Python for Loops

What Is a Loop?

A loop is a programming structure that allows us to execute a block of code repeatedly.

Imagine that you need to print the numbers from 1 to 100. Without a loop, you would have to write 100 separate print() statements.

With a loop, the same task can be completed using only a few lines of code.

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

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

1
2
3
4
5

Why Loops Matter

Loops are one of the most important concepts in programming. Real applications constantly process collections of data.

For example, a program may need to:

• Process thousands of customer records.
• Calculate student marks.
• Display products from a database.
• Search through a list of names.
• Calculate totals and averages.
• Generate reports.
• Process files or API responses.

Instead of writing the same instruction repeatedly, we write the instruction once and let the loop handle repetition.

The for Loop

Python's for loop is used to iterate over an iterable such as a list, string, tuple, set, dictionary, or range.

Basic Syntax

for item in collection:
    # code to execute

For example:

fruits = ["apple", "banana", "mango"]

for fruit in fruits:
    print(fruit)

apple
banana
mango

During each iteration, the variable fruit receives the next item from the list.

Looping Through Lists

Lists are one of the most common data structures used with for loops.

students = ["Ali", "Sara", "Ahmed", "Fatima"]

for student in students:
    print(student)

Ali
Sara
Ahmed
Fatima

Processing List Values

prices = [100, 250, 500, 750]

for price in prices:
    print(price * 1.10)

Here every price is increased by 10%.

Looping Through Strings

A string is an iterable sequence of characters. Therefore, you can use a for loop to process every character.

word = "Python"

for character in word:
    print(character)

P
y
t
h
o
n

Counting Characters

message = "hello"

count = 0

for character in message:
    count += 1

print(count)

5

Looping Through Tuples

Tuples can also be processed using a for loop.

coordinates = (10, 20, 30)

for value in coordinates:
    print(value)

10
20
30

The loop works because tuples are iterable.

Looping Through Sets

Sets can also be iterated using a for loop.

colors = {"red", "green", "blue"}

for color in colors:
    print(color)

Looping Through Dictionaries

Dictionaries require special attention because they contain keys and values.

Looping Through Keys

student = {
    "name": "Ali",
    "age": 21,
    "grade": "A"
}

for key in student:
    print(key)

name
age
grade

Looping Through Values

for value in student.values():
    print(value)

Ali
21
A

Looping Through Keys and Values

for key, value in student.items():
    print(key, ":", value)

name : Ali
age : 21
grade : A

The range() Function

The range() function is commonly used with for loops when you need a sequence of numbers.

Basic range()

for number in range(5):
    print(number)

0
1
2
3
4

Starting From Another Number

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

1
2
3
4
5

Start, Stop & Step in range()

The complete form of range() is:

range(start, stop, step)

Using a Step

for number in range(2, 11, 2):
    print(number)

2
4
6
8
10

Counting Backwards

for number in range(5, 0, -1):
    print(number)

5
4
3
2
1

Looping with Indexes

Sometimes you need both the position of an item and the item itself.

A simple approach is to use range() with len().

fruits = ["apple", "banana", "mango"]

for index in range(len(fruits)):
    print(index, fruits[index])

0 apple
1 banana
2 mango

Using enumerate()

Python provides a cleaner and more readable approach with enumerate().

fruits = ["apple", "banana", "mango"]

for index, fruit in enumerate(fruits):
    print(index, fruit)

0 apple
1 banana
2 mango

Using for with if

Loops become much more powerful when combined with conditions.

For example, we can print only even numbers.

for number in range(1, 11):

    if number % 2 == 0:
        print(number)

2
4
6
8
10

Filtering Data

marks = [45, 78, 91, 32, 66]

for mark in marks:

    if mark >= 50:
        print(mark)

78
91
66

for with else

Python allows an else block to be attached directly to a for loop.

The else block runs when the loop finishes normally without encountering a break.

for number in range(3):
    print(number)
else:
    print("Loop completed.")

0
1
2
Loop completed.

How break Changes It

for number in range(5):

    if number == 3:
        break

    print(number)

else:
    print("Loop completed.")

0
1
2

Because break stopped the loop, the else block did not execute.

Nested for Loops

A loop inside another loop is called a nested loop.

for row in range(3):

    for column in range(3):
        print(row, column)

0 0
0 1
0 2
1 0
1 1
1 2
2 0
2 1
2 2

For every iteration of the outer loop, the inner loop runs completely.

Multiplication Table

for number in range(1, 4):

    for multiplier in range(1, 6):

        print(number * multiplier, end=" ")

    print()

1 2 3 4 5
2 4 6 8 10
3 6 9 12 15

Practical Real-World Examples

Example 1 — Calculate Total Sales

sales = [1200, 2500, 1800, 3200]

total = 0

for sale in sales:
    total += sale

print("Total sales:", total)

Total sales: 8700

Example 2 — Calculate an Average

marks = [75, 82, 68, 91, 88]

total = 0

for mark in marks:
    total += mark

average = total / len(marks)

print("Average:", average)

Average: 80.8

Example 3 — Search for a Product

products = ["Laptop", "Mouse", "Keyboard", "Monitor"]

search = "Mouse"

for product in products:

    if product == search:
        print("Product found!")

Product found!

Example 4 — Generate a Report

employees = {
    "Ali": 75000,
    "Sara": 82000,
    "Ahmed": 68000
}

for name, salary in employees.items():
    print(f"{name}: Rs. {salary:,}")

Ali: Rs. 75,000
Sara: Rs. 82,000
Ahmed: Rs. 68,000

Common Mistakes

1. Forgetting the Colon

for number in range(5)
    print(number)

The loop statement must end with :.

2. Incorrect Indentation

for number in range(5):
print(number)

The loop body must be indented.

3. Forgetting That range() Excludes Stop

for number in range(1, 5):
    print(number)

This prints 1 through 4, not 5.

4. Modifying a Collection While Iterating

5. Using the Wrong Variable

fruits = ["apple", "banana", "mango"]

for fruit in fruits:
    print(fruit)

Choose meaningful loop variable names whenever possible. fruit communicates more clearly than a generic name such as x.

Practice Exercises

Mini Challenge — Student Marks Analyzer

Build a small program that analyzes a list of student marks.

Your program should:

1. Store several student marks in a list.
2. Print every mark.
3. Calculate the total.
4. Calculate the average.
5. Count the number of passing students.
6. Count the number of failing students.
7. Display the highest mark.

marks = [78, 45, 91, 63, 38, 84, 55]

total = 0
passed = 0
failed = 0

for mark in marks:

    print("Mark:", mark)

    total += mark

    if mark >= 50:
        passed += 1
    else:
        failed += 1

average = total / len(marks)

print("Total:", total)
print("Average:", average)
print("Passed:", passed)
print("Failed:", failed)

Quick Knowledge Check

Lesson Summary

The for loop is one of Python's most important tools for processing repeated data.

• Loops repeat a block of code.
• for loops iterate through iterable objects.
• Lists, tuples, strings and sets can be iterated directly.
• Dictionaries can be processed using keys, values or items.
• range() generates sequences of numbers.
• The stop value of range() is excluded.
• enumerate() provides indexes and values together.
• Loops can be combined with if statements.
• for...else can detect normal loop completion.
• Nested loops place one loop inside another.