Python Loops: for, while, range & Examples Tutorial

Module 10: Loops in Python

Loops are one of the fundamental concepts in Python programming. They allow you to execute the same block of code repeatedly without writing the same statements again and again.

For example, suppose you want to print numbers from 1 to 100.

Without a loop:

pythonCopy
1print(1)
2print(2)
3print(3)
4# ...
5print(100)

This approach is repetitive, difficult to maintain, and impractical for larger tasks.

With a for loop:

pythonCopy
1for number in range(1, 101):
2    print(number)

The same task requires only a few lines of code.

Loops are useful for:

• Processing lists and other collections
• Repeating calculations
• Reading user input until a condition is satisfied
• Searching for values
• Generating patterns
• Processing files
• Building menus and interactive programs
• Performing repetitive data-processing tasks

By the end of this module, you will understand Python for loops, while loops, nested loops, range(), enumerate(), zip(), break, continue, pass, loop else, and practical loop-based programs.

What Is a Loop?

A loop repeatedly executes a block of code.

Python provides two primary loop structures:

A for loop usually processes items one by one:

pythonCopy
1for item in collection:
2    print(item)

A while loop continues while its condition evaluates to True:

pythonCopy
1while condition:
2    print("Running")

Why Are Loops Important?

Imagine printing "Python" five times.

Without a loop:

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

With a loop:

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1for _ in range(5):
2    print("Python")

Output:

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1Python
2Python
3Python
4Python
5Python

The loop makes the program shorter and easier to modify.

For example, changing:

pythonCopy
1range(5)

to:

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1range(1000)

allows the program to repeat the operation 1,000 times without adding hundreds of lines of code.

Understanding Iteration

An iteration is one execution of a loop body.

Consider:

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1for number in range(1, 4):
2    print(number)

The loop performs three iterations:

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1Iteration 1 → number = 1
2Iteration 2 → number = 2
3Iteration 3 → number = 3

Output:

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11
22
33

Understanding iterations makes it easier to reason about loop execution.

The for Loop

The for loop is used to iterate over an iterable.

Common iterables include:

• Lists
• Tuples
• Strings
• Sets
• Dictionaries
• Ranges
• Other iterable objects

Syntax

pythonCopy
1for variable in iterable:
2    statement

The loop takes one item from the iterable at a time and assigns it to the loop variable.

Example: Print Numbers

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

Output:

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11
22
33
44
55

Here, number receives each value produced by range(1, 6).

Iterating Over a String

Strings are iterable, so you can process each character individually.

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1word = "Python"
2
3for letter in word:
4    print(letter)

Output:

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1P
2y
3t
4h
5o
6n

This is useful when processing individual characters.

Iterating Over a List

pythonCopy
1fruits = ["Apple", "Banana", "Orange"]
2
3for fruit in fruits:
4    print(fruit)

Output:

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1Apple
2Banana
3Orange

You can also perform an operation for every item:

pythonCopy
1prices = [100, 250, 75]
2
3for price in prices:
4    print(f"Price: ₹{price}")

Output:

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1Price: ₹100
2Price: ₹250
3Price: ₹75

Iterating Over a Tuple

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1coordinates = (10, 20, 30)
2
3for value in coordinates:
4    print(value)

Output:

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110
220
330

Iterating Over a Set

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1languages = {"Python", "Java", "C++"}
2
3for language in languages:
4    print(language)

The order of elements in a set should not be relied upon.

Iterating Over a Dictionary

When you iterate directly over a dictionary, Python gives you its keys.

pythonCopy
1student = {
2    "name": "Ankit",
3    "age": 22,
4    "course": "Python"
5}
6
7for key in student:
8    print(key)

Output:

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1name
2age
3course

To access both keys and values, use .items():

pythonCopy
1for key, value in student.items():
2    print(f"{key}: {value}")

Output:

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1name: Ankit
2age: 22
3course: Python

The while Loop

A while loop repeatedly executes a block of code as long as its condition is True.

Syntax

pythonCopy
1while condition:
2    statement

For example:

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1count = 1
2
3while count <= 5:
4    print(count)
5    count += 1

Output:

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11
22
33
44
55

The important part is:

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1count += 1

It changes the condition variable so that the loop eventually stops.

