CONTROL STRUCTURES

Control structures allow you to control the flow of execution in a program. There are three basic types of control structures in Python:

Types of Control Structures

1. Sequential

Default mode: Code statements are executed sequentially, one line after another.

2. Selection (Decision-Making Statements)

Used for decisions and branching: Choose between two or more alternative paths. Python offers the following types of selection statements:

• if
• if..else
• if..elif
• Nested if

3. Iteration (Loops)

Used for looping: Repeat a piece of code multiple times while a condition evaluates to True.

• for
• while

IF Statement

Selection/Decision-Making Statements
Decision-making statements control the flow of the program by evaluating a Boolean expression and executing specific code blocks based on the result.

Example:

# IF Statement Example
num = 8
 
if num > 5:
    print("Value is above 5")
else:
    print("Value is less than 5")

Output:

Value is above 5

Concept of Indentation

Indentation refers to the spaces given at the beginning of the code. It is Python’s way of defining code blocks. All statements within the same block must have the same level of indentation. For example:

if True:
    print("This is inside the if block")
    print("Indented properly")

If..Else Statement

# If..Else Example
num = int(input("Enter a number: "))
 
if num % 2 == 0:
    print(f"{num} is even")
else:
    print(f"{num} is odd")

Output:

Enter a number: 4  
4 is even

If..Elif Statement

# If..Elif Example
marks = int(input("Enter your marks: "))
 
if marks >= 90:
    print("Grade: A")
elif marks >= 75:
    print("Grade: B")
elif marks >= 50:
    print("Grade: C")
else:
    print("Grade: F")

Output:

Enter your marks: 85  
Grade: B

Nested If Statement

# Nested If Example
age = int(input("Enter your age: "))
 
if age > 18:
    if age < 60:
        print("You are eligible to work")
    else:
        print("You are eligible for retirement")
else:
    print("You are too young to work")

Output:

Enter your age: 45  
You are eligible to work

Try It Yourself

Problem 1: Check Divisibility

Write a program to check whether a given number is divisible by both 3 and 5.

num = int(input("Enter a number: "))
 
if num % 3 == 0 and num % 5 == 0:
    print(f"{num} is divisible by both 3 and 5")
else:
    print(f"{num} is not divisible by both 3 and 5")

Problem 2: Determine Leap Year

Write a program to determine whether a given year is a leap year or not.

year = int(input("Enter a year: "))
 
if year % 4 == 0:
    if year % 100 == 0:
        if year % 400 == 0:
            print(f"{year} is a leap year")
        else:
            print(f"{year} is not a leap year")
    else:
        print(f"{year} is a leap year")
else:
    print(f"{year} is not a leap year")

Problem 3: Determine the Largest of Three Numbers

Write a program to find the largest among three numbers.

a = int(input("Enter first number: "))
b = int(input("Enter second number: "))
c = int(input("Enter third number: "))
 
if a >= b and a >= c:
    print(f"The largest number is {a}")
elif b >= a and b >= c:
    print(f"The largest number is {b}")
else:
    print(f"The largest number is {c}")