Branching and Loops in Practice

We've learned a lot of theory — now it's time to build some real projects. This is a hands-on lesson. Using the if statements, for loops, while loops, break, continue, and other concepts we've learned, we'll build four complete programs step by step.

For each project, we'll analyze the requirements, write the code, and discuss possible improvements.

Project 1: Number Guessing Game (Enhanced)

In Lesson 08, we wrote a basic number guessing game. Now let's upgrade it with difficulty selection, attempt limits, and a "play again" feature.

Requirements

TEXT

1. Player selects difficulty: Easy (1-50, unlimited), Normal (1-100, 10 tries), Hard (1-200, 7 tries)
2. System randomly generates a target number
3. After each guess, hint "too high" or "too low" and display remaining attempts
4. After guessing correctly or running out of attempts, ask "play again?"
5. Enter q to quit

Complete Code (Star-Star-Star)

Example: Number Guessing Game

PYTHON

import random

print("=" * 30)
print("Number Guessing Game (Enhanced)")
print("=" * 30)

while True:  # Outer loop: controls "play again"
    # Select difficulty
    print("\nSelect difficulty:")
    print("1. Easy (1~50, unlimited)")
    print("2. Normal (1~100, 10 tries)")
    print("3. Hard (1~200, 7 tries)")

    choice = input("Enter 1/2/3: ")

    if choice == "1":
        max_num = 50
        max_attempts = float("inf")  # Unlimited
    elif choice == "2":
        max_num = 100
        max_attempts = 10
    elif choice == "3":
        max_num = 200
        max_attempts = 7
    else:
        print("Invalid choice, defaulting to Normal")
        max_num = 100
        max_attempts = 10

    # Generate target number
    target = random.randint(1, max_num)
    attempts = 0
    print(f"\nNumber generated between 1~{max_num}. Start guessing!")

    # Inner loop: game main logic
    while True:
        # Check if attempts are exhausted
        if attempts >= max_attempts:
            print(f"\nOut of attempts! The answer was {target}")
            break

        # Get player input
        guess_str = input(f"{max_attempts - attempts} tries left, enter your guess: ")

        if guess_str.lower() == "q":
            print("Quitting game.")
            max_attempts = -1  # Flag to notify outer loop
            break

        # Validate input
        if not guess_str.isdigit():
            print("Please enter a valid number!")
            continue

        guess = int(guess_str)
        attempts += 1

        # Check guess
        if guess < target:
            print("Too low, try higher!")
        elif guess > target:
            print("Too high, try lower!")
        else:
            print(f"\nCongratulations! The answer was {target}!")
            print(f"You got it in {attempts} tries.")
            break

    # Check if we should exit entirely
    if max_attempts == -1:  # Player entered q
        break

    # Ask for another round
    again = input("\nPlay again? (y/n): ")
    if again.lower() != "y":
        print("Thanks for playing, goodbye!")
        break

▶ Try it Yourself

Design highlight: The outer loop controls "play again," the inner loop controls the "guess process." The max_attempts = -1 flag solves the issue of breaking out of both loops when the player enters q. A more elegant approach would be to encapsulate the game logic in a function and use return to exit once.

Project 2: Multiplication Table (Multiple Styles)

In Lesson 08, we printed a standard multiplication table. A good programmer should be able to flexibly control loop structures to output any style.

Style 1: Standard Lower Triangle (Star-Star)

Example: Standard Lower Triangle

PYTHON

print("=== Standard Lower Triangle ===")
for i in range(1, 10):
    for j in range(1, i + 1):
        print(f"{j}x{i}={i*j}", end="\t")
    print()

▶ Try it Yourself

Style 2: Full Rectangle (Star-Star)

Example: Full Rectangle

PYTHON

print("=== Full Rectangle ===")
for i in range(1, 10):
    for j in range(1, 10):
        print(f"{j}x{i}={i*j}", end="\t")
    print()

▶ Try it Yourself

Style 3: Upper Triangle (Star-Star-Star)

Example: Upper Triangle

PYTHON

print("=== Upper Triangle ===")
for i in range(1, 10):
    # Print leading spaces for alignment
    for k in range(1, i):
        print(end="\t\t")
    # Print multiplication formulas
    for j in range(i, 10):
        print(f"{i}x{j}={i*j}", end="\t")
    print()

▶ Try it Yourself

Output:

TEXT

=== Upper Triangle ===
1x1=1    1x2=2    1x3=3    ...    1x9=9
          2x2=4    2x3=6    ...    2x9=18
                    3x3=9    ...    3x9=27
                              ...
                                        9x9=81

Key insight: The challenge of the upper triangle is the "space alignment" — each row needs (i-1) * 2 tab characters of indentation. This uses two inner loops: the first prints spaces, the second prints the formulas.