How a while Loop Works

Consider:

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1count = 1
2
3while count <= 3:
4    print(count)
5    count += 1

The execution is:

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1count = 1
2↓
31 <= 3 → True → print 1
4↓
5count = 2
6↓
72 <= 3 → True → print 2
8↓
9count = 3
10↓
113 <= 3 → True → print 3
12↓
13count = 4
14↓
154 <= 3 → False
16↓
17Loop ends

This demonstrates why the condition and state update are both important in a while loop.

Countdown Example

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1number = 5
2
3while number > 0:
4    print(number)
5    number -= 1
6
7print("Blast off!")

Output:

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15
24
33
42
51
6Blast off!

Using a while Loop for User Input

A while loop is useful when you don't know how many attempts a user will need.

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1password = ""
2
3while password != "python":
4    password = input("Enter password: ")
5
6print("Access granted.")

The loop continues until the user enters the correct password.

For real applications, never store production passwords as plain text. Use a secure authentication system and password hashing.

Infinite Loops

An infinite loop is a loop that never reaches a condition that stops it.

For example:

pythonCopy
1while True:
2    print("Running...")

The condition:

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1True

never becomes false.

An intentional infinite loop is sometimes useful when combined with break:

pythonCopy
1while True:
2    command = input("Enter command: ")
3
4    if command == "exit":
5        break
6
7    print(f"You entered: {command}")

When the user enters exit, the break statement terminates the loop.

for Loop vs while Loop

Choosing the correct loop makes your code easier to understand.

Example using for:

pythonCopy
1for number in range(10):
2    print(number)

Example using while:

pythonCopy
1number = 0
2
3while number < 10:
4    print(number)
5    number += 1

Both can produce similar results, but the for loop is generally clearer when iterating over a known range.

The range() Function

The range() function generates a sequence of integers commonly used with for loops.

It supports three forms:

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1range(stop)

pythonCopy
1range(start, stop)

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1range(start, stop, step)

The stop value is exclusive.

range(stop)

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

Output:

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10
21
32
43
54

The value 5 is not included.

range(start, stop)

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

Output:

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11
22
33
44
55

range(start, stop, step)

The step controls how much the value changes after each iteration.

pythonCopy
1for number in range(0, 11, 2):
2    print(number)

Output:

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10
22
34
46
58
610

Using a Negative Step

A negative step can be used to count backward.

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1for number in range(10, 0, -1):
2    print(number)

Output:

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110
29
38
47
56
65
74
83
92
101

enumerate()

When iterating over a collection, you sometimes need both the index and the value.

Instead of manually managing a counter, use enumerate().

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1fruits = ["Apple", "Banana", "Orange"]
2
3for index, fruit in enumerate(fruits):
4    print(index, fruit)

Output:

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10 Apple
21 Banana
32 Orange

You can choose a different starting index:

pythonCopy
1for index, fruit in enumerate(fruits, start=1):
2    print(index, fruit)

Output:

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11 Apple
22 Banana
33 Orange

Using enumerate() is usually cleaner than:

pythonCopy
1index = 0
2
3for fruit in fruits:
4    print(index, fruit)
5    index += 1

zip()

The zip() function allows you to iterate over multiple iterables simultaneously.

pythonCopy
1names = ["Ankit", "Rahul", "Aman"]
2marks = [90, 85, 95]
3
4for name, mark in zip(names, marks):
5    print(f"{name}: {mark}")

Output:

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1Ankit: 90
2Rahul: 85
3Aman: 95

By default, zip() stops when the shortest iterable is exhausted.

Using zip() With Three Lists

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1names = ["Ankit", "Rahul"]
2ages = [22, 21]
3cities = ["Delhi", "Lucknow"]
4
5for name, age, city in zip(names, ages, cities):
6    print(f"{name} | {age} | {city}")

Output:

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1Ankit | 22 | Delhi
2Rahul | 21 | Lucknow

This is particularly useful when related data is stored in separate sequences.

The break Statement

The break statement immediately terminates the nearest enclosing loop.

pythonCopy
1for number in range(1, 11):
2    if number == 6:
3        break
4
5    print(number)

Output:

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11
22
33
44
55

When number becomes 6, Python executes break and exits the loop.

