C4 · Validation

Spec reference: Section C - Programming Paradigms
Key idea: Check that data entered by a user is acceptable before processing it.

---

What is validation?

Validation is the process of checking that data entered into a program is acceptable - that it is of the right type, within the right range, and in the right format - before the program processes it.

Key definition

Validation - an automated check performed by a program to ensure that data entered is reasonable, complete and within acceptable limits.

Validation ≠ Verification

• Validation checks that data is acceptable (e.g. age is between 0 and 120).
• Verification checks that data was entered correctly - e.g. typing your email address twice to confirm it matches.

---

Validation check techniques

1. Range check

Ensures a value falls within an acceptable minimum and maximum.

IF age < 0 OR age > 120 THEN
    SEND "Invalid age" TO DISPLAY
END IF

Examples:

• Age must be between 0 and 120
• Score must be between 0 and 100
• Month must be between 1 and 12

---

2. Type check

Ensures the data is of the correct data type.

try:
    age = int(input("Enter your age: "))
except ValueError:
    print("Error: Age must be a whole number.")

Examples:

• Age must be an integer, not a string like "twenty"
• Price must be a float, not text

---

3. Length check

Ensures a string has an acceptable number of characters.

IF LEN(password) < 8 OR LEN(password) > 20 THEN
    SEND "Password must be 8-20 characters" TO DISPLAY
END IF

Examples:

• Password must be between 8 and 20 characters
• UK postcode must be 6-8 characters

---

4. Presence check

Ensures that a required field is not left empty.

IF username = "" THEN
    SEND "Username cannot be empty" TO DISPLAY
END IF

---

5. Format check

Ensures data matches an expected pattern or structure.

# Email must contain @ and a dot
IF NOT ("@" IN email AND "." IN email) THEN
    SEND "Invalid email address" TO DISPLAY
END IF

Examples:

• Email must contain @ and a .
• Date entered as DD/MM/YYYY
• UK phone number starts with 07 or 01

---

6. Lookup check

Ensures the entered value exists in a list of acceptable values.

valid_grades = ["A", "B", "C", "D", "U"]
IF grade NOT IN valid_grades THEN
    SEND "Invalid grade" TO DISPLAY
END IF

---

Post-check actions

Once a validation check fails, the program must respond appropriately:

Response	When to use
Display an error message	Always - tell the user what went wrong
Re-prompt for input	Use a loop to keep asking until valid data is entered
Reject and move on	Log the error and skip that record (batch processing)
Use a default value	Replace invalid data with a safe default

Validation loop (best practice)

REPEAT
    RECEIVE age FROM KEYBOARD
    IF age < 0 OR age > 120 THEN
        SEND "Please enter a valid age (0-120)" TO DISPLAY
    END IF
UNTIL age >= 0 AND age <= 120

This keeps asking until valid data is entered - the user cannot proceed with invalid input.

---

Worked example: Registration form validation

def validate_username(username):
    if len(username) < 3:
        return False, "Username must be at least 3 characters"
    if len(username) > 20:
        return False, "Username must be no more than 20 characters"
    if " " in username:
        return False, "Username cannot contain spaces"
    return True, "Valid"

def validate_age(age_str):
    try:
        age = int(age_str)
    except ValueError:
        return False, "Age must be a whole number"
    if age < 16 or age > 100:
        return False, "Age must be between 16 and 100"
    return True, "Valid"

# Usage
valid, msg = validate_username("alice_99")
print(msg)  # Valid

valid, msg = validate_age("abc")
print(msg)  # Age must be a whole number

---

Summary

Check type	What it validates
Range check	Value is between a min and max
Type check	Data is the correct type (int, float, string, etc.)
Length check	String has acceptable number of characters
Presence check	Field is not empty
Format check	Data matches a required pattern (e.g. email, date)
Lookup check	Value exists in a list of allowed values

---

Test Yourself

Question 1 of 5

A form requires that a user's age is between 16 and 99. Which type of validation check is this?