Project 3: Simple Calculator (Enhanced)

In the Lesson 08 exercises, we wrote a basic calculator. Now let's make an enhanced version supporting continuous operations with history.

Requirements

TEXT

1. Start with current result = 0
2. User enters: operator number, e.g., "+ 5"
3. Supported operators: + - * / ** // % clear exit
4. clear resets the result to 0
5. exit quits the program
6. After each operation, show the current result and history

Complete Code (Star-Star-Star)

Example: Enhanced Calculator

PYTHON

print("=" * 40)
print("Enhanced Calculator")
print("Format: operator number, e.g., + 5")
print("Supported: + - * / ** // %")
print("Special commands: clear reset exit quit")
print("=" * 40)

result = 0.0
history = []  # Store operation history

while True:
    # Display current result
    print(f"\nCurrent result: {result}")

    # Get user input
    cmd = input(">>> ").strip()

    # Handle special commands
    if cmd.lower() in ("exit", "quit"):
        print("Thanks for using!")
        break

    if cmd.lower() == "clear":
        result = 0.0
        history.clear()
        print("Reset to 0")
        continue

    # Parse regular operation commands
    parts = cmd.split()
    if len(parts) != 2:
        print("Invalid format! Enter: operator number")
        continue

    op, num_str = parts

    # Validate number
    if not num_str.replace(".", "").isdigit() or num_str.count(".") > 1:
        print("Invalid number!")
        continue

    num = float(num_str)

    # Execute operation
    old_result = result
    if op == "+":
        result += num
    elif op == "-":
        result -= num
    elif op == "*":
        result *= num
    elif op == "/":
        if num == 0:
            print("Error: Cannot divide by zero!")
            continue
        result /= num
    elif op == "**":
        result **= num
    elif op == "//":
        if num == 0:
            print("Error: Cannot divide by zero!")
            continue
        result //= num
    elif op == "%":
        if num == 0:
            print("Error: Cannot divide by zero!")
            continue
        result %= num
    else:
        print(f"Invalid operator: {op}")
        continue

    # Record history
    history.append(f"{old_result} {op} {num} = {result}")

    # Display result
    print(f"= {result}")

    # Show last 3 history entries
    if len(history) >= 1:
        print("--- Recent History ---")
        for item in history[-3:]:
            print(f"  {item}")

▶ Try it Yourself

Sample run:

TEXT

>>> + 10
= 10.0
--- Recent History ---
  0.0 + 10.0 = 10.0

>>> * 3
= 30.0
--- Recent History ---
  0.0 + 10.0 = 10.0
  10.0 * 3.0 = 30.0

>>> / 0
Error: Cannot divide by zero!

>>> clear
Reset to 0

Programming mindset: This example shows the classic interplay of loops and conditions — while True keeps the program running, if-elif-else handles various inputs, continue skips invalid input, and break exits normally.

Project 4: Prime Number Finder

Let the user input a range, output all prime numbers within that range, and count them.

Requirements

TEXT

1. User inputs start and end values
2. Program outputs all primes in that range
3. Display 10 per line, neatly aligned
4. Show the total count of primes

Complete Code (Star-Star-Star)

Example: Prime Number Finder

PYTHON

print("Prime Number Finder")
print("Enter a range, and I'll find all primes within it.")

# Get range
start_str = input("Start value (>=2): ")
end_str = input("End value: ")

# Validate input
if not (start_str.isdigit() and end_str.isdigit()):
    print("Please enter valid positive integers!")
else:
    start = int(start_str)
    end = int(end_str)

    if start < 2:
        start = 2
        print("Start value adjusted to 2 (primes start from 2)")

    if start > end:
        print("Start value cannot be greater than end value!")
    else:
        primes = []  # Store found primes

        # Check each number in the range
        for num in range(start, end + 1):
            # Check if num is prime
            is_prime = True
            for i in range(2, int(num ** 0.5) + 1):
                if num % i == 0:
                    is_prime = False
                    break

            if is_prime:
                primes.append(num)

        # Output results
        print(f"\nFound {len(primes)} primes between {start} and {end}:")

        for i, prime in enumerate(primes, 1):
            print(f"{prime:5d}", end="")
            if i % 10 == 0:  # Newline every 10
                print()

        print()  # Final newline

▶ Try it Yourself

Sample output:

TEXT

Start value (>=2): 50
End value: 200

Found 34 primes between 50 and 200:
   53   59   61   67   71   73   79   83   89   97
  101  103  107  109  113  127  131  137  139  149
  151  157  163  167  173  179  181  191  193  197
  199

Optimization tip: When checking if a number is prime, you don't need to divide up to num-1 — only up to sqrt(num). If num has a factor larger than sqrt(num), it must have a paired factor smaller than sqrt(num). This optimization is very effective for large ranges (100,000+).

