Input Validation Techniques

This lesson covers the specific input validation techniques required by OCR J277 Section 2.4: range check, type check, presence check, format check, length check, and lookup check. Input validation is a core part of defensive design and ensures that data entered into a program is reasonable, complete, and in the correct format before it is processed.

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What Is Input Validation?

Input validation is the process of checking that data entered by a user (or received from another source) meets certain criteria before the program processes it. Validation does NOT check whether data is correct — it checks whether it is reasonable and acceptable.

For example, a validation check can confirm that an age is between 0 and 150, but it cannot confirm that the user has actually entered their real age.

OCR Exam Tip: Validation checks that data is reasonable — it does NOT check that data is accurate. This distinction between validation and verification is a common exam question. Validation = reasonable; Verification = accurate/correct.

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The Six Validation Checks

1. Range Check

A range check ensures that a value falls within an acceptable range (minimum and maximum).

OCR Pseudocode:

do
    mark = int(input("Enter mark (0-100): "))
    if mark < 0 OR mark > 100 then
        print("Error: mark must be between 0 and 100")
    endif
until mark >= 0 AND mark <= 100

Python:

while True:
    mark = int(input("Enter mark (0-100): "))
    if 0 <= mark <= 100:
        break
    print("Error: mark must be between 0 and 100")

Examples: age must be 0–150, exam mark must be 0–100, month must be 1–12.

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2. Type Check

A type check ensures that the data is of the correct data type (e.g. integer, string, real).

OCR Pseudocode:

do
    ageInput = input("Enter your age: ")
    if NOT ageInput.isNumeric() then
        print("Error: please enter a whole number")
    endif
until ageInput.isNumeric()
age = int(ageInput)

Python:

while True:
    age_input = input("Enter your age: ")
    if age_input.isdigit():
        age = int(age_input)
        break
    print("Error: please enter a whole number")

Examples: age should be an integer, price should be a real number, name should contain only letters.

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3. Presence Check

A presence check ensures that data has actually been entered — the field is not left blank.

OCR Pseudocode:

do
    name = input("Enter your name: ")
    if name == "" then
        print("Error: name cannot be empty")
    endif
until name != ""

Python:

while True:
    name = input("Enter your name: ")
    if name.strip() != "":
        break
    print("Error: name cannot be empty")

Examples: username must not be blank, email address must be provided.

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4. Format Check

A format check ensures that the data follows a specific pattern or format.

OCR Pseudocode:

do
    date = input("Enter date (DD/MM/YYYY): ")
    valid = true
    if date.length != 10 then
        valid = false
    elseif date.substring(2, 3) != "/" OR date.substring(5, 6) != "/" then
        valid = false
    endif
    if valid == false then
        print("Error: date must be in DD/MM/YYYY format")
    endif
until valid == true

Python: