beginner⏱60 minLesson 6 of 10

Control Flow: Loops

Master Python loops: while loops, for loops, range(), break, continue, nested loops, and iteration patterns.

Control Flow: Loops

Loops allow you to execute a block of code multiple times. They are essential for processing collections, repeating tasks, and implementing algorithms.

What Are Loops?

A loop repeats a block of code until a condition is met. Python provides two main loop types: while and for.

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The while Loop

The while loop repeats as long as a condition remains True.

Basic Syntax

python

while condition:
    # Code executes repeatedly while condition is True
    pass

Simple while Example

python

# Countdown from 5
count = 5

while count > 0:
    print(f"Count: {count}")
    count -= 1  # IMPORTANT: Update the counter!

print("Liftoff! 🚀")

Output:

Count: 5
Count: 4
Count: 3
Count: 2
Count: 1
Liftoff! 🚀

⚠️Warning

Always ensure the loop condition eventually becomes False! Forgetting to update the loop variable creates an infinite loop:

python

# INFINITE LOOP - don't do this!
count = 5
while count > 0:
    print(count)
    # Forgot: count -= 1

while Loop with User Input

python

# number_guessing.py
import random

def guess_the_number():
    """Simple number guessing game using while loop."""
    
    secret = random.randint(1, 20)
    attempts = 0
    max_attempts = 5
    
    print("=== Number Guessing Game ===")
    print("I'm thinking of a number between 1 and 20.")
    print(f"You have {max_attempts} attempts.\n")
    
    while attempts < max_attempts:
        guess = int(input(f"Attempt {attempts + 1}/{max_attempts}: "))
        attempts += 1
        
        if guess == secret:
            print(f"🎉 Correct! You got it in {attempts} attempts!")
            return
        elif guess < secret:
            print("Too low! Try higher.")
        else:
            print("Too high! Try lower.")
    
    print(f"\n😔 Game over! The number was {secret}.")

# Run the game
guess_the_number()

Sample output:

=== Number Guessing Game ===
I'm thinking of a number between 1 and 20.
You have 5 attempts.

Attempt 1/5: 10
Too high! Try lower.
Attempt 2/5: 5
Too low! Try higher.
Attempt 3/5: 7
🎉 Correct! You got it in 3 attempts!

The for Loop

The for loop iterates over a sequence (list, string, range, etc.).

Basic Syntax

python

for item in sequence:
    # Code executes once for each item in the sequence
    pass

Iterating Over Different Sequences

python

# Iterating over a list
fruits = ["apple", "banana", "cherry"]
print("Fruits:")
for fruit in fruits:
    print(f"  - {fruit}")

# Iterating over a string
print("\nLetters in 'Python':")
for letter in "Python":
    print(f"  {letter}")

# Iterating over a tuple
coordinates = (10, 20, 30)
print("\nCoordinates:")
for coord in coordinates:
    print(f"  {coord}")

Output:

Fruits:
  - apple
  - banana
  - cherry

Letters in 'Python':
  P
  y
  t
  h
  o
  n

Coordinates:
  10
  20
  30

The range() Function

range() generates a sequence of numbers, commonly used with for loops.

range() Variants

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range() Examples

python

# range(stop) - starts at 0, stops before stop
print("range(5):")
for i in range(5):
    print(i, end=" ")
# Output: 0 1 2 3 4

print("\n\nrange(2, 6):")
for i in range(2, 6):
    print(i, end=" ")
# Output: 2 3 4 5

print("\n\nrange(0, 10, 2):")
for i in range(0, 10, 2):
    print(i, end=" ")
# Output: 0 2 4 6 8

print("\n\nrange(5, 0, -1):")
for i in range(5, 0, -1):
    print(i, end=" ")
# Output: 5 4 3 2 1

Practical range() Uses

python

# Sum of first n numbers
def sum_first_n(n):
    total = 0
    for i in range(1, n + 1):
        total += i
    return total

print(f"Sum of 1 to 100: {sum_first_n(100)}")  # 5050

# Multiplication table
def print_multiplication_table(number, up_to=10):
    print(f"\nMultiplication Table for {number}:")
    for i in range(1, up_to + 1):
        print(f"  {number} × {i:2d} = {number * i:3d}")

print_multiplication_table(7)

Output:

