Understanding Python's if...else Statement with Examples

Conditional statements are an essential part of programming. They allow a program to execute certain blocks of code only when specific conditions are met. This article will cover the if, if...else, and if...elif...else statements in Python, complete with examples for clarity.

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1. Python if Statement

The if statement is used to execute a block of code only if a condition evaluates to True.

Syntax:

if condition:
    # Code to execute if the condition is True

• The condition is a Boolean expression (e.g., number > 0) that evaluates to either True or False.

Example:

number = int(input('Enter a number: '))

if number > 0:
    print(f'{number} is a positive number.')

print('This statement always executes.')

Output 1:

Enter a number: 10
10 is a positive number.
This statement always executes.

Output 2:

Enter a number: -5
This statement always executes.

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2. Indentation in Python

Python uses indentation to define code blocks. Failing to use proper indentation will raise an error.

Correct Example:

x = 1
total = 0

if x != 0:
    total += x
    print(total)  

print("This statement always executes.")

Incorrect Example:

x = 1
total = 0

if x != 0:
total += x
print(total)  # This will cause an indentation error

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3. Python if...else Statement

The else clause can be used with if to specify an alternative block of code to execute if the condition evaluates to False.

Syntax:

if condition:
    # Code block if condition is True
else:
    # Code block if condition is False

Example:

number = int(input('Enter a number: '))

if number > 0:
    print('Positive number')
else:
    print('Not a positive number')

print('This statement always executes.')

Output 1:

Enter a number: 10
Positive number
This statement always executes.

Output 2:

Enter a number: -2
Not a positive number
This statement always executes.

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4. Python if...elif...else Statement

When there are multiple conditions to evaluate, the if...elif...else statement is used.

Syntax:

if condition1:
    # Code block 1
elif condition2:
    # Code block 2
else:
    # Code block 3

Example:

number = int(input('Enter a number: '))

if number > 0:
    print('Positive number')
elif number < 0:
    print('Negative number')
else:
    print('Zero')

print('This statement always executes.')

Output:

Enter a number: -5
Negative number
This statement always executes.

• Only one block of code is executed, even if multiple conditions are True.

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5. Python Nested if Statements

You can nest an if statement inside another if statement to handle more complex scenarios.

Example:

number = int(input('Enter a number: '))

if number >= 0:
    if number == 0:
        print('Number is 0')
    else:
        print('Number is positive')
else:
    print('Number is negative')

Output:

Enter a number: 5
Number is positive

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6. Real-World Example: Grading System

Let’s apply these concepts to a real-world scenario: assigning grades to students based on their scores.

Example:

score = int(input('Enter your score: '))

if score > 90:
    grade = 'A'
elif score > 75:
    grade = 'B'
elif score > 65:
    grade = 'C'
else:
    grade = 'F'

print(f'Your grade is: {grade}')

Output 1:

Enter your score: 88
Your grade is: B

Output 2:

Enter your score: 64
Your grade is: F

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Key Takeaways

• if executes code only when the condition is True.
• if...else provides an alternative when the condition is False.
• if...elif...else handles multiple conditions.
• Proper indentation is crucial in Python to define code blocks.

With these examples and explanations, you should now have a clear understanding of how to use conditional statements in Python effectively!