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Python – Conditional Statements (if, elif, else) #

Conditional statements are the backbone of decision-making in programming. They allow your Python programs to execute certain blocks of code based on whether specific conditions are met. This guide will delve into Python’s conditional statements: if, elif, and else, providing detailed explanations and practical examples.

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Table of Contents #

1. Introduction to Conditional Statements
2. if Statement
   • Syntax
   • Example
3. elif Statement
   • Syntax
   • Example
4. else Statement
   • Syntax
   • Example
5. Combining if, elif, and else
   • Example
6. Nested Conditional Statements
   • Example
7. Logical Operators in Conditions
   • Operators
   • Example
8. Best Practices
9. Common Mistakes to Avoid
10. Conclusion

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1. Introduction to Conditional Statements #

Conditional statements allow your program to perform different actions based on different conditions. They enable decision-making, making your code dynamic and responsive to varying inputs or states.

In Python, the primary conditional statements are:

• if: Executes a block of code if a specified condition is true.
• elif (short for “else if”): Checks another condition if the previous if or elif conditions were false.
• else: Executes a block of code if none of the preceding if or elif conditions were true.

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2. if Statement #

Syntax #

if condition:
    # code block to execute if condition is True

• if: The keyword that starts the conditional statement.
• condition: A boolean expression that evaluates to True or False.
• Code Block: Indented block of code that runs if the condition is True.

Example #

# Example of an if statement

age = 18

if age >= 18:
    print("You are eligible to vote.")

Output:

You are eligible to vote.

Explanation:

• The condition age >= 18 is evaluated.
• Since age is 18, which is equal to 18, the condition is True.
• Therefore, the message “You are eligible to vote.” is printed.

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3. elif Statement #

Syntax #

if condition1:
    # code block if condition1 is True
elif condition2:
    # code block if condition1 is False and condition2 is True

• elif: Stands for “else if,” allowing multiple conditions to be checked sequentially.
• You can have multiple elif statements following an if.

Example #

# Example of using elif

score = 85

if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")

Output:

Grade: B

Explanation:

• First, the condition score >= 90 is checked. Since score is 85, this is False.
• The elif condition score >= 80 is then checked. Since 85 >= 80 is True, “Grade: B” is printed.
• No further conditions are checked after a True condition is met.

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4. else Statement #

Syntax #

if condition1:
    # code block if condition1 is True
elif condition2:
    # code block if condition1 is False and condition2 is True
else:
    # code block if all above conditions are False

• else: Captures any case where none of the preceding if or elif conditions are True.
• There can only be one else statement in a conditional block, and it must come last.

Example #

# Example of using else

temperature = 30

if temperature > 30:
    print("It's a hot day.")
elif temperature > 20:
    print("It's a nice day.")
else:
    print("It's cold.")

Output:

It's a nice day.

Explanation:

• temperature > 30 is False since 30 is not greater than 30.
• temperature > 20 is True since 30 > 20.
• Therefore, “It’s a nice day.” is printed.
• The else block is skipped because a True condition was already met.

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5. Combining if, elif, and else #

You can combine if, multiple elif, and else to handle various conditions sequentially.

Example #

# Comprehensive example using if, elif, and else

day = "Sunday"

if day == "Saturday" or day == "Sunday":
    print("It's the weekend!")
elif day == "Friday":
    print("It's almost the weekend.")
else:
    print("It's a weekday.")

Output:

It's the weekend!

Explanation:

• The if condition checks if day is either “Saturday” or “Sunday”. Since day is “Sunday”, it’s True.
• “It’s the weekend!” is printed.
• The elif and else blocks are not evaluated because a True condition was already met.

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6. Nested Conditional Statements #

Conditional statements can be nested within each other to handle more complex decision-making scenarios.

Example #

# Example of nested if statements

number = 15

if number > 0:
    print("The number is positive.")
    if number % 2 == 0:
        print("The number is even.")
    else:
        print("The number is odd.")
else:
    print("The number is not positive.")

Output:

The number is positive.
The number is odd.

Explanation:

1. The first if checks if number > 0. Since 15 > 0, it’s True, and “The number is positive.” is printed.
2. Inside the first if, there’s another if that checks if number % 2 == 0 (i.e., if the number is even).
3. Since 15 % 2 is 1, the condition is False, so the else block executes, printing “The number is odd.”