Sum of 1 to 100: 5050

Multiplication Table for 7:
  7 ×  1 =   7
  7 ×  2 =  14
  7 ×  3 =  21
  7 ×  4 =  28
  7 ×  5 =  35
  7 ×  6 =  42
  7 ×  7 =  49
  7 ×  8 =  56
  7 ×  9 =  63
  7 × 10 =  70

Loop Control: break and continue

Control statements modify loop behavior.

break - Exit the Loop Immediately

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python

# break example: find first even number
numbers = [1, 3, 5, 8, 9, 10, 11]

for num in numbers:
    if num % 2 == 0:
        print(f"First even number found: {num}")
        break
    print(f"  Checking {num}... (odd)")

Output:

  Checking 1... (odd)
  Checking 3... (odd)
  Checking 5... (odd)
First even number found: 8

continue - Skip to Next Iteration

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python

# continue example: skip odd numbers
for num in range(1, 11):
    if num % 2 != 0:
        continue  # Skip odd numbers
    print(num, end=" ")
# Output: 2 4 6 8 10

break and continue Comparison

python

# Demonstration of break vs continue
print("Using break (stops at 5):")
for i in range(1, 11):
    if i == 5:
        break
    print(i, end=" ")
# Output: 1 2 3 4

print("\n\nUsing continue (skips 5):")
for i in range(1, 11):
    if i == 5:
        continue
    print(i, end=" ")
# Output: 1 2 3 4 6 7 8 9 10

Nested Loops

Loops inside loops are called nested loops. The inner loop completes all its iterations for each iteration of the outer loop.

Nested Loop Flow

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Nested Loop Examples

python

# Multiplication table grid
print("Multiplication Table (1-5):")
print("    ", end="")
for j in range(1, 6):
    print(f"{j:4d}", end="")
print("\n" + "-" * 25)

for i in range(1, 6):
    print(f"{i:2d} |", end="")
    for j in range(1, 6):
        print(f"{i * j:4d}", end="")
    print()

Output:

Multiplication Table (1-5):
       1   2   3   4   5
-------------------------
 1 |   1   2   3   4   5
 2 |   2   4   6   8  10
 3 |   3   6   9  12  15
 4 |   4   8  12  16  20
 5 |   5  10  15  20  25

Pattern Printing with Nested Loops

python

# Print a pyramid pattern
def print_pyramid(height):
    """Print a pyramid of stars."""
    for i in range(1, height + 1):
        # Print spaces
        spaces = " " * (height - i)
        # Print stars
        stars = "*" * (2 * i - 1)
        print(spaces + stars)

print("Pyramid of height 5:")
print_pyramid(5)

Output:

Pyramid of height 5:
    *
   ***
  *****
 *******
*********

The else Clause with Loops

Python allows an else clause with loops. It executes when the loop completes normally (not via break).

Loop else Syntax

python

for item in sequence:
    if condition:
        break
else:
    # Executes if loop completed without break
    pass

Loop else Example

python

# Search with else clause
def search_number(numbers, target):
    """Search for a number and report if found."""
    for num in numbers:
        if num == target:
            print(f"Found {target}!")
            break
    else:
        print(f"{target} not found in the list.")

search_number([1, 3, 5, 7, 9], 5)   # Found
search_number([1, 3, 5, 7, 9], 10)  # Not found

Output:

Found 5!
10 not found in the list.

Enumerate and zip

Python provides useful functions for working with loops.

enumerate() - Get Index and Value

python

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

# Without enumerate
print("Without enumerate:")
for i in range(len(fruits)):
    print(f"  {i}: {fruits[i]}")

# With enumerate (cleaner!)
print("\nWith enumerate:")
for index, fruit in enumerate(fruits):
    print(f"  {index}: {fruit}")

# Starting index at 1
print("\nStarting at 1:")
for index, fruit in enumerate(fruits, start=1):
    print(f"  {index}. {fruit}")

zip() - Iterate Over Multiple Sequences

python

names = ["Alice", "Bob", "Charlie"]
ages = [25, 30, 35]
cities = ["NYC", "London", "Tokyo"]

print("Using zip:")
for name, age, city in zip(names, ages, cities):
    print(f"  {name} is {age} years old and lives in {city}")

Output:

Using zip:
  Alice is 25 years old and lives in NYC
  Bob is 30 years old and lives in London
  Charlie is 35 years old and lives in Tokyo

