print('n It's not zero ')
10. else:
11. print('n It's zero ')
13. s = ""
14. if s:
15. print('s Not an empty string ')
16. else:
17. print('s Is an empty string ')
19. l = []
20. if l:
21. print('l It's not an empty list ')
22. else:
23. print('l It's an empty list ')
25. d = {}
26. if d:
27. print('d It's not an empty dictionary ')
28. else:
29. print('d It's an empty dictionary ')
31. def func():
32. print(" Function called ")
34. if func():
35. print('func() The return value is not empty ')
36. else:
37. print('func() The return value is null ')
Running results ：
b yes False
n It's zero
s Is an empty string
l It's an empty list
d It's an empty dictionary
Function called
func() The return value is null
explain ： For no return Function of statement , The return value is null , That is to say None.
