Loops are programming constructs that allow you to repeat a block of code multiple times. Instead of writing the same code over and over, loops let you execute code repeatedly while a condition is true or for a specific number of iterations.
Python provides two main types of loops: for loops and while loops. Each serves different purposes and is useful in different situations.
example:
# Without a loop - repetitive and inefficient
print("Welcome to Python!")
print("Welcome to Python!")
print("Welcome to Python!")
print("Welcome to Python!")
print("Welcome to Python!")
# With a loop - clean and efficient
for i in range(5):
print("Welcome to Python!")
The for loop is used to iterate over a sequence (like a list, string, or range) or any iterable object. It's perfect when you know how many times you want to repeat something or when you want to process each item in a collection.
# Print numbers from 0 to 4
for i in range(5):
print(f"Number: {i}")
# Print numbers from 1 to 10
for i in range(1, 11):
print(f"Count: {i}")
# Print even numbers from 2 to 20
for i in range(2, 21, 2):
print(f"Even number: {i}")
# Countdown from 10 to 1
for i in range(10, 0, -1):
print(f"Countdown: {i}")
# Iterate through a list of names
students = ["Alice", "Bob", "Charlie", "Diana", "Emma"]
for student in students:
print(f"Hello, {student}!")
# Process a list of numbers
scores = [85, 92, 78, 96, 88]
for score in scores:
if score >= 90:
print(f"Excellent score: {score}")
elif score >= 80:
print(f"Good score: {score}")
else:
print(f"Score needs improvement: {score}")
# Calculate total and average
total = 0
for score in scores:
total += score
average = total / len(scores)
print(f"Total: {total}, Average: {average:.1f}")
# Iterate through each character in a string
name = "Python"
for letter in name:
print(f"Letter: {letter}")
# Count vowels in a string
message = "Hello World"
vowels = "aeiouAEIOU"
vowel_count = 0
for char in message:
if char in vowels:
vowel_count += 1
print(f"Number of vowels in '{message}': {vowel_count}")
# Create a reversed string
original = "Programming"
reversed_string = ""
for char in original:
reversed_string = char + reversed_string
print(f"Original: {original}")
print(f"Reversed: {reversed_string}")
# Get both index and value while looping
fruits = ["apple", "banana", "cherry", "date"]
for index, fruit in enumerate(fruits):
print(f"{index}: {fruit}")
# Start enumeration from 1 instead of 0
for position, fruit in enumerate(fruits, 1):
print(f"Position {position}: {fruit}")
# Process items with their positions
shopping_list = ["milk", "bread", "eggs", "cheese"]
print("Shopping List:")
for item_number, item in enumerate(shopping_list, 1):
print(f"{item_number}. {item}")
The while loop continues executing as long as a condition remains true. It's useful when you don't know exactly how many iterations you need or when the loop should continue based on changing conditions.
# Count from 1 to 5
count = 1
while count <= 5:
print(f"Count: {count}")
count += 1
# User input validation
password = ""
while password != "secret123":
password = input("Enter password: ")
if password != "secret123":
print("Incorrect password. Try again.")
print("Access granted!")
# Generate random numbers until we get 5
import random
number = 0
attempts = 0
while number != 5:
number = random.randint(1, 10)
attempts += 1
print(f"Attempt {attempts}: Got {number}")
print(f"Found 5 after {attempts} attempts!")
# Bank account simulation
balance = 1000
while balance > 0:
print(f"Current balance: ${balance}")
withdrawal = float(input("Enter withdrawal amount (0 to quit): "))
if withdrawal == 0:
break
elif withdrawal > balance:
print("Insufficient funds!")
else:
balance -= withdrawal
print(f"Withdrew ${withdrawal}")
print(f"Final balance: ${balance}")
# Number guessing game
import random
secret_number = random.randint(1, 100)
guess = 0
attempts = 0
max_attempts = 7
while guess != secret_number and attempts < max_attempts:
guess = int(input(f"Guess the number (1-100). Attempt {attempts + 1}/{max_attempts}: "))
attempts += 1
if guess < secret_number:
print("Too low!")
elif guess > secret_number:
print("Too high!")
else:
print(f"Congratulations! You guessed it in {attempts} attempts!")
if guess != secret_number:
print(f"Game over! The number was {secret_number}")
You can place loops inside other loops to handle more complex repetitive tasks.
