Python教程-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决策运行特定的代码块。在这里，决策是根据特定条件的有效性而做出的。条件检查是决策制定的基础。

在Python中，使用以下语句执行决策制定。

语句	描述
if 语句	if 语句用于测试特定条件。如果条件为真，则会执行一段代码块（if-块）。
if - else 语句	if-else 语句与 if 语句类似，只是还提供了条件的假情况的代码块。如果 if 语句中提供的条件为假，则会执行 else 语句。
嵌套的 if 语句	嵌套的 if 语句使我们可以在外部 if 语句内部使用 if - else 语句。

Python 中的缩进

为了编程方便和实现简单性，Python 不允许使用括号来表示块级别的代码。在Python中，缩进用于声明一个代码块。如果两个语句具有相同的缩进级别，那么它们是同一代码块的一部分。

通常，给出四个空格来缩进语句，这是Python中典型的缩进量。

缩进是Python语言中最常用的部分，因为它声明了代码块。一个块的所有语句都具有相同级别的缩进。我们将看到实际的缩进如何在决策制定和其他Python内容中发生。

if 语句

if 语句用于测试特定条件，如果条件为真，则执行一个称为 if-块 的代码块。if 语句的条件可以是任何有效的逻辑表达式，可以计算为真或假。

if 语句的语法如下：

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：编写一个程序，找出三个数中的最大数。

# 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 语句提供了一个与 if 语句结合的 else 块，该块在条件的假情况下执行。

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：

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

示例 1：编写一个程序，判断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：编写一个程序，判断一个数字是偶数还是奇数。

# 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 语句允许我们检查多个条件，并根据它们之间的真实条件执行特定的语句块。根据需要，我们的程序可以具有任意数量的 elif 语句。然而，使用 elif 是可选的。

elif 语句的工作方式类似于C中的 if-else-if 梯形语句。它必须跟在 if 语句后面。

elif 语句的语法如下：

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 ...