Top-Down Design and Modular Programming

Student Management System
        |
        +-- Add Student
        |       +-- Get student details
        |       +-- Validate input
        |       +-- Save to file
        |
        +-- Search Student
        |       +-- Get search criteria
        |       +-- Read file
        |       +-- Display results
        |
        +-- Delete Student
        |       +-- Find student
        |       +-- Confirm deletion
        |       +-- Remove from file
        |
        +-- Display All Students
                +-- Read file
                +-- Format output
                +-- Print records

# Example: Program to manage student grades
# Step 1: Main problem
def manage_grades():
    while True:
        choice = display_menu()
        if choice == 1:
            add_grade()
        elif choice == 2:
            view_grades()
        elif choice == 3:
            calculate_average()
        elif choice == 4:
            break

# Step 2: Sub-problems as functions
def display_menu():
    print("1. Add Grade")
    print("2. View Grades")
    print("3. Calculate Average")
    print("4. Exit")
    return int(input("Choice: "))

def add_grade():
    # Get student name and grade
    # Validate input
    # Store in list/file

def view_grades():
    # Read all grades
    # Format and display

def calculate_average():
    # Sum all grades
    # Divide by count
    # Return result

# File: calculator_module.py

def add(a, b):
    return a + b

def subtract(a, b):
    return a - b

def multiply(a, b):
    return a * b

def divide(a, b):
    if b == 0:
        raise ValueError("Cannot divide by zero")
    return a / b

# File: main.py
from calculator_module import add, subtract, multiply, divide

print(add(10, 5))       # 15
print(multiply(3, 4))   # 12

# Step 1: High-level description
# "Read a file and count word frequencies"

# Step 2: Break into steps
def count_word_frequencies(filename):
    text = read_file(filename)
    words = split_into_words(text)
    frequencies = count_occurrences(words)
    display_results(frequencies)

# Step 3: Refine each function
def read_file(filename):
    with open(filename, 'r') as f:
        return f.read()

def split_into_words(text):
    return text.lower().split()

def count_occurrences(words):
    freq = {}
    for word in words:
        freq[word] = freq.get(word, 0) + 1
    return freq

def display_results(frequencies):
    for word, count in sorted(frequencies.items()):
        print(f"{word}: {count}")

# Library Management System using Top-Down Design

# Module 1: data_handler.py
books = []

def add_book(title, author):
    books.append({"title": title, "author": author, "available": True})

def search_book(title):
    return [b for b in books if title.lower() in b["title"].lower()]

def get_all_books():
    return books

# Module 2: display.py
def show_menu():
    print("\n=== Library Menu ===")
    print("1. Add Book")
    print("2. Search Book")
    print("3. Display All Books")
    print("4. Exit")

def show_books(book_list):
    if not book_list:
        print("No books found.")
        return
    for i, book in enumerate(book_list, 1):
        status = "Available" if book["available"] else "Issued"
        print(f"{i}. {book['title']} by {book['author']} [{status}]")

# Module 3: main.py
from data_handler import add_book, search_book, get_all_books
from display import show_menu, show_books

def main():
    while True:
        show_menu()
        choice = input("Enter choice: ")
        if choice == "1":
            t = input("Title: ")
            a = input("Author: ")
            add_book(t, a)
            print("Book added!")
        elif choice == "2":
            t = input("Search title: ")
            results = search_book(t)
            show_books(results)
        elif choice == "3":
            show_books(get_all_books())
        elif choice == "4":
            break

if __name__ == "__main__":
    main()