# Multiplication table
print("Multiplication Table:")
for i in range(1, 6):
for j in range(1, 6):
result = i * j
print(f"{i} x {j} = {result:2d}", end=" ")
print() # New line after each row
# Pattern printing
print("\nStar Pattern:")
for row in range(1, 6):
for col in range(row):
print("*", end=" ")
print() # New line after each row
# Number pyramid
print("\nNumber Pyramid:")
for row in range(1, 6):
for col in range(1, row + 1):
print(col, end=" ")
print()
# Student grades matrix
grades = [
[85, 92, 78], # Alice's grades
[90, 88, 95], # Bob's grades
[79, 83, 91] # Charlie's grades
]
student_names = ["Alice", "Bob", "Charlie"]
subjects = ["Math", "Science", "English"]
# Calculate average for each student
for i, student in enumerate(student_names):
total = 0
for grade in grades[i]:
total += grade
average = total / len(grades[i])
print(f"{student}'s average: {average:.1f}")
# Calculate average for each subject
print("\nSubject Averages:")
for j, subject in enumerate(subjects):
total = 0
for i in range(len(grades)):
total += grades[i][j]
average = total / len(grades)
print(f"{subject} average: {average:.1f}")
Python provides statements to control loop execution: break, continue, and pass.
# Exit loop early
numbers = [1, 3, 7, 2, 9, 4, 6]
for num in numbers:
if num == 9:
print("Found 9! Stopping search.")
break
print(f"Checking: {num}")
# Menu system with exit option
while True:
print("\n=== Main Menu ===")
print("1. View Profile")
print("2. Edit Settings")
print("3. Logout")
choice = input("Enter choice (1-3): ")
if choice == "1":
print("Displaying profile...")
elif choice == "2":
print("Opening settings...")
elif choice == "3":
print("Logging out...")
break
else:
print("Invalid choice!")
# Find first even number
numbers = [1, 3, 5, 8, 11, 12, 15]
for num in numbers:
if num % 2 == 0:
print(f"First even number found: {num}")
break
else:
print("No even number found")
# Skip specific iterations
for i in range(1, 11):
if i % 2 == 0:
continue # Skip even numbers
print(f"Odd number: {i}")
# Process only valid data
data = [10, -5, 8, 0, 15, -2, 20]
positive_sum = 0
for number in data:
if number <= 0:
continue # Skip non-positive numbers
positive_sum += number
print(f"Added {number}, sum now: {positive_sum}")
print(f"Sum of positive numbers: {positive_sum}")
# Skip empty or invalid inputs
users = ["Alice", "", "Bob", " ", "Charlie", None]
print("Valid users:")
for user in users:
if not user or not user.strip():
continue # Skip empty or whitespace-only names
print(f"- {user}")
# Placeholder for future implementation
for i in range(5):
if i == 2:
pass # TODO: Add special handling for i=2
else:
print(f"Processing: {i}")
# Empty loop body (rare but sometimes needed)
for item in some_list:
pass # Just iterate without doing anything
# Placeholder in conditional structures within loops
for user in users:
if user.is_admin():
pass # Admin handling to be implemented
elif user.is_moderator():
print(f"Moderator: {user.name}")
else:
print(f"Regular user: {user.name}")
# Sum of numbers
numbers = [10, 20, 30, 40, 50]
total = 0
for number in numbers:
total += number
print(f"Total: {total}")
# Product of numbers
numbers = [2, 3, 4, 5]
product = 1
for number in numbers:
product *= number
print(f"Product: {product}")
# Concatenating strings
words = ["Python", "is", "awesome"]
sentence = ""
for word in words:
sentence += word + " "
print(f"Sentence: {sentence.strip()}")
# Finding maximum
scores = [85, 92, 78, 96, 88]
max_score = scores[0]
for score in scores:
if score > max_score:
max_score = score
print(f"Highest score: {max_score}")
# Count specific items
grades = ['A', 'B', 'A', 'C', 'B', 'A', 'D']
a_count = 0
for grade in grades:
if grade == 'A':
a_count += 1
print(f"Number of A grades: {a_count}")
# Count multiple categories
text = "Hello World"
vowel_count = 0
consonant_count = 0
for char in text.lower():
if char.isalpha():
if char in 'aeiou':
vowel_count += 1
else:
consonant_count += 1
print(f"Vowels: {vowel_count}, Consonants: {consonant_count}")
# Count words by length
sentence = "Python programming is fun and educational"
words = sentence.split()
short_words = 0 # 1-4 letters
medium_words = 0 # 5-7 letters
long_words = 0 # 8+ letters
for word in words:
if len(word) <= 4:
short_words += 1
elif len(word) <= 7:
medium_words += 1
else:
long_words += 1
print(f"Short: {short_words}, Medium: {medium_words}, Long: {long_words}")
# Filter positive numbers
numbers = [10, -5, 8, -2, 15, -1, 20]
positive_numbers = []
for number in numbers:
if number > 0:
positive_numbers.append(number)
print(f"Positive numbers: {positive_numbers}")
# Filter students by grade
students = [
{"name": "Alice", "grade": 92},
{"name": "Bob", "grade": 76},