Project 5: BMI Health Index + Password Strength Checker

This project ties together arithmetic operators, comparison operators, logical operators, conditionals, and string methods to solve two common real-world problems.

Part 1: BMI Health Index (Star-Star)

BMI (Body Mass Index) is an internationally used measure of body fat. The formula is simple:

BMI = weight(kg) / height(m)^2

Example: BMI Calculator

PYTHON

# BMI Health Index Calculator

# Input data
weight = 68
height = 1.75

# Calculate BMI
bmi = weight / (height ** 2)

print("=== BMI Health Index ===")
print(f"Weight: {weight} kg")
print(f"Height: {height} m")
print(f"BMI: {bmi:.1f}")

# Use chained comparison and if-elif to determine category
if bmi < 18.5:
    print("Category: Underweight")
elif 18.5 <= bmi < 24:
    print("Category: Normal")
elif 24 <= bmi < 28:
    print("Category: Overweight")
else:
    print("Category: Obese")

▶ Try it Yourself

Output:

TEXT

=== BMI Health Index ===
Weight: 68 kg
Height: 1.75 m
BMI: 22.2
Category: Normal

Tip: Chained comparisons like 18.5 <= bmi < 24 are much cleaner than bmi >= 18.5 and bmi < 24.

Part 2: Password Strength Checker (Star-Star-Star)

Write a program to determine if a user's password is secure. Password strength depends on its length and the types of characters it contains.

Example: Password Strength Checker

PYTHON

# Password Strength Checker

password = "Py3#thon"

# Analyze password characteristics
length = len(password)                    # Length
has_digit = any(c.isdigit() for c in password)     # Contains digit
has_letter = any(c.isalpha() for c in password)    # Contains letter
has_special = not password.isalnum()               # Contains special character

print("=== Password Strength Checker ===")
print(f"Password: {password}")
print(f"Length: {length}")
print(f"Contains digit: {has_digit}")
print(f"Contains letter: {has_letter}")
print(f"Contains special char: {has_special}")

# Combine conditions with logical operators to determine strength
if length < 6:
    strength = "Weak"
elif length >= 10 and has_digit and has_letter and has_special:
    strength = "Strong"
elif length >= 8 and has_digit and has_letter:
    strength = "Medium"
else:
    strength = "Weak"

print(f"Password strength: {strength}")

▶ Try it Yourself

Output:

TEXT

=== Password Strength Checker ===
Password: Py3#thon
Length: 8
Contains digit: True
Contains letter: True
Contains special char: True
Password strength: Medium

Details: len() gets string length, isdigit() checks for digits, isalpha() checks for letters, isalnum() checks if only letters and digits (returns False if special chars exist — negated with not becomes "contains special char"). These methods will be covered systematically in the strings lesson.

Common Branching/Looping Mistakes

Everyone makes mistakes when writing code, especially loop beginners. Here are the most typical pitfalls.

Mistake 1: Infinite Loop — Forgetting to Update the Condition Variable

PYTHON

# Wrong way
n = 1
while n <= 10:
    print(n)
    # forgot n += 1 — never ends

# Correct way
n = 1
while n <= 10:
    print(n)
    n += 1

Mistake 2: Confusing `=` with `==`

PYTHON

# Wrong way
if score = 100:  # This is assignment, not comparison!
    print("Perfect!")

# Correct way
if score == 100:
    print("Perfect!")

Mistake 3: Modifying a List While Iterating

PYTHON

# Wrong way — deleting during iteration causes skipping
numbers = [1, 2, 3, 4, 5]
for num in numbers:
    if num % 2 == 0:
        numbers.remove(num)
print(numbers)  # May not be what you expect

# Correct way — create a new list
numbers = [1, 2, 3, 4, 5]
odd_numbers = [num for num in numbers if num % 2 != 0]
print(odd_numbers)

Mistake 4: Misunderstanding range() End Value

PYTHON

# Wrong: expecting range(5) to output 1 2 3 4 5
for i in range(5):
    print(i)  # Actual output: 0 1 2 3 4

# Correct way
for i in range(1, 6):
    print(i)  # Output: 1 2 3 4 5

Common Use Cases

Scenario	Loop/Branch Technique	Description
Data processing (filtering, cleaning)	`for` + `continue`	Skip invalid data
User interaction menu	`while True` + `break`	Show menu repeatedly until exit
Pagination loading	`while` + condition	Until no more data
Search/lookup	`for` + `break` + `else`	Stop on found; else if not found
Batch file processing	`for` + nested logic	Iterate every file in a directory
Game main loop	`while True` + event handling	Continuous rendering and input response

FAQ

Q I keep getting infinite loops. What should I check?

