Python if-else语句

决策是几乎所有编程语言中最重要的方面。顾名思义，决策允许我们针对特定的条件运行特定的代码块。在这里，决策是基于特定条件的有效性进行的。条件检查是决策制定的基础。

在Python中，使用以下语句进行决策制定。

语句	描述
If语句	If语句用于测试特定的条件。如果条件为真，则执行一段代码块（if-block）。
If-else语句	If-else语句与if语句类似，只是它还提供了用于检查条件为假时的代码块。如果if语句中的条件为假，则执行else语句。
嵌套if语句	嵌套if语句使我们能够在外部if语句内部使用if-else语句。

Python中的缩进

为了编程方便和简单性，Python不允许使用圆括号来标记代码块的级别。在Python中，缩进用于声明一个代码块。如果两个语句处于相同的缩进级别，则它们属于同一个代码块。

通常，在Python中使用四个空格进行缩进是一种典型的缩进量。

缩进是Python语言中最常用的部分，因为它声明了代码块。一个代码块中的所有语句都以相同级别的缩进进行缩进。我们将看到实际的缩进如何在Python中进行决策制定和其他事情。

if语句

if语句用于测试特定条件，如果条件为真，则执行一段被称为if-block的代码块。if语句的条件可以是任何有效的逻辑表达式，该表达式可以求值为真或假。

if语句的语法如下所示:

if expression:  
    statement  

示例 1

# Simple Python program to understand the if statement  
num = int(input("enter the number:"))         
# Here, we are taking an integer num and taking input dynamically  
if num%2 == 0:      
# Here, we are checking the condition. If the condition is true, we will enter the block  
    print("The Given number is an even number")    

输出:

enter the number: 10
The Given number is an even number 

示例2：编写程序打印三个数字中的最大值

# Simple Python Program to print the largest of the three numbers.  
a = int (input("Enter a: "));    
b = int (input("Enter b: "));    
c = int (input("Enter c: "));    
if a>b and a>c:    
# Here, we are checking the condition. If the condition is true, we will enter the block  
    print ("From the above three numbers given a is largest");    
if b>a and b>c:    
# Here, we are checking the condition. If the condition is true, we will enter the block  
    print ("From the above three numbers given b is largest");    
if c>a and c>b:    
# Here, we are checking the condition. If the condition is true, we will enter the block  
    print ("From the above three numbers given c is largest");    

输出:

Enter a: 100
Enter b: 120
Enter c: 130
From the above three numbers given c is largest 

if-else语句

if-else语句结合了if语句和else语句，在条件不满足时执行else块。

如果条件为真，则执行if块。否则，执行else块。

if-else语句的语法如下所示:

if condition:  
    #block of statements   
else:   
    #another block of statements (else-block)   

示例1：检查一个人是否有资格投票

# Simple Python Program to check whether a person is eligible to vote or not.  
age = int (input("Enter your age: "))    
# Here, we are taking an integer num and taking input dynamically  
if age>=18:    
# Here, we are checking the condition. If the condition is true, we will enter the block  
    print("You are eligible to vote !!");    
else:    
    print("Sorry! you have to wait !!");    

输出:

Enter your age: 90
You are eligible to vote !!

示例2：检查一个数字是否为偶数

# Simple Python Program to check whether a number is even or not.  
num = int(input("enter the number:"))         
# Here, we are taking an integer num and taking input dynamically  
if num%2 == 0:      
# Here, we are checking the condition. If the condition is true, we will enter the block  
    print("The Given number is an even number")    
else:    
    print("The Given Number is an odd number")    

输出:

enter the number: 10
The Given number is even number

elif语句

elif语句使我们能够检查多个条件，并根据其中一个条件的真实性执行特定的代码块。根据需要，我们的程序中可以有任意数量的elif语句。但是，使用elif是可选的。

elif语句在C语言中类似于if-else-if梯形语句。它必须跟在if语句之后。

elif语句的语法如下所示:

if expression 1:   
    # block of statements   

elif expression 2:   
    # block of statements   

elif expression 3:   
    # block of statements   

else:   
    # block of statements  

示例 1

# Simple Python program to understand elif statement  
number = int(input("Enter the number?"))    
# Here, we are taking an integer number and taking input dynamically  
if number==10:    
# Here, we are checking the condition. If the condition is true, we will enter the block  
    print("The given number is equals to 10")    
elif number==50:  
# Here, we are checking the condition. If the condition is true, we will enter the block    
    print("The given number is equal to 50");    
elif number==100:    
# Here, we are checking the condition. If the condition is true, we will enter the block  
    print("The given number is equal to 100");    
else:    
    print("The given number is not equal to 10, 50 or 100");    

输出:

Enter the number?15
The given number is not equal to 10, 50 or 100

示例 2

# Simple Python program to understand elif statement  
marks = int(input("Enter the marks? "))    
# Here, we are taking an integer marks and taking input dynamically  
if marks > 85 and marks <= 100:  
# Here, we are checking the condition. If the condition is true, we will enter the block    
   print("Congrats ! you scored grade A ...")    
elif marks > 60 and marks <= 85:    
# Here, we are checking the condition. If the condition is true, we will enter the block  
   print("You scored grade B + ...")    
elif marks > 40 and marks <= 60:  
# Here, we are checking the condition. If the condition is true, we will enter the block    
   print("You scored grade B ...")    
elif (marks > 30 and marks <= 40):    
# Here, we are checking the condition. If the condition is true, we will enter the block  
   print("You scored grade C ...")    
else:    
   print("Sorry you are fail ?")    

输出:

Enter the marks? 89
Congrats ! you scored grade A ...