if and else Statements

The if statement executes a block of code when a condition is true. To handle the scenario when the condition is false, use the else clause.

Syntax

if condition:
    # Execute this block if condition is True
    statement_a1
    statement_a2
else:
    # Execute this block if condition is False
    statement_b1
    statement_b2

Example: Checking a Ticket

has_ticket = 1  # 1 represents having a ticket

if has_ticket == 1:
    print("Ticket verified. Boarding allowed.")
    print("Looking forward to the journey!")
else:
    print("No ticket. Boarding denied.")
    print("Better luck next time.")

Output with Ticket:

Ticket verified. Boarding allowed.
Looking forward to the journey!

Output without Ticket:

No ticket. Boarding denied.
Better luck next time.

Exercise: Write a program that checks if a knife's blade length is 10 cm or less. If it is, print "Security check passed." Otherwies, print "Item confiscated."

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The elif Statement

For checking multiple exclusive conditions sequentially, use elif (short for "else if").

Syntax

if condition_1:
    # Block for condition_1
elif condition_2:
    # Block for condition_2
elif condition_3:
    # Block for condition_3
else:
    # Block if no conditions are met

The flow stops at the first condition that evaluates to True.

Example: Grading System

exam_score = 77

if 90 <= exam_score <= 100:
    print("Grade: A")
elif 80 <= exam_score < 90:
    print("Grade: B")
elif 70 <= exam_score < 80:
    print("Grade: C")
elif 60 <= exam_score < 70:
    print("Grade: D")
elif 0 <= exam_score < 60:
    print("Grade: F")

Note: elif must follow an initial if statement. It can be combined with a final else block.

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Nested Conditional Statements

Conditional statements can be placed inside other conditional statements to create more complex logic.

Syntax

if outer_condition:
    # Outer block
    if inner_condition:
        # Inner block
        inner_statement
    else:
        alternative_inner_statement
else:
    # Alternative outer block
    outer_alternative_statement

Example: Security Check

ticket_status = 1  # 1 means valid ticket
blade_length_cm = 9  # Knife blade length

if ticket_status == 1:
    print("Valid ticket. Proceed to security.")
    if blade_length_cm < 10:
        print("Security clearance granted.")
        print("Enjoy your trip!")
    else:
        print("Security alert: Prohibited item detected.")
        print("Police notification required.")
else:
    print("Invalid ticket. Entry forbidden.")

Exercise: Simulate boarding a bus. Check if the passenger's card balance is over 2 units. If they board, check if there are vacant seats (seats_available > 0) and print an appropriate message.

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while Loops

A while loop repeats a block of code as long as its condition remains True.

Syntax

while condition:
    # Loop body
    repeated_statement_1
    repeated_statement_2

Example: Simple Counter

counter = 0
while counter < 5:
    print(f"Iteration number: {counter}")
    counter += 1  # Increment counter

Applications

1. Sum of Numbers from 1 to 100

total = 0
num = 1
while num <= 100:
    total += num
    num += 1
print(f"The cumulative sum from 1 to 100 is: {total}")

2. Sum of Even Numbers from 1 to 100

even_sum = 0
value = 1
while value <= 100:
    if value % 2 == 0:
        even_sum += value
    value += 1
print(f"The sum of even numbers from 1 to 100 is: {even_sum}")

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Nested while Loops

A while loop can contain another while loop.

Example: Printing a Pattern

row = 1
while row <= 5:
    col = 1
    while col <= row:
        print("* ", end="")
        col += 1
    print()  # New line after each row
    row += 1

Output:

*
* *
* * *
* * * *
* * * * *

Example: Multiplication Table

a = 1
while a <= 9:
    b = 1
    while b <= a:
        product = a * b
        print(f"{b}*{a}={product:2d} ", end="")
        b += 1
    print()
    a += 1

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for Loops

The for loop iterates over items in a sequence (like a string, list, or range).

Syntax

for element in sequence:
    # Code to execute for each element
    process(element)
else:
    # Optional: executes after loop finishes normally (not via `break`)
    final_action()

Example: Iterating Over a String

word = "Python"
for char in word:
    print(char)

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Loop Control: break and continue

• break: Immediately terminates the entire loop.
• continue: Skips the rest of the current iteration and proceeds to the next one.

break Example

for i in range(10):
    if i == 5:
        break  # Loop stops when i is 5
    print(i)

continue Example

for i in range(10):
    if i % 2 == 0:
        continue  # Skip even numbers
    print(i)  # Prints only odd numbers

Important: break and continue only affect the innermost loop in which they are placed.

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Rock-Paper-Scissors Game Example

import random

player_input = input("Enter choice: scissors (0), rock (1), paper (2): ")
player_choice = int(player_input)
computer_choice = random.randint(0, 2)

# Winning conditions: (0,2), (1,0), (2,1)
if (player_choice == 0 and computer_choice == 2) or \
   (player_choice == 1 and computer_choice == 0) or \
   (player_choice == 2 and computer_choice == 1):
    print("You win!")
elif player_choice == computer_choice:
    print("It's a tie!")
else:
    print("You lose!")