Typing errors that a check digit detects

Name:

Date:

Class:

Quick Questions

Shade in the A or B circle for each question.

1. A user types the number 4512 as 4152. Which typing error is this?

A Omitted digit

B Transposition error

2. A user types 7934 as 7834. Which typing error is this?

A Incorrect digit

B Transposition error

3. A user types 12345 as 1235. Which typing error is this?

A Extra digit

B Omitted digit

4. A user types 2468 as 24668. Which typing error is this?

A Extra digit

B Omitted 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

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1Typed 4519 instead of 4529		ATyped 459 instead of 4529
2Typed 4592 instead of 4529		BA transposition error: two digits were swapped, so the check digit calculation gives a different result
3An omitted digit error: a digit is left out when the number is typed, so the number is too short		CThe 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.
4An extra digit error: an additional digit is typed that should not be there, so the number is too long		DA 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.
5A student says: "4529 typed as 4259 is not an error because it has the same digits"		ETyped 45529 instead of 