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7. Logical Operators in Conditions #

Logical operators allow you to combine multiple conditions within a single if, elif, or else statement. The primary logical operators in Python are and, or, and not.

Operators #

Operator	Description	Example
and	Returns True if both operands are True.	if a > 0 and b > 0:
or	Returns True if at least one operand is True.	if a > 0 or b > 0:
not	Reverses the boolean value of the operand.	if not a > 0:

Example #

# Example using logical operators

username = "admin"
password = "secret123"

if username == "admin" and password == "secret123":
    print("Access granted.")
elif username == "admin" and not password == "secret123":
    print("Incorrect password.")
else:
    print("Invalid username.")

Output:

Access granted.

Explanation:

• The first if condition checks if both username is “admin” and password is “secret123”. Both conditions are True, so “Access granted.” is printed.
• The elif and else blocks are not evaluated because the if condition was met.

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8. Best Practices #

1. Use Meaningful Conditions: Ensure your conditions are clear and meaningful to enhance readability.pythonCopy code# Good if user_age >= 18: print("Eligible for voting.") # Bad if a >= 18: print("Eligible for voting.")
2. Limit Nesting: Avoid deep nesting of conditional statements as it can make code hard to read. Consider using logical operators or refactoring code.pythonCopy code# Deep nesting (Not Recommended) if condition1: if condition2: if condition3: do_something() # Refactored with logical operators if condition1 and condition2 and condition3: do_something()
3. Use elif Instead of Multiple if: When checking multiple exclusive conditions, use elif to ensure only one block is executed.pythonCopy code# Using elif if score >= 90: grade = 'A' elif score >= 80: grade = 'B' else: grade = 'C' # Using multiple if (Not Recommended for exclusive conditions) if score >= 90: grade = 'A' if score >= 80: grade = 'B' else: grade = 'C'
4. Keep It Simple: Aim for simplicity in your conditions to make the code easy to understand and maintain.
5. Use Parentheses for Clarity: When combining multiple logical operators, use parentheses to make the order of evaluation explicit.pythonCopy codeif (age >= 18 and has_license) or is_supervisor: allow_entry()

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9. Common Mistakes to Avoid #

1. Incorrect Indentation: Python relies on indentation to define code blocks. Improper indentation can lead to IndentationError or unexpected behavior.pythonCopy code# Incorrect Indentation if condition: print("Condition is True") # This will raise an IndentationError
2. Using Assignment = Instead of Equality ==: The single = is for assignment, while == is for comparison.pythonCopy code# Incorrect if x = 10: print("x is 10") # Correct if x == 10: print("x is 10")
3. Forgetting the Colon :: Each if, elif, and else statement must end with a colon.pythonCopy code# Incorrect if x > 0 print("Positive") # Correct if x > 0: print("Positive")
4. Overusing Nested Conditionals: Deeply nested conditionals can make code hard to read and maintain. Use logical operators or refactor when possible.
5. Not Covering All Cases: Ensure that all possible scenarios are handled, especially when using if, elif, and else.
6. Logical Errors in Conditions: Ensure that the logical conditions accurately represent the intended logic.pythonCopy code# Logical Error if age > 18 and age < 65: print("Working age.") # Intended to include 18 and 65 if age >= 18 and age <= 65: print("Working age.")

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10. Conclusion #

Conditional statements are fundamental to controlling the flow of your Python programs. By effectively using if, elif, and else, you can create dynamic and responsive applications that react appropriately to different inputs and states.

Key Takeaways:

• if: Checks a condition and executes a block of code if it’s True.
• elif: Checks another condition if the previous if or elif was False.
• else: Executes a block of code if all preceding conditions are False.
• Logical Operators: Combine multiple conditions for more complex decision-making.
• Best Practices: Keep conditions clear, limit nesting, and use elif appropriately.
• Avoid Common Mistakes: Watch out for indentation errors, misuse of operators, and logical inaccuracies.

Mastering conditional statements will enhance your ability to write effective and efficient Python code, enabling you to tackle a wide range of programming challenges.