Chapter 18: For Loops¶

🚀 Open Notebook¶

📺 Video Tutorial¶

Learn Python for loops in 5 minutes! 🔁 (5:47)

📚 What You’ll Learn¶

Master the for loop - Python’s most elegant way to iterate through sequences and repeat code!

🎯 Learning Objectives¶

Understand for loop syntax and usage
Use range() function effectively
Iterate through strings, lists, and sequences
Use reversed() and step parameters
Apply enumerate() for indexed iteration
Combine loops with conditionals

📖 Concept Explanation¶

What is a For Loop?¶

A for loop iterates through a sequence (like a list, string, or range) and executes code for each item.

Syntax¶

for variable in sequence:
    # code to execute for each item

How It Works¶

Take first item from sequence
Execute the code block
Take next item
Repeat until no items left

For Loop vs While Loop¶

For loop: When you know how many times to repeat
While loop: When you repeat until a condition is false

💡 Examples¶

Example 1: Basic For Loop¶

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

# Output: 0, 1, 2, 3, 4

Example 2: Iterating Through a String¶

name = "Python"

for letter in name:
    print(letter)

# Output:
# P
# y
# t
# h
# o
# n

Example 3: Iterating Through a List¶

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

for fruit in fruits:
    print(f"I like {fruit}")

# Output:
# I like apple
# I like banana
# I like cherry

Example 4: Using range() with Different Parameters¶

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

print()

# range(start, stop)
for i in range(2, 7):
    print(i, end=" ")  # 2 3 4 5 6

print()

# range(start, stop, step)
for i in range(0, 10, 2):
    print(i, end=" ")  # 0 2 4 6 8

Example 5: Countdown with Reversed¶

# Countdown from 10 to 1
for i in range(10, 0, -1):
    print(i)
print("Blastoff!")

# Alternative using reversed()
for i in reversed(range(1, 11)):
    print(i)
print("Blastoff!")

Example 6: Sum of Numbers¶

total = 0

for num in range(1, 11):
    total += num

print(f"Sum of 1-10: {total}")  # 55

Example 7: Multiplication Table¶

number = 7

print(f"Multiplication table for {number}:\n")

for i in range(1, 11):
    result = number * i
    print(f"{number} × {i} = {result}")

Example 8: Enumerate for Index and Value¶

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

for index, fruit in enumerate(fruits):
    print(f"{index}: {fruit}")

# Output:
# 0: apple
# 1: banana
# 2: cherry

# Start index from 1
for index, fruit in enumerate(fruits, start=1):
    print(f"{index}. {fruit}")

Example 9: Iterating Through Dictionary¶

student = {
    "name": "Alice",
    "age": 20,
    "grade": "A"
}

# Iterate through keys
for key in student:
    print(f"{key}: {student[key]}")

# Iterate through key-value pairs
for key, value in student.items():
    print(f"{key}: {value}")

Example 10: Nested For Loops (Preview)¶

for i in range(3):
    for j in range(3):
        print(f"({i}, {j})", end=" ")
    print()

# Output:
# (0, 0) (0, 1) (0, 2) 
# (1, 0) (1, 1) (1, 2) 
# (2, 0) (2, 1) (2, 2)

🔍 Common Mistakes¶

Mistake 1: Off-by-One Errors¶

# Wrong: range stops BEFORE the end
for i in range(10):
    print(i)  # Prints 0-9, NOT 0-10

# Correct: Add 1 to include the number
for i in range(11):
    print(i)  # Prints 0-10

# Or use appropriate start/stop
for i in range(1, 11):
    print(i)  # Prints 1-10

Mistake 2: Modifying List While Iterating¶

# Dangerous!
numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num % 2 == 0:
        numbers.remove(num)  # Can skip elements!

# Better approach
numbers = [1, 2, 3, 4, 5]
numbers = [num for num in numbers if num % 2 != 0]

Mistake 3: Using Wrong Variable Name¶

fruits = ["apple", "banana"]

# Confusing
for fruit in fruits:
    print(fruits)  # Prints whole list twice!

# Clear
for fruit in fruits:
    print(fruit)  # Prints each fruit

Mistake 4: Forgetting range() Around Numbers¶

# Wrong
for i in 5:  # TypeError!
    print(i)

# Correct
for i in range(5):
    print(i)

✍️ Practice Exercises¶

Easy¶

Print numbers 1-20
Print even numbers from 2-20
Print your name 10 times
Sum numbers from 1-100
Print each character in a word

Medium¶

Create a multiplication table (1-12)
Count vowels in a string
Find factorial of a number
Print Fibonacci sequence (first 10 numbers)
Calculate average of a list of numbers

Hard¶

Print prime numbers up to 50
Reverse a string using a loop
Find all divisors of a number
Create a pattern printer
Build a grade calculator for multiple students

🎮 Real-World Applications¶

Application 1: Grade Calculator¶

print("=== Grade Calculator ===\n")

num_students = int(input("How many students? "))
total_average = 0

for i in range(1, num_students + 1):
    print(f"\nStudent {i}:")
    grade = float(input("Enter grade: "))
    total_average += grade
    
    if grade >= 90:
        letter = "A"
    elif grade >= 80:
        letter = "B"
    elif grade >= 70:
        letter = "C"
    elif grade >= 60:
        letter = "D"
    else:
        letter = "F"
    
    print(f"Letter grade: {letter}")

class_average = total_average / num_students
print(f"\nClass average: {class_average:.2f}")

