Chapter 6: Looping Structures

6.1 Chapter Overview

Looping structures allow a Python program to repeat instructions automatically. Instead of writing the same code many times, a loop can repeat the task for a specific number of times or while a condition remains true.

Loops are used in attendance systems, marks processing, menu-driven programs, billing systems, data analysis, automation scripts, games, and many real-world applications.

6.2 Chapter Objectives

• Understand the purpose of looping structures.
• Use for loops to repeat tasks for a fixed number of times.
• Use the range() function to generate number sequences.
• Use while loops to repeat tasks based on conditions.
• Apply loop control statements such as break and continue.
• Use nested loops for repeated patterns and grouped processing.
• Develop interactive Python applications using loops.

Learning Outcomes

• Write Python programs using for and while loops.
• Apply loops to process marks, totals, counters, and menus.
• Identify and prevent infinite loop errors.
• Create simple automation programs using repeated execution.

6.3 What is a Loop?

A loop is a programming structure that repeats a block of code. The repetition continues either for a fixed number of times or until a condition becomes false.

6.4 Why Loops are Important

Without loops, programmers must write repeated code manually. This makes programs longer, harder to maintain, and more likely to contain errors.

6.5 The for Loop

A for loop is used when the programmer knows how many times a task should repeat. It is commonly used with lists, strings, and the range() function.

Syntax

for variable in sequence:
    statement

Example: Print a Message 5 Times

for i in range(5):
    print("Welcome to PDTC Python Programming")

6.6 Understanding range()

The range() function generates a sequence of numbers. It is often used with for loops.

Example

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

6.7 Looping Through a List

A for loop can repeat through each item in a list.

courses = ["Python", "AI", "Data Science", "Cyber Security"]

for course in courses:
    print(course)

6.8 Calculating Total Marks with a for Loop

This example uses a loop to calculate total marks from a list.

marks = [80, 75, 90, 85]
total = 0

for mark in marks:
    total = total + mark

print("Total Marks:", total)

6.9 The while Loop

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

Syntax

while condition:
    statement

Example: Print Numbers 1 to 5

number = 1

while number <= 5:
    print(number)
    number = number + 1

6.10 Infinite Loop

An infinite loop happens when the loop condition never becomes False. This causes the program to repeat forever.

Wrong Example

number = 1

while number <= 5:
    print(number)

In this example, number is never increased, so the condition remains True forever.

Correct Example

number = 1

while number <= 5:
    print(number)
    number = number + 1

6.11 Loop Control Statements

Loop control statements change the normal flow of a loop.

Example using break

for number in range(1, 10):
    if number == 5:
        break
    print(number)

Example using continue

for number in range(1, 6):
    if number == 3:
        continue
    print(number)

6.12 Nested Loops

A nested loop means a loop inside another loop. Nested loops are commonly used for tables, patterns, grids, and repeated grouped data.

Example: Multiplication Table Pattern

for row in range(1, 4):
    for column in range(1, 4):
        print(row, column)

6.13 Interactive Example: Login Attempts

This example allows the user to enter a password until it is correct.

password = ""

while password != "pdtc123":
    password = input("Enter password: ")

print("Login successful")

6.14 Interactive Example: Student Marks Entry

This program asks how many subject marks the user wants to enter, then calculates the total and average.

number_of_subjects = int(input("How many subjects? "))
total = 0

for i in range(number_of_subjects):
    marks = float(input("Enter marks: "))
    total = total + marks

average = total / number_of_subjects

print("Total Marks:", total)
print("Average Marks:", average)

6.15 Common Beginner Mistakes

6.16 Hands-On Practice

6.17 Final Assessment Quiz

Answer the following questions. Correct Answer = +1 Mark Wrong Answer = -0.5 Mark

Final Practical Assessment

Develop an interactive Python program named marks_loop_calculator.py. The program must ask how many subjects the student has, use a loop to collect marks, calculate total marks, calculate average marks, and display the final result clearly.

6.18 Chapter Summary

In this chapter, learners studied looping structures in Python. They learned how to repeat tasks using for loops and while loops, how to use range(), how to stop or skip loop cycles using break and continue, and how loops support automation in interactive Python applications.