Searching With break

A common use of break is stopping a search when the required value is found.

pythonCopy
1numbers = [4, 8, 15, 16, 23, 42]
2target = 23
3
4for number in numbers:
5    if number == target:
6        print("Number found!")
7        break

Output:

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1Number found!

The continue Statement

The continue statement skips the remaining code in the current iteration and moves to the next iteration.

pythonCopy
1for number in range(1, 6):
2    if number == 3:
3        continue
4
5    print(number)

Output:

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11
22
34
45

When number is 3, the print() statement is skipped.

Printing Only Odd Numbers

pythonCopy
1for number in range(1, 11):
2    if number % 2 == 0:
3        continue
4
5    print(number)

Output:

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11
23
35
47
59

The pass Statement

The pass statement does nothing.

It is useful when Python requires a statement but you intentionally want the block to remain empty.

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1for number in range(5):
2    pass
3
4print("Loop completed.")

Output:

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1Loop completed.

It is also useful while designing code that will be implemented later:

pythonCopy
1def calculate_tax():
2    pass

Unlike break and continue, pass does not change loop execution.

Loop else

Python allows an else block after a for or while loop.

The loop's else block executes when the loop finishes normally, meaning it was not terminated by break.

Example:

pythonCopy
1for number in range(1, 6):
2    print(number)
3else:
4    print("Loop completed normally.")

Output:

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11
22
33
44
55
6Loop completed normally.

The difference becomes clearer with break.

pythonCopy
1for number in range(1, 6):
2    if number == 3:
3        break
4
5    print(number)
6else:
7    print("Loop completed normally.")

Output:

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11
22

The else block does not execute because the loop ended with break.

Practical Example: Prime Number Checker

Loop else can be useful when searching for a factor.

pythonCopy
1number = int(input("Enter a number: "))
2
3if number < 2:
4    print("Not a prime number.")
5else:
6    for divisor in range(2, int(number ** 0.5) + 1):
7        if number % divisor == 0:
8            print("Not a prime number.")
9            break
10    else:
11        print("Prime number.")

The loop searches for a divisor.

If a divisor is found, break runs.

If the loop finishes without finding one, the else block runs.

Nested Loops

A nested loop is a loop inside another loop.

Example:

pythonCopy
1for row in range(3):
2    for column in range(4):
3        print("*", end=" ")
4    print()

Output:

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1* * * *
2* * * *
3* * * *

The outer loop controls the rows.

The inner loop controls the columns.

How Nested Loops Work

Consider:

pythonCopy
1for row in range(2):
2    for column in range(3):
3        print(f"({row}, {column})")

Output:

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1(0, 0)
2(0, 1)
3(0, 2)
4(1, 0)
5(1, 1)
6(1, 2)

For every iteration of the outer loop, the entire inner loop executes.

This makes nested loops useful for:

• Tables
• Grids
• Matrices
• Pattern printing
• Comparing combinations of values

Multiplication Grid

pythonCopy
1for row in range(1, 4):
2    for column in range(1, 4):
3        print(row * column, end="\t")
4
5    print()

Output:

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11	2	3
22	4	6
33	6	9

Pattern Printing

Nested loops are commonly used to generate patterns.

Increasing Triangle

pythonCopy
1rows = 5
2
3for row in range(1, rows + 1):
4    for _ in range(row):
5        print("*", end=" ")
6
7    print()

Output:

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1*
2* *
3* * *
4* * * *
5* * * * *

Decreasing Triangle

pythonCopy
1rows = 5
2
3for row in range(rows, 0, -1):
4    for _ in range(row):
5        print("*", end=" ")
6
7    print()

Output:

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1* * * * *
2* * * *
3* * *
4* *
5*

Number Pattern

pythonCopy
1rows = 5
2
3for row in range(1, rows + 1):
4    for number in range(1, row + 1):
5        print(number, end=" ")
6
7    print()

Output:

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11
21 2
31 2 3
41 2 3 4
51 2 3 4 5

Practice Project: Multiplication Table

Let's build a program that generates the multiplication table for any number.

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1number = int(input("Enter a number: "))
2
3for multiplier in range(1, 11):
4    result = number * multiplier
5    print(f"{number} × {multiplier} = {result}")

Example:

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1Enter a number: 7
2
37 × 1 = 7
47 × 2 = 14
57 × 3 = 21
67 × 4 = 28
77 × 5 = 35
87 × 6 = 42
97 × 7 = 49
107 × 8 = 56
117 × 9 = 63
127 × 10 = 70

Practice Project: Sum of Numbers

You can use a loop to calculate the sum of a range of numbers.

