IG Math Space

By the end of this class session, students will be able to:

• Understand the concept and necessity of loops in programming.
• Use for loops to iterate over sequences and range().
• Use while loops for condition-based repetition.
• Differentiate between for and while loops and choose the appropriate one.
• Recognize and avoid infinite loops.
• Understand the loop else clause and when it executes.
• Solve practical problems using loops (factorial, prime, Fibonacci, sum of digits).

A loop is a programming construct that repeats a block of code multiple times. Loops are essential when you need to perform the same operation on multiple items or repeat an action until a condition is met.

Why do we need loops?

Without loops, if you wanted to print numbers 1 to 100, you would need 100 print() statements. With a loop, just 2 lines:

for i in range(1, 101):
    print(i)

Types of Loops in Python

3.1 Definition

A for loop in Python iterates over a sequence (such as a list, tuple, string, dictionary, set, or range) and executes a block of code once for each item in the sequence.

3.2 Syntax

for variable in sequence:
    statement(s)

• variable — takes the value of each item in the sequence, one at a time.
• sequence — any iterable object (list, tuple, string, range, etc.).
• The block is indented (usually 4 spaces).

3.3 How it Works

1. Python picks the first item from the sequence and assigns it to the variable.
2. The indented block executes.
3. Python picks the next item, and so on.
4. When the sequence is exhausted, the loop ends.

Conceptual Flowchart — for Loop

Fig 1. Flowchart of a for loop — iterate over each item until the sequence is exhausted.

3.4 The range() Function

The range() function is commonly used with for loops to generate a sequence of numbers.

range(5)        # 0, 1, 2, 3, 4
range(1, 6)     # 1, 2, 3, 4, 5
range(0, 10, 2) # 0, 2, 4, 6, 8
range(5, 0, -1) # 5, 4, 3, 2, 1

3.5 Example Programs — for Loop

Example 1: Print numbers 1 to 5

for i in range(1, 6):
    print(i)

Example 2: Print each character of a string

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

Example 3: Sum of first N natural numbers

n = int(input("Enter N: "))
total = 0
for i in range(1, n + 1):
    total = total + i
print("Sum of first", n, "natural numbers is:", total)

Example 4: Multiplication table

num = int(input("Enter a number: "))
for i in range(1, 11):
    print(num, "x", i, "=", num * i)

Example 5: Iterate over a list

fruits = ["apple", "banana", "cherry", "mango"]
for fruit in fruits:
    print("I like", fruit)

Example 6: Factorial of a number

n = int(input("Enter a number: "))
fact = 1
for i in range(1, n + 1):
    fact = fact * i
print("Factorial of", n, "is", fact)

Example 7: Print even numbers from 1 to 20

for i in range(2, 21, 2):
    print(i)

Example 8: for loop with else clause

The else block executes only if the loop completes normally (without hitting break).

for i in range(1, 5):
    print(i)
else:
    print("Loop completed successfully.")

4.1 Definition

A while loop repeats a block of code as long as a condition remains True. It is used when the number of iterations is not known in advance.

4.2 Syntax

while condition:
    statement(s)

• The condition is checked before each iteration.
• If the condition is True, the block executes.
• If the condition is False, the loop ends.
• Important: The block must contain something that eventually makes the condition False; otherwise, you get an infinite loop.

4.3 How it Works

1. Evaluate the condition.
2. If True, execute the block.
3. Go back to step 1.
4. If False, exit the loop.

Conceptual Flowchart — while Loop

Fig 2. Flowchart of a while loop — condition checked before each iteration; body must update the variable.

4.4 Example Programs — while Loop

Example 9: Print numbers 0 to 4

n = 0
while n < 5:
    print(n)
    n += 1

Example 10: Sum of digits of a number

num = int(input("Enter a number: "))
temp = num
total = 0
while temp > 0:
    digit = temp % 10
    total = total + digit
    temp = temp // 10
print("Sum of digits of", num, "is", total)

Example 11: Reverse a number

num = int(input("Enter a number: "))
temp = num
reverse = 0
while temp > 0:
    digit = temp % 10
    reverse = reverse * 10 + digit
    temp = temp // 10
print("Reverse of", num, "is", reverse)

Example 12: Check whether a number is prime

num = int(input("Enter a number: "))
if num < 2:
    print(num, "is not a prime number.")
else:
    i = 2
    is_prime = True
    while i <= num // 2:
        if num % i == 0:
            is_prime = False
            break
        i += 1
    if is_prime:
        print(num, "is a prime number.")
    else:
        print(num, "is not a prime number.")

Example 13: Sentinel-controlled loop (sum until 0)

total = 0
num = int(input("Enter a number (0 to stop): "))
while num != 0:
    total = total + num
    num = int(input("Enter a number (0 to stop): "))
print("Total sum is:", total)

Example 14: while loop with else

n = 0
while n < 3:
    print(n)
    n += 1
else:
    print("While loop ended normally.")

An infinite loop occurs when the condition of a while loop never becomes False.

Example: Infinite loop (do not run without break)

while True:
    print("This will run forever!")

To stop it: Use Ctrl + C in the terminal, or include a break statement.

Useful infinite loop pattern with break

while True:
    command = input("Enter command (quit to exit): ")
    if command == "quit":
        break
    print("You entered:", command)

Both for and while loops can have an else block. The else block executes only if the loop completes normally (i.e., without hitting a break).

Syntax

for variable in sequence:
    statement(s)
else:
    statement(s)   # executes if no break

Example 15: for-else with break

numbers = [10, 20, 30, 40, 50]
target = int(input("Enter number to search: "))
for num in numbers:
    if num == target:
        print(target, "found!")
        break
else:
    print(target, "not found.")

Example 16: while-else with break

n = 0
while n < 5:
    print(n)
    n += 1
else:
    print("Loop ended normally (no break).")

1. What is a loop? Why are loops used in programming?
2. Differentiate between for and while loops with suitable examples.
3. Explain the range() function with all three forms.
4. What is an infinite loop? How can you avoid it?
5. When does the else clause of a loop execute?
6. Write a Python program to print the multiplication table of a number using a for loop.
7. Write a Python program to check whether a number is prime using a while loop.
8. Write a Python program to find the sum of digits of a number using a while loop.
9. Write a Python program to print the reverse of a number using a while loop.
10. Explain the difference between for-else and while-else with examples.