Defensive Design and Validation

1. The five kinds of validation check

All four boards expect you to know these by name and apply them.

# Range check
age = int(input("Age: "))
if age < 0 or age > 130:
    print("Age must be between 0 and 130.")

# Presence check
name = input("Name: ")
if name == "":
    print("Name cannot be empty.")

# Length check
password = input("Password: ")
if len(password) < 8 or len(password) > 20:
    print("Password must be 8-20 characters.")

# Format check
email = input("Email: ")
if "@" not in email or "." not in email:
    print("That does not look like a valid email.")

# Type check (using try/except)
try:
    qty = int(input("Quantity: "))
except ValueError:
    print("Quantity must be a whole number.")

// Range check
int age = int.Parse(Console.ReadLine());
if (age < 0 || age > 130) Console.WriteLine("Age must be between 0 and 130.");

// Presence check
string name = Console.ReadLine();
if (string.IsNullOrEmpty(name)) Console.WriteLine("Name cannot be empty.");

// Length check
string password = Console.ReadLine();
if (password.Length < 8 || password.Length > 20)
    Console.WriteLine("Password must be 8-20 characters.");

// Format check
string email = Console.ReadLine();
if (!email.Contains("@") || !email.Contains("."))
    Console.WriteLine("That does not look like a valid email.");

// Type check
if (!int.TryParse(Console.ReadLine(), out int qty))
    Console.WriteLine("Quantity must be a whole number.");

2. Validate-until-valid using a while loop

Printing an error and continuing is rarely enough. The standard pattern is to loop until the user gives valid input. This combines validation with exception handling.

age = -1
while age < 0 or age > 130:
    try:
        age = int(input("Enter age (0-130): "))
        if age < 0 or age > 130:
            print("Out of range, try again.")
    except ValueError:
        print("Whole numbers only.")
        age = -1

print(f"Age accepted: {age}")

int age = -1;
while (age < 0 || age > 130)
{
    Console.Write("Enter age (0-130): ");
    if (!int.TryParse(Console.ReadLine(), out age))
    {
        Console.WriteLine("Whole numbers only.");
        age = -1;
    }
    else if (age < 0 || age > 130)
    {
        Console.WriteLine("Out of range, try again.");
    }
}
Console.WriteLine($"Age accepted: {age}");

3. Authentication: a basic login

Authentication asks "are you who you say you are?". The simplest version compares a typed password to a stored one. Real systems hash passwords (Lesson 6 of the CS series), but the GCSE concept is the comparison plus the design choices around it.

users = {
    "alice": "Sunset!42",
    "ben":   "Pa55word!"
}

attempts = 0
while attempts < 3:
    username = input("Username: ")
    password = input("Password: ")
    if username in users and users[username] == password:
        print("Login successful.")
        break
    attempts = attempts + 1
    print("Username or password incorrect.")
else:
    print("Account locked. Try again later.")

Dictionary<string, string> users = new Dictionary<string, string>
{
    {"alice", "Sunset!42"},
    {"ben",   "Pa55word!"}
};

int attempts = 0;
while (attempts < 3)
{
    string u = Console.ReadLine();
    string p = Console.ReadLine();
    if (users.ContainsKey(u) && users[u] == p)
    {
        Console.WriteLine("Login successful.");
        break;
    }
    attempts++;
    Console.WriteLine("Username or password incorrect.");
}

4. Anticipating misuse

Defensive design is more than validation. The exam wants you to think about what users will do wrong, not just what they should do right. Examples:

• What if the user closes the program halfway through entering data? (Save progress; do not corrupt the file.)
• What if the user enters their date of birth as 30 February 2010? (Validate dates.)
• What if two users press "Save" at the same time? (Lock the file or warn one of them.)
• What if the user types their password with caps lock on? (Warn them; do not silently fail.)
• What if a teacher accidentally deletes a class? (Confirm dialog; soft-delete with undo.)

Each scenario is a defensive-design decision the developer makes before the user encounters it.

5. Maintainability matters too

Defensive design is not only about input. The exam also wants you to talk about maintainability: code that other people can read and change without breaking it.

6. A six-mark question, fully marked

Question: A school is designing an online form to register new pupils. Discuss the defensive-design features the developer should include. [6 marks]