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1total = 0
2
3for number in range(1, 101):
4    total += number
5
6print("Sum:", total)

Output:

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1Sum: 5050

For this particular problem, Python also provides a built-in sum() function:

pythonCopy
1total = sum(range(1, 101))
2print("Sum:", total)

The loop version is still useful for learning how accumulation works.

Practice Project: Factorial

The factorial of a positive integer n is:

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1n! = n × (n - 1) × (n - 2) × ... × 1

For example:

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15! = 5 × 4 × 3 × 2 × 1 = 120

Python implementation:

pythonCopy
1number = int(input("Enter a non-negative integer: "))
2
3if number < 0:
4    print("Factorial is not defined for negative integers.")
5else:
6    factorial = 1
7
8    for value in range(2, number + 1):
9        factorial *= value
10
11    print("Factorial:", factorial)

Practice Project: Count Vowels

Loops can process every character in a string.

pythonCopy
1text = input("Enter text: ")
2
3vowels = "aeiou"
4count = 0
5
6for character in text.lower():
7    if character in vowels:
8        count += 1
9
10print("Number of vowels:", count)

Example:

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1Enter text: Python Programming
2Number of vowels: 4

Practice Project: Find the Largest Number

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1numbers = [25, 12, 89, 42, 67]
2
3largest = numbers[0]
4
5for number in numbers[1:]:
6    if number > largest:
7        largest = number
8
9print("Largest:", largest)

Output:

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1Largest: 89

Python also has a built-in max() function:

pythonCopy
1print(max(numbers))

Learning the loop-based approach is valuable because it demonstrates how comparison and state tracking work internally.

Practice Project: Number Guessing Game

A while loop is useful for interactive programs.

pythonCopy
1secret_number = 7
2
3while True:
4    guess = int(input("Guess the number: "))
5
6    if guess == secret_number:
7        print("Correct! You guessed the number.")
8        break
9    elif guess < secret_number:
10        print("Too low. Try again.")
11    else:
12        print("Too high. Try again.")

This example combines:

• while
• if
• elif
• else
• break
• User input

Practice Project: Menu-Driven Program

Loops and conditional statements can be combined to create a simple menu.

pythonCopy
1while True:
2    print("\n1. Say Hello")
3    print("2. Show Python")
4    print("3. Exit")
5
6    choice = input("Choose an option: ").strip()
7
8    if choice == "1":
9        print("Hello!")
10    elif choice == "2":
11        print("Python is powerful.")
12    elif choice == "3":
13        print("Goodbye!")
14        break
15    else:
16        print("Invalid choice.")

The loop keeps displaying the menu until the user chooses 3.

Common Loop Mistakes

Forgetting to Update a while Loop Variable

Incorrect:

pythonCopy
1count = 1
2
3while count <= 5:
4    print(count)

The value of count never changes, so the condition remains true.

Correct:

pythonCopy
1count = 1
2
3while count <= 5:
4    print(count)
5    count += 1

Using the Wrong range() Endpoint

Remember that the stop value is exclusive.

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

Output:

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11
22
33
44

It does not print 5.

If you need 5, use:

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1range(1, 6)

Incorrect Indentation

Incorrect:

pythonCopy
1for number in range(5):
2print(number)

Correct:

pythonCopy
1for number in range(5):
2    print(number)

Python uses indentation to identify the loop body.

Using break Outside a Loop

Incorrect:

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1break

break can only be used inside a loop.

Confusing continue With break

break:

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1Stops the entire loop

continue:

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1Skips the current iteration

Example:

pythonCopy
1for number in range(1, 6):
2    if number == 3:
3        continue
4
5    print(number)

The loop continues after skipping 3.

Avoid Modifying a Collection While Iterating Over It

Modifying a list while directly iterating over it can produce unexpected behavior.

For example:

pythonCopy
1numbers = [1, 2, 3, 4, 5, 6]
2
3for number in numbers:
4    if number % 2 == 0:
5        numbers.remove(number)
6
7print(numbers)

This can skip elements because the list changes while the loop is traversing it.

