Loops in Python

In real programming, we often need to repeat tasks multiple times. For example:

• Printing student names
• Processing thousands of records
• Reading files line by line
• Running calculations repeatedly

Writing the same code again and again is inefficient. Python solves this problem using loops.

A loop allows a block of code to execute repeatedly until a condition becomes false or data ends.

Why Loops Are Important

Imagine printing numbers from 1 to 5 without loops:

print(1)
print(2)
print(3)
print(4)
print(5)

This works — but what if you need 10,000 numbers? Loops automate repetition safely and efficiently.

Types of Loops in Python

Python mainly provides two looping mechanisms:

• for loop → repeat over a sequence
• while loop → repeat while a condition is true

The for Loop

A for loop is used when we know how many times we want to repeat something or when iterating through data collections.

Basic syntax:

# for loop syntax

for variable in sequence:
    # code block

Loop Using range()

The most common way to use a for loop is with the built-in range() function.

range() generates numbers automatically.

# Printing numbers from 1 to 5

for i in range(1, 6):
    print(i)   # prints each value of i

1
2
3
4
5

• range(1,6) starts from 1 and stops before 6.
• The loop runs once for each generated number.
• i stores the current value during each iteration.

Understanding range() Clearly

Looping Through a List

Loops are commonly used to process collections like lists.

# Iterating through a list

courses = ["Python", "SQL", "AI"]

for course in courses:
    print("Learning:", course)

Learning: Python
Learning: SQL
Learning: AI

• The loop automatically accesses each item.
• No indexing is required.
• This pattern is heavily used in real applications.

The while Loop

A while loop runs as long as a condition remains true.

Use while loops when repetition depends on logic, not fixed counts.

# while loop example

count = 1

while count <= 5:
    print(count)
    count += 1   # updating condition

1
2
3
4
5

• The loop continues while condition is True.
• Updating the variable prevents infinite looping.

Infinite Loops (Important Concept)

If the condition never becomes False, the loop runs forever.

# Infinite loop example (avoid)

while True:
    print("Running forever")

Always ensure loop conditions eventually stop.

break Statement

The break statement stops a loop immediately.

# Using break

for num in range(1,10):

    if num == 5:
        break   # exit loop

    print(num)

1
2
3
4

• Loop terminates once condition matches.

continue Statement

The continue statement skips the current iteration only.

# Using continue

for num in range(1,6):

    if num == 3:
        continue   # skip 3

    print(num)

1
2
4
5

Nested Loops

A loop inside another loop is called a nested loop. Used in matrices, tables, and pattern generation.

# Nested loop example

for i in range(1,4):
    for j in range(1,4):
        print(i, j)

1 1
1 2
1 3
2 1
2 2
2 3
3 1
3 2
3 3

Real-World Example: Order Processing System

Loops commonly process repeated business data.

# Processing customer orders

orders = [250, 450, 300, 150]

total_sales = 0

for order in orders:
    total_sales += order   # accumulate total

print("Total Sales:", total_sales)

Total Sales: 1150

• Each order is processed automatically.
• Loops eliminate repetitive calculations.

Practice

Quick Quiz

Loop Concepts Summary

Recap: Loops automate repetition, process data efficiently, and form the backbone of automation and large-scale programs.

Next up: Strings in Python.