A Check three things: 1) Will the loop condition ever become False? 2) Does the loop body modify the condition variable? 3) Does while True have a corresponding break? Adding print() to output key variable values is the most effective way to debug infinite loops.

Q Which is more efficient — range() or iterating over a list?

A range() returns a lazy sequence without generating all numbers at once. range(1000000) uses almost no memory, while list(range(1000000)) uses a lot. For large iteration counts, use range() directly for better memory efficiency.

Q The project code is much longer than the previous examples. How should I read it?

A Three steps: First, understand the overall structure (how many loops/conditions). Second, understand the main flow (how variables change). Third, focus on edge cases (input validation, division by zero, etc.). Good code is self-documenting — meaningful variable names and comments at key points.

Summary

Enhanced Guessing Game: Outer loop for "play again," inner loop for "guessing," use while True + break for multi-level loop exit management
Multiplication Table Styles: Control inner loop's range() boundaries to output lower triangle, upper triangle, rectangle, and more
Enhanced Calculator: continue to skip invalid input, list to store history, split() to parse user input
Prime Number Finder: Optimize to check up to sqrt(num) for efficiency; enumerate(primes, 1) for numbered output
Debugging Tips: Three elements for infinite loop checks, distinguish = from ==, don't modify lists while iterating

Exercises

Star: Narcissistic Numbers

A "narcissistic number" is a 3-digit number where the sum of the cubes of its digits equals the number itself. For example: 153 = 1^3 + 5^3 + 3^3.

Use a for loop to find all narcissistic numbers (there are 4).

Hint: Use range(100, 1000) to iterate all 3-digit numbers; use // and % to extract hundreds, tens, and units digits.

Star-Star: Print a Diamond

Accept an odd number n from the user, then output a diamond made of * in the console.

Example (n=5):

TEXT

  *
 ***
*
 ***
  *

Hint: A diamond has an upper and lower half. For the upper half, loop i from 1 to n//2 + 1, decreasing spaces and increasing asterisks.

Star-Star-Star: Student Score Management System

Write a program that implements the following features:

TEXT

1. Enter student scores (enter q to stop)
2. Calculate total, average, highest, and lowest scores
3. Count students in each range: 90-100 (Excellent), 80-89 (Good), 70-79 (Average), 60-69 (Pass), <60 (Fail)
4. Output scores sorted from highest to lowest

Requirements:

Use while True loop to collect scores
Use a list to store scores
Use for loop to count score ranges
Use sorted(..., reverse=True) for sorting

This project comprehensively applies loops, lists, sorting, and statistics. Completing it means you truly master Python's basic control flow.

Branching and Loops in Practice

Branching and Loops in Practice

Project 1: Number Guessing Game (Enhanced)

Requirements

Complete Code (Star-Star-Star)

Example: Number Guessing Game

Project 2: Multiplication Table (Multiple Styles)

Style 1: Standard Lower Triangle (Star-Star)

Example: Standard Lower Triangle

Style 2: Full Rectangle (Star-Star)

Example: Full Rectangle

Style 3: Upper Triangle (Star-Star-Star)

Example: Upper Triangle

Project 3: Simple Calculator (Enhanced)

Requirements

Complete Code (Star-Star-Star)

Example: Enhanced Calculator

Project 4: Prime Number Finder

Requirements

Complete Code (Star-Star-Star)

Example: Prime Number Finder

Project 5: BMI Health Index + Password Strength Checker

Part 1: BMI Health Index (Star-Star)

Example: BMI Calculator

Part 2: Password Strength Checker (Star-Star-Star)

Example: Password Strength Checker

Common Branching/Looping Mistakes

Mistake 1: Infinite Loop — Forgetting to Update the Condition Variable

Mistake 2: Confusing = with ==

Mistake 3: Modifying a List While Iterating

Mistake 4: Misunderstanding range() End Value

Common Use Cases

FAQ

Summary

Exercises

🙏 帮我们做得更好

Mistake 2: Confusing `=` with `==`