Application 2: Shopping Receipt¶

print("=== Shopping Receipt ===\n")

items = []
prices = []
quantities = []

num_items = int(input("How many items? "))

for i in range(num_items):
    print(f"\nItem {i + 1}:")
    item = input("Item name: ")
    price = float(input("Price: $"))
    qty = int(input("Quantity: "))
    
    items.append(item)
    prices.append(price)
    quantities.append(qty)

print("\n" + "="*40)
print("RECEIPT")
print("="*40)

total = 0
for i in range(num_items):
    subtotal = prices[i] * quantities[i]
    total += subtotal
    print(f"{items[i]:<20} {quantities[i]}× ${prices[i]:.2f} = ${subtotal:.2f}")

print("="*40)
print(f"{'TOTAL:':<20} ${total:.2f}")
print("="*40)

Application 3: Password Generator¶

import random

print("=== Password Generator ===\n")

length = int(input("Password length: "))
password = ""

lowercase = "abcdefghijklmnopqrstuvwxyz"
uppercase = "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
digits = "0123456789"
symbols = "!@#$%^&*"

all_chars = lowercase + uppercase + digits + symbols

for i in range(length):
    password += random.choice(all_chars)

print(f"Generated password: {password}")

Application 4: Data Analysis¶

# Temperature analysis
temperatures = [72, 75, 68, 70, 73, 71, 69, 74, 76, 72]

total = 0
highest = temperatures[0]
lowest = temperatures[0]

for temp in temperatures:
    total += temp
    if temp > highest:
        highest = temp
    if temp < lowest:
        lowest = temp

average = total / len(temperatures)

print(f"Average temperature: {average:.1f}°F")
print(f"Highest: {highest}°F")
print(f"Lowest: {lowest}°F")
print(f"Range: {highest - lowest}°F")

Application 5: ASCII Art Generator¶

# Triangle pattern
height = int(input("Enter height: "))

for i in range(1, height + 1):
    print("* " * i)

# Output for height=5:
# * 
# * * 
# * * * 
# * * * * 
# * * * * *

🚀 Challenge Projects¶

Challenge 1: Prime Number Finder¶

Create a program that:

Finds all prime numbers up to N
Shows how many primes were found
Calculates sum of all primes
Displays them in rows of 10

Challenge 2: Fibonacci Sequence¶

Build a program that:

Generates first N Fibonacci numbers
Shows the ratio between consecutive numbers
Calculates sum of even Fibonacci numbers
Finds Fibonacci numbers under a limit

Challenge 3: Statistics Calculator¶

Create a tool that:

Accepts multiple numbers from user
Calculates mean, median, mode
Finds range and standard deviation
Shows frequency distribution

Challenge 4: Text Analyzer¶

Build a program that:

Counts words, sentences, paragraphs
Finds most common words
Calculates average word length
Identifies longest and shortest words

Challenge 5: Pattern Creator¶

Create patterns like:

    *
   ***
  *****
 *******
*********

Allow user to choose height
Create different shapes (diamond, pyramid, etc.)
Use different characters
Add colors (using ANSI codes)

📚 Range() Function Reference¶

# Basic usage
range(stop)           # 0 to stop-1
range(start, stop)    # start to stop-1
range(start, stop, step)  # start to stop-1, by step

# Examples
range(5)              # 0, 1, 2, 3, 4
range(2, 8)           # 2, 3, 4, 5, 6, 7
range(0, 10, 2)       # 0, 2, 4, 6, 8
range(10, 0, -1)      # 10, 9, 8, 7, 6, 5, 4, 3, 2, 1
range(5, 0, -2)       # 5, 3, 1

🔄 Loop Control Statements¶

Break - Exit Loop Early¶

for i in range(10):
    if i == 5:
        break
    print(i)
# Prints: 0, 1, 2, 3, 4

Continue - Skip Current Iteration¶

for i in range(6):
    if i == 3:
        continue
    print(i)
# Prints: 0, 1, 2, 4, 5

Else with For Loop¶

for i in range(5):
    print(i)
else:
    print("Loop completed!")
# Runs if loop wasn't broken

🎯 Common Patterns¶

Count Occurrences¶

text = "hello world"
count = 0

for char in text:
    if char == 'l':
        count += 1

print(f"'l' appears {count} times")

Build a String¶

result = ""

for i in range(5):
    result += str(i) + " "

print(result)  # "0 1 2 3 4 "

Find Maximum¶

numbers = [34, 78, 12, 90, 45]
maximum = numbers[0]

for num in numbers:
    if num > maximum:
        maximum = num

print(f"Maximum: {maximum}")

💪 Try It Yourself¶

Modify main.py to:

Create a countdown timer
Build a times table generator
Make a number pattern printer
Calculate factorial
Find all multiples of 3 in a range

🎓 Key Takeaways from Video¶

Strings are text data enclosed in quotes
Understanding the proper syntax is important
Use loops to repeat actions
Use if-elif-else for conditional logic

💡 These points cover the main concepts from the video tutorial to help reinforce your learning.

🔗 Next Chapter¶

Continue to Chapter 19: Countdown Timer to build a real countdown timer application!