A safer approach is to create a new list:

pythonCopy
1numbers = [1, 2, 3, 4, 5, 6]
2
3odd_numbers = []
4
5for number in numbers:
6    if number % 2 != 0:
7        odd_numbers.append(number)
8
9print(odd_numbers)

Output:

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1[1, 3, 5]

For simple filtering, a list comprehension is even more concise:

pythonCopy
1numbers = [1, 2, 3, 4, 5, 6]
2
3odd_numbers = [number for number in numbers if number % 2 != 0]
4
5print(odd_numbers)

Avoid Unnecessary Nested Loops

Nested loops are sometimes necessary, but they can become expensive for large datasets.

For example:

pythonCopy
1for i in range(100):
2    for j in range(100):
3        print(i, j)

The inner loop runs for every outer-loop iteration, resulting in 10,000 iterations.

When working with large data, look for ways to reduce unnecessary repeated work or use appropriate data structures and algorithms.

Useful Loop Patterns

Accumulator Pattern

Use a variable to accumulate a result.

pythonCopy
1total = 0
2
3for number in [10, 20, 30]:
4    total += number
5
6print(total)

Output:

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160

Counter Pattern

Use a variable to count matching items.

pythonCopy
1numbers = [10, 15, 20, 25, 30]
2count = 0
3
4for number in numbers:
5    if number % 5 == 0:
6        count += 1
7
8print("Count:", count)

Search Pattern

Use break when the target is found.

pythonCopy
1names = ["Ankit", "Rahul", "Aman"]
2
3target = "Rahul"
4
5for name in names:
6    if name == target:
7        print("Found")
8        break

Filtering Pattern

Create a new collection containing only matching values.

pythonCopy
1numbers = [1, 2, 3, 4, 5, 6]
2
3even_numbers = []
4
5for number in numbers:
6    if number % 2 == 0:
7        even_numbers.append(number)
8
9print(even_numbers)

Output:

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1[2, 4, 6]

Loop Control Statements Summary

Example:

pythonCopy
1for number in range(1, 6):
2    if number == 2:
3        continue
4
5    if number == 5:
6        break
7
8    print(number)

Output:

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11
23
34

Practice Exercises

Exercise 1: Print Even Numbers

Write a program that prints all even numbers from 1 to 50.

Expected output begins with:

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12
24
36
48
510
6...

Exercise 2: Calculate the Sum of Even Numbers

Calculate the sum of all even numbers between 1 and 100.

Exercise 3: Count Digits

Write a program that counts how many digits are present in an integer.

Example:

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1Input: 12345
2Output: 5

Exercise 4: Reverse a String

Write a loop that reverses a string without using slicing.

Example:

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1Input: Python
2Output: nohtyP

Exercise 5: Find Common Elements

Given two lists, use loops to find values that appear in both lists.

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1first = [1, 2, 3, 4, 5]
2second = [3, 4, 5, 6, 7]

Expected result:

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1[3, 4, 5]

Exercise 6: Fibonacci Sequence

Write a program that prints the first n Fibonacci numbers.

For example:

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10 1 1 2 3 5 8 13 21 34

Exercise 7: Prime Numbers

Write a program that prints all prime numbers between 1 and 100.

Exercise 8: Multiplication Tables

Use nested loops to print multiplication tables from 1 to 10.

Key Takeaways

• Loops allow you to execute code repeatedly.
• Python provides for and while loops.
• Use for when iterating over an iterable or a known sequence.
• Use while when repetition depends on a condition.
• range() generates integer sequences and uses an exclusive stop value.
• enumerate() provides both indexes and values.
• zip() allows multiple iterables to be processed together.
• break exits a loop immediately.
• continue skips the current iteration.
• pass acts as a placeholder and performs no operation.
• A loop can have an else block that runs when the loop finishes without break.
• Nested loops are useful for grids, tables, patterns, and combinations.
• Always make sure while loops have a way to eventually terminate.
• Avoid modifying a collection directly while iterating over it unless you fully understand the consequences.
• Built-in functions such as sum(), max(), and min() can often replace manual loops in production code, but understanding loops is essential for learning programming.

Loops become significantly more powerful when combined with conditional statements, functions, lists, dictionaries, comprehensions, and exception handling. These concepts form the foundation for building real-world Python applications.