Real-World Example: Data Analysis with Loops

python

# data_analysis.py
def analyze_grades(student_data):
    """Analyze student grades using loops."""
    
    print("=" * 55)
    print("         STUDENT GRADE ANALYSIS")
    print("=" * 55)
    
    total_students = 0
    total_grade = 0
    highest_grade = 0
    highest_student = ""
    lowest_grade = 100
    lowest_student = ""
    passed = 0
    failed = 0
    
    for name, grade in student_data:
        total_students += 1
        total_grade += grade
        
        # Track highest and lowest
        if grade > highest_grade:
            highest_grade = grade
            highest_student = name
        
        if grade < lowest_grade:
            lowest_grade = grade
            lowest_student = name
        
        # Count pass/fail
        if grade >= 60:
            passed += 1
        else:
            failed += 1
        
        # Print individual result
        status = "PASS" if grade >= 60 else "FAIL"
        print(f"  {name:15s} {grade:6.1f}  {status}")
    
    # Summary
    print("-" * 55)
    average = total_grade / total_students
    print(f"  Total Students: {total_students}")
    print(f"  Average Grade:  {average:.1f}")
    print(f"  Highest:        {highest_student} ({highest_grade:.1f})")
    print(f"  Lowest:         {lowest_student} ({lowest_grade:.1f})")
    print(f"  Passed:         {passed}")
    print(f"  Failed:         {failed}")
    print(f"  Pass Rate:      {passed/total_students*100:.1f}%")
    print("=" * 55)

# Sample data
students = [
    ("Alice", 92.5),
    ("Bob", 78.0),
    ("Charlie", 45.0),
    ("Diana", 88.5),
    ("Eve", 95.0),
    ("Frank", 62.0),
    ("Grace", 71.5),
    ("Henry", 55.0),
]

analyze_grades(students)

Output:

=======================================================
         STUDENT GRADE ANALYSIS
=======================================================
  Alice            92.5  PASS
  Bob              78.0  PASS
  Charlie          45.0  FAIL
  Diana            88.5  PASS
  Eve              95.0  PASS
  Frank            62.0  PASS
  Grace            71.5  PASS
  Henry            55.0  FAIL
-------------------------------------------------------
  Total Students: 8
  Average Grade:  73.4
  Highest:        Eve (95.0)
  Lowest:         Charlie (45.0)
  Passed:         6
  Failed:         2
  Pass Rate:      75.0%
=======================================================

Practice Exercises

Exercise 1: Sum of Even Numbers

Write a loop that calculates the sum of all even numbers from 1 to 100.

Exercise 2: Factorial Calculator

Write a program that calculates the factorial of a number using a for loop. (5! = 5 × 4 × 3 × 2 × 1 = 120)

Exercise 3: FizzBuzz

Write a program that prints numbers 1 to 100, but:

For multiples of 3, print "Fizz"
For multiples of 5, print "Buzz"
For multiples of both, print "FizzBuzz"

Exercise 4: Prime Number Checker

Write a program that checks if a number is prime using a loop.

Exercise 5: Reverse a String

Write a loop that reverses a string without using built-in reverse functions.

Exercise 6: Pattern Printing

Use nested loops to print this pattern:

Exercise 7: Number Guessing Game

Create a complete number guessing game where the computer picks a random number and the user has to guess it. Provide hints (too high/too low) and count attempts.

Exercise 8: Password Validator

Write a program that asks for a password and uses a loop to check:

At least 8 characters
Contains at least one uppercase letter
Contains at least one digit Keep asking until a valid password is entered.

Summary

In this lesson, you learned:

How to use while loops for condition-based repetition
How to use for loops for sequence iteration
How range() generates number sequences
How break exits a loop immediately
How continue skips to the next iteration
How nested loops work and when to use them
How the else clause works with loops
How enumerate() and zip() enhance iteration
How to apply loops to real-world data processing

Loops are fundamental to programming. Master them to efficiently process data and implement algorithms.

Progress60%

Control Flow: Conditionals

Functions

Course Content

1. How Computers Work 2. Python Setup & First Program 3. Variables & Data Types 4. Operators & Expressions 5. Control Flow: Conditionals 6. Control Flow: Loops 7. Functions 8. Data Structures: Lists & Tuples 9. Data Structures: Dictionaries & Sets 10. Imperative Algorithm Projects