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Control Flow.md

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Control Flow

Table of Contents

Basic if-else Conditions

The if statement allows you to execute a block of code only if a specified condition is true. The else statement provides an alternative block of code if the condition is false.

Syntax:

if condition:
    # Code to execute if condition is true
else:
    # Code to execute if condition is false

Example:

age = 18
if age >= 18:
    print("You are eligible to vote.")
else:
    print("You are not eligible to vote.")

Here, the code checks if age is greater than or equal to 18. If true, it prints a message about eligibility. If false, it provides an alternate message.

Nested and Multiple Conditions

Python allows you to nest if statements within each other for more complex conditions or use elif (short for “else if”) for multiple conditions.

Syntax:

if condition1:
    # Code for condition1
elif condition2:
    # Code for condition2
else:
    # Code if neither condition1 nor condition2 is true

Example of elif:

score = 85
if score >= 90:
    print("Grade: A")
elif score >= 80:
    print("Grade: B")
elif score >= 70:
    print("Grade: C")
else:
    print("Grade: D")

This code assigns a grade based on score. It checks each condition in order, and once a true condition is found, the corresponding block is executed, skipping any remaining conditions.

Example of nested conditions:

age = 20
has_permission = True
if age >= 18:
    if has_permission:
        print("Access granted.")
    else:
        print("Permission denied.")
else:
    print("Access denied due to age.")

Here, has_permission is checked only if the first condition (age >= 18) is true. This allows for handling complex scenarios with dependencies.

Logical Expressions in Control Flow

Logical expressions use operators (and, or, not) to combine multiple conditions in a single if statement, enhancing flexibility.

Operators

  • and: Evaluates to true if both conditions are true.
  • or: Evaluates to true if at least one condition is true.
  • not: Inverts the boolean value of a condition.

Use Cases Examples

  • Example 1:

    age = 20
has_ID = True
if age >= 18 and has_ID:
    print("Allowed entry.")
else:
    print("Entry not allowed.")

    Here, both age >= 18 and has_ID must be true for entry to be allowed.

  • Example 2:

    has_ticket = False
vip_pass = True
if has_ticket or vip_pass:
    print("Entry granted.")
else:
    print("Entry denied.")

    This example allows entry if the user has either a ticket or a VIP pass.

  • Example 3:

    is_member = False
if not is_member:
    print("Please sign up to become a member.")

    The not operator inverts is_member. If is_member is False, the message is shown.

Ternary Expressions

A ternary expression is a shorthand way to write an if-else statement in a single line. It’s useful for simple conditions and assignments.

Syntax:

value_if_true if condition else value_if_false

Example:

age = 17
status = "Adult" if age >= 18 else "Minor"
print(status)  # Output: Minor

Here, status is assigned "Adult" if age >= 18 is true; otherwise, it’s assigned "Minor". Ternary expressions are concise and useful when assigning values based on conditions.

Another Example:

is_even = "Even" if num % 2 == 0 else "Odd"

This checks if a number is even or odd, assigning the appropriate string based on the condition num % 2 == 0.