← IGCSE Computer Science Paper 1

Typing errors that a check digit detects

An extra digit worked out from the other digits in a number. It catches typing errors: a wrong digit, two digits swapped (transposition), a missing digit or an extra digit.

Part of: IGCSE Computer Science 0478 — 2.2 Methods of Error Detection — Module 33, IGCSE Computer Science 0478 — 7a Program Development Life Cycle — Module 59

Understanding Typing errors that a check digit detects in IGCSE Computer Science Paper 1

What it does: The 2 is missing, giving a shorter number, which is an omitted digit error that the check digit catches. One digit (9) was replaced by a different digit (8). No digits swapped places, so it is an incorrect digit, not a transposition. Check digits catch data entry mistakes such as a transposition. Interference during transmission is handled by methods like parity or checksum.

Common mistakes: 4719 became 4179, so two digits were swapped. That is a transposition error, not an omitted digit. The output should be "Transposition error". The typed number 479 is missing the digit 1, so it is an omitted digit error. An extra digit error would mean a digit had been added.

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This module's key terms are also in a printable crossword (Modules 168–178) and word search (Modules 168–188).

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Learning
Duo
A user types the number 4512 as 4152. Which typing error is this?
Transposition errorOmitted digit
Matches
A student says: "4529 typed as 4259 is not an error because it has the same digits"
↓
The same digits in the wrong order is a transposition error. The check digit calculation uses weights by position, so the result changes and the error is detected.
Applying
Meanings
Term
check digit
Meaning
An extra digit worked out from the other digits in a number. It catches typing errors: a wrong digit, two digits swapped (transposition), a missing digit or an extra digit.
Practice
INPUT Entered
Testing
Gaps
IF (entered) <> 10 THEN
Check Answer
Spots
3// digits 2 and 3 are swapped
4OUTPUT "Omitted digit error"

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Quick Questions

1. A user types the number 4512 as 4152. Which typing error is this?
A. Omitted digit
B. Transposition error ✓
Two digits (1 and 5) have swapped places, so it is a transposition error. A check digit is designed to detect this.
2. A user types 7934 as 7834. Which typing error is this?
A. Incorrect digit ✓
B. Transposition error
One digit (9) was replaced by a different digit (8). No digits swapped places, so it is an incorrect digit, not a transposition.
3. A user types 12345 as 1235. Which typing error is this?
A. Extra digit
B. Omitted digit ✓
The digit 4 is missing, so the number is shorter than it should be. That is an omitted digit.
4. A user types 2468 as 24668. Which typing error is this?
A. Extra digit ✓
B. Omitted digit
An additional 6 was typed, making the number longer than it should be. That is an extra digit.
5. Which typing error is a check digit designed to detect?
A. Two digits swapped by mistake ✓
B. Interference changing a signal on a cable
Check digits catch data entry mistakes such as a transposition. Interference during transmission is handled by methods like parity or checksum.

Match the Pairs

Typed 4519 instead of 4529 → An incorrect digit error: one digit was typed wrongly, so the check digit no longer matches
Typed 4592 instead of 4529 → A transposition error: two digits were swapped, so the check digit calculation gives a different result
An omitted digit error: a digit is left out when the number is typed, so the number is too short → Typed 459 instead of 4529
An extra digit error: an additional digit is typed that should not be there, so the number is too long → Typed 45529 instead of 4529
A student says: "4529 typed as 4259 is not an error because it has the same digits" → The same digits in the wrong order is a transposition error. The check digit calculation uses weights by position, so the result changes and the error is detected.
A student says: "A check digit only checks that the number has the right length" → A check digit does not just check length. It recalculates from the digits, so it can detect a wrong digit and swapped digits too, as well as missing or extra digits.

Key Term

check digit
An extra digit worked out from the other digits in a number. It catches typing errors: a wrong digit, two digits swapped (transposition), a missing digit or an extra digit.

Fill in the Blanks

1. IF LENGTH(entered) <> 10 THEN OUTPUT "Digit missing or extra" ENDIF
Hint: Type the function that counts how many characters were entered, to spot an omitted or extra digit.
2. IF calculated <> checkDigit THEN OUTPUT "Wrong digit entered" ENDIF
Hint: Type the operator meaning 'is not equal to', used when the recalculated check digit does not match.
3. IF LENGTH(entered) < 10 THEN OUTPUT "Omitted digit" ENDIF
Hint: Type the comparison operator meaning 'fewer than', which catches a missing digit.
4. IF LENGTH(entered) > 10 THEN OUTPUT "Extra digit typed" ENDIF
Hint: Type the comparison operator meaning 'more than', which catches an extra digit.
5. IF calculated <> checkDigit THEN OUTPUT "Digits swapped: transposition" ENDIF
Hint: Type the keyword that follows an IF condition and starts the code that runs when the check fails.

Spot the Error

Question 1
A number ← "4719"
B typed ← "4179"
C // digits 2 and 3 are swapped
D OUTPUT "Omitted digit error"
Line D — 4719 became 4179, so two digits were swapped. That is a transposition error, not an omitted digit. The output should be "Transposition error".
Question 2
A number ← "4719"
B typed ← "479"
C OUTPUT "Extra digit error"
Line C — The typed number 479 is missing the digit 1, so it is an omitted digit error. An extra digit error would mean a digit had been added.
Question 3
A // A check digit fixes typos
B correct ← "4719"
C typed ← "4729"
D OUTPUT "Wrong digit error"
Line A — A check digit only detects typing errors. It cannot fix them. The comment should say it detects typos, and the user must re-enter the number.
Question 4
A correct ← "4719"
B typed ← "47195"
C IF LENGTH(typed) < 4 THEN
D OUTPUT "Extra digit"
E ENDIF
Line C — An extra digit makes the typed number longer than the correct one. The test should be LENGTH(typed) > 4, not < 4.
Question 5
A correct ← "4719"
B typed ← "47195"
C IF LENGTH(typed) < 4 THEN
D OUTPUT "Omitted digit"
E ENDIF
Line B — An omitted digit makes the typed number shorter, so typed should be "479" (the 1 is missing). "47195" has an extra digit, so the test for a shorter number would never be true.

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