Python Conditional Statements: if, elif, else for Precise Control Flow

In programming, conditional statements are essential tools for controlling the execution flow of a program. Python provides three types of conditional statements: if, elif, and else, allowing us to execute different code blocks based on various conditions. This article will detail the usage of these three statements and provide example code to help readers understand.

1. if Statement

The most basic conditional statement is the if statement. When the given condition is true, the program will execute the corresponding code block.

1.1 Basic Structure

if condition:    # Execute code block

1.2 Example

Here is a simple example that checks if the number input by the user is positive:

number = int(input("Please enter a number: "))
if number > 0:    print("This number is positive")

In this example, if the user inputs a number greater than 0, the program will output “This number is positive”.

2. elif Statement

When there are multiple conditions to check, the elif (short for “else if”) statement can be used to add additional conditions. If the previous if condition is not met, the program will check the elif condition.

2.1 Basic Structure

if condition1:    # Execute code block 1
elif condition2:    # Execute code block 2

2.2 Example

The following example extends the previous functionality to determine if the input is a positive number, negative number, or zero:

number = int(input("Please enter a number: "))
if number > 0:    print("This number is positive")
elif number < 0:    print("This number is negative")
else:    print("This number is zero")

In this example, we first check if the user input is greater than 0; if not, we check if it is less than 0. If neither condition is met, it defaults to outputting “This number is zero”.

3. else Statement

When none of the previous if and elif conditions are satisfied, the else statement can be used to handle other cases. It is typically placed after all other conditions.

3.1 Basic Structure

if condition1:    # Execute code block 1
elif condition2:    # Execute code block 2
else:    # Execute default code block

3.2 Example

age = int(input("Please enter your age: "))
if age < 13:    print("You are still a child")
elif age < 20:    print("You are a teenager")
else:    print("You are an adult")

In this example, we classify based on the user’s input age. When the age is less than 13, it outputs “You are still a child”; if less than 20, it outputs “You are a teenager”; otherwise, it outputs “You are an adult”.

Conclusion

Through the above content, we have learned how to use the basic control flow structures in Python: if, elif, and else. They allow us to flexibly control the execution path of a program based on different situations. In practical development, this foundational knowledge is crucial for building complex logic. We hope that everyone can deepen their understanding of the Python programming language and its control flow mechanisms by practicing these concepts.