{"name": "Charlie", "grade": 88},
{"name": "Diana", "grade": 95}
]
honor_students = []
for student in students:
if student["grade"] >= 90:
honor_students.append(student["name"])
print(f"Honor students: {honor_students}")
# Filter valid email addresses
emails = ["user@example.com", "invalid-email", "test@test.com", "bad@", "good@domain.org"]
valid_emails = []
for email in emails:
if "@" in email and "." in email:
valid_emails.append(email)
print(f"Valid emails: {valid_emails}")
# Iterate through dictionary keys
student_grades = {"Alice": 92, "Bob": 85, "Charlie": 90, "Diana": 88}
print("Student names:")
for name in student_grades:
print(f"- {name}")
# Iterate through values
print("\nGrades:")
for grade in student_grades.values():
print(f"- {grade}")
# Iterate through key-value pairs
print("\nStudent grades:")
for name, grade in student_grades.items():
print(f"{name}: {grade}")
# Update dictionary values
for name in student_grades:
if student_grades[name] >= 90:
student_grades[name] = "A"
elif student_grades[name] >= 80:
student_grades[name] = "B"
else:
student_grades[name] = "C"
print("\nLetter grades:")
for name, letter_grade in student_grades.items():
print(f"{name}: {letter_grade}")
def check_password_strength():
"""Check password strength with detailed feedback."""
while True:
password = input("Enter a password to check (or 'quit' to exit): ")
if password.lower() == 'quit':
break
# Initialize criteria checks
has_upper = False
has_lower = False
has_digit = False
has_special = False
special_chars = "!@#$%^&*()_+-=[]{}|;:,.<>?"
# Check each character
for char in password:
if char.isupper():
has_upper = True
elif char.islower():
has_lower = True
elif char.isdigit():
has_digit = True
elif char in special_chars:
has_special = True
# Calculate strength score
score = 0
feedback = []
if len(password) >= 8:
score += 1
else:
feedback.append("Add more characters (minimum 8)")
if has_upper:
score += 1
else:
feedback.append("Add uppercase letters")
if has_lower:
score += 1
else:
feedback.append("Add lowercase letters")
if has_digit:
score += 1
else:
feedback.append("Add numbers")
if has_special:
score += 1
else:
feedback.append("Add special characters")
# Determine strength level
if score == 5:
strength = "Very Strong"
elif score >= 4:
strength = "Strong"
elif score >= 3:
strength = "Moderate"
elif score >= 2:
strength = "Weak"
else:
strength = "Very Weak"
print(f"\nPassword strength: {strength} ({score}/5)")
if feedback:
print("Suggestions:")
for suggestion in feedback:
print(f"- {suggestion}")
print("-" * 40)
check_password_strength()
def generate_sales_report():
"""Generate a comprehensive sales report."""
# Sample sales data
sales_data = [
{"product": "Laptop", "quantity": 5, "price": 1200, "month": "January"},
{"product": "Mouse", "quantity": 20, "price": 25, "month": "January"},
{"product": "Keyboard", "quantity": 15, "price": 75, "month": "January"},
{"product": "Laptop", "quantity": 8, "price": 1200, "month": "February"},
{"product": "Mouse", "quantity": 25, "price": 25, "month": "February"},
{"product": "Monitor", "quantity": 10, "price": 300, "month": "February"},
]
print("=== SALES REPORT ===\n")
# Calculate total revenue by product
product_revenue = {}
for sale in sales_data:
product = sale["product"]
revenue = sale["quantity"] * sale["price"]
if product in product_revenue:
product_revenue[product] += revenue
else:
product_revenue[product] = revenue
print("Revenue by Product:")
total_revenue = 0
for product, revenue in product_revenue.items():
print(f"{product}: ${revenue:,}")
total_revenue += revenue
print(f"\nTotal Revenue: ${total_revenue:,}")
# Calculate monthly totals
monthly_revenue = {}
for sale in sales_data:
month = sale["month"]
revenue = sale["quantity"] * sale["price"]
if month in monthly_revenue:
monthly_revenue[month] += revenue
else:
monthly_revenue[month] = revenue
print("\nRevenue by Month:")
for month, revenue in monthly_revenue.items():
print(f"{month}: ${revenue:,}")
# Find best-selling product by quantity
product_quantities = {}
for sale in sales_data:
product = sale["product"]
quantity = sale["quantity"]
if product in product_quantities:
product_quantities[product] += quantity
else:
product_quantities[product] = quantity
best_seller = ""
max_quantity = 0
for product, quantity in product_quantities.items():
if quantity > max_quantity:
max_quantity = quantity
best_seller = product
print(f"\nBest Selling Product: {best_seller} ({max_quantity} units)")
# Calculate average sale value
total_sales_count = len(sales_data)
average_sale = total_revenue / total_sales_count
print(f"Average Sale Value: ${average_sale:.2f}")
generate_sales_report()
def analyze_text():
"""Comprehensive text analysis tool."""
text = input("Enter text to analyze: ")
if not text.strip():
print("No text provided!")
return
print(f"\n=== ANALYSIS OF: '{text}' ===\n")
# Basic statistics
char_count = len(text)
char_count_no_spaces = len(text.replace(" ", ""))
word_count = len(text.split())
sentence_count = text.count('.') + text.count('!') + text.count('?')
print("Basic Statistics:")
print(f"Characters (with spaces): {char_count}")
print(f"Characters (without spaces): {char_count_no_spaces}")
print(f"Words: {word_count}")
print(f"Sentences: {sentence_count}")
# Character frequency analysis
char_frequency = {}
for char in text.lower():
if char.isalpha():
if char in char_frequency:
char_frequency[char] += 1
else:
char_frequency[char] = 1
print(f"\nCharacter Frequencies:")
for char in sorted(char_frequency.keys()):
print(f"'{char}': {char_frequency[char]}")
# Word frequency analysis
words = text.lower().split()
word_frequency = {}
for word in words:
# Remove punctuation
clean_word = ""
for char in word:
if char.isalpha():
clean_word += char
if clean_word:
if clean_word in word_frequency:
word_frequency[clean_word] += 1
else:
word_frequency[clean_word] = 1
print(f"\nWord Frequencies:")
for word in sorted(word_frequency.keys()):
print(f"'{word}': {word_frequency[word]}")
# Find longest and shortest words
longest_word = ""
shortest_word = None
for word in word_frequency.keys():
if len(word) > len(longest_word):
longest_word = word
if shortest_word is None or len(word) < len(shortest_word):
shortest_word = word
print(f"\nLongest word: '{longest_word}' ({len(longest_word)} letters)")
print(f"Shortest word: '{shortest_word}' ({len(shortest_word)} letters)")
# Vowel and consonant analysis
vowels = "aeiou"
vowel_count = 0
consonant_count = 0
for char in text.lower():
if char.isalpha():
if char in vowels:
vowel_count += 1
else:
consonant_count += 1
print(f"\nVowels: {vowel_count}")
print(f"Consonants: {consonant_count}")
if vowel_count + consonant_count > 0:
vowel_percentage = (vowel_count / (vowel_count + consonant_count)) * 100
print(f"Vowel percentage: {vowel_percentage:.1f}%")
analyze_text()
# Avoid recalculating inside loops
# Inefficient
numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
for number in numbers:
if number % 2 == 0:
print(f"{number} is even")
# More efficient for complex calculations
def is_even(n):
return n % 2 == 0
for number in numbers:
if is_even(number):
print(f"{number} is even")
# Use list comprehensions for simple transformations
# Traditional loop
squares = []
for i in range(10):
squares.append(i ** 2)
# List comprehension (more Pythonic)
squares = [i ** 2 for i in range(10)]
# Use built-in functions when possible
numbers = [1, 2, 3, 4, 5]
# Instead of manual sum
total = 0
for number in numbers:
total += number
# Use built-in sum()
total = sum(numbers)
Loops are fundamental for repetitive tasks in programming:
Key Concepts:
- for loops iterate over sequences and collections
- while loops continue based on conditions
- range() generates number sequences
- enumerate() provides index and value pairs
- break exits loops early
- continue skips to next iteration
- pass serves as a placeholder
Common Patterns:
- Accumulator pattern for sums and products
- Counter pattern for counting occurrences
- Filter pattern for selecting items
- Nested loops for 2D data processing
Best Practices:
- Choose the right loop type for your task
- Use descriptive variable names
- Avoid infinite loops with proper exit conditions
- Consider list comprehensions for simple transformations
- Use built-in functions when appropriate
Mastering loops will enable you to process data efficiently, automate repetitive tasks, and build more sophisticated programs!