Validation & Verification

🎯 Syllabus & Goals 3 min

Cambridge 7.5 · Validation and verification Paper 2 · Algorithms, Programming and Logic

By the end of this lesson you can:

1. Explain the difference between validation and verification, and why both are needed.
2. Describe and write pseudocode for range, length, type, presence and format checks, and describe a check digit.
3. Describe the verification methods double entry and screen/visual check, and choose suitable checks for given data.

Textbook: Chapter 7, §7.5 (pp. 276–281)

Recap / Warm-Up 5 min

You can now loop until a condition is met. That is exactly how a program refuses bad data: it keeps asking until the input passes a check.

Quick starter

A form accepts the date of birth 31/02/2010. What is wrong, and should a computer be able to spot it?

🧠 Key Concept 14 min

1 · Two checks with similar names

2 · The six validation checks

Each check is a loop that repeats until the data passes. Two examples:

OUTPUT "Enter the mark "
REPEAT
  INPUT Mark
  IF Mark < 0 OR Mark > 100
    THEN
      OUTPUT "Mark must be 0 to 100, re-enter "
  ENDIF
UNTIL Mark >= 0 AND Mark <= 100

OUTPUT "Choose a password "
REPEAT
  INPUT Password
  IF LENGTH(Password) < 8 OR LENGTH(Password) > 12
    THEN
      OUTPUT "8 to 12 characters, re-enter "
  ENDIF
UNTIL LENGTH(Password) >= 8 AND LENGTH(Password) <= 12

OUTPUT "How many siblings? "
REPEAT
  INPUT Siblings
  IF Siblings <> DIV(Siblings, 1)
    THEN
      OUTPUT "Whole number only, re-enter "
  ENDIF
UNTIL Siblings = DIV(Siblings, 1)

OUTPUT "Enter your email address "
REPEAT
  INPUT Email
  IF Email = ""
    THEN
      OUTPUT "*=Required "
  ENDIF
UNTIL Email <> ""

Format check — a room code such as B204 (one capital letter, then three digits)

REPEAT
  OUTPUT "Room code (e.g. B204) "
  INPUT RoomCode
  Valid ← FALSE
  IF LENGTH(RoomCode) = 4
    THEN
      IF SUBSTRING(RoomCode, 1, 1) >= "A" AND SUBSTRING(RoomCode, 1, 1) <= "Z"
        THEN
          Valid ← TRUE
      ENDIF
  ENDIF
UNTIL Valid

(A full format check also tests that characters 2–4 are digits, in the same way. String handling is taught in Unit 8.)

3 · Check digits

A check digit is the final digit of a code, calculated from all the other digits. When the code is typed or scanned, the computer recalculates it. If the result does not match, the code is rejected. It detects:

• an incorrect digit — 5327 entered instead of 5307;
• a transposition error — two digits swapped: 5037 instead of 5307;
• omitted or extra digits — 537 or 53107 instead of 5307;
• phonetic errors — 13 (thirteen) heard instead of 30 (thirty).

4 · Verification: double entry and visual check

A screen/visual check is manual. When entry is complete, the data is shown on screen and the user confirms it is correct — either by comparing with the paper form, or from their own knowledge.

Worked Example 12 min

Worked example 1 · Choosing checks for an online form

A youth club's online sign-up form asks for four items. Choose validation and verification for each.

1. Ask "what could be impossible?" — blank, too long, wrong type, out of range. Each answer names a validation check.
2. Ask "what could be mistyped but still look valid?" — that needs verification.a mistyped password is valid but unknown to the user, so they could never log in.
3. Give a reason tied to the data, e.g. "range 11–18 because the club only takes those ages"."with reasons" questions give a mark for the reason, not just the check.

Worked example 2 · Presence, type and range in one loop

A cinema app lets a user book 1 to 6 tickets. The input must not be blank, must be a whole number, and must be in range.

REPEAT
  OUTPUT "Tickets (1-6): "
  INPUT Entry
  Valid ← FALSE
  IF Entry = ""
    THEN
      OUTPUT "Required"
    ELSE
      IF Entry <> DIV(Entry, 1)
        THEN
          OUTPUT "Whole number please"
        ELSE
          IF Entry < 1 OR Entry > 6
            THEN
              OUTPUT "1 to 6 only"
            ELSE
              Valid ← TRUE
          ENDIF
      ENDIF
  ENDIF
UNTIL Valid

valid = False
while not valid:
    entry = input("Tickets (1-6): ")
    if entry == "":
        print("Required")
    elif not entry.isdigit():
        print("Whole number please")
    elif int(entry) < 1 or int(entry) > 6:
        print("1 to 6 only")
    else:
        valid = True
tickets = int(entry)
print("Booked", tickets, "tickets")

Tickets (1-6):
Required
Tickets (1-6): 2.5
Whole number please
Tickets (1-6): 9
1 to 6 only
Tickets (1-6): 4
Booked 4 tickets

1. Presence first. A blank entry cannot be tested for type or range.the checks are nested so each runs only if the one before it passed.
2. Then type — 2.5 is rejected as not a whole number.
3. Then range — 9 is rejected.
4. Each rejection outputs why, then the loop asks again. 4 passes all three, so Valid becomes TRUE.

Worked example 3 · Checking an ISBN-13 check digit

A librarian types the ISBN 978-1-91502-463-? . Calculate the check digit (the 13th digit).

1. Multiply the 12 digits by 1, 3, 1, 3, … in turn (the weights).
2. Add the products: 9 + 21 + 8 + 3 + 9 + 3 + 5 + 0 + 2 + 12 + 6 + 9 = 87.
3. Find the remainder ÷ 10: 87 MOD 10 = 7.
4. Subtract from 10: 10 − 7 = 3 (if the remainder is 0, the check digit is 0). Full ISBN: 978-1-91502-463-3.
5. Test a transposition: typing …9510… instead of …9150… gives a total of 95 and a check digit of 5, not 3 — so the error is detected.the weights 1 and 3 make swapped neighbours change the total.

Try It Yourself 12 min

📝 Exam Practice 10 min

🗝️ Recap & Key Terms 3 min

Validation lets a program reject unreasonable data automatically. Verification checks data was copied accurately. Good systems use several validation checks on one item, plus double entry or a visual check.

Validation

Automated checks by a program that data is reasonable before it is accepted into a computer system.

Verification

Checking that data has been accurately copied from another source or transferred within a system.

Range check

Checks that a number is between an upper and a lower value.

Length check

Checks that data is an exact number of characters, or between two lengths.

Type check / presence check

Checks that data is of a given data type / that a data item has been entered.

Format check

Checks that the characters entered conform to a pre-defined pattern.

Check digit

An extra digit, calculated from the others, appended to a number to check it has been entered without error.

Double entry / visual check

Entering data twice and comparing the copies / the user checking data on screen before confirming it.

Homework 1 min

Task (≤ 15 min): A phone app splits a restaurant bill between diners. It works for 2 to 12 diners and bills from $10 to $500. (a) State the validation checks needed for each input, with reasons. [4] (b) Write pseudocode for the check on the number of diners. [3]

REPEAT
  OUTPUT "Number of diners (2-12): "
  INPUT Diners
  IF Diners < 2 OR Diners > 12
    THEN
      OUTPUT "Must be 2 to 12"
  ENDIF
UNTIL Diners >= 2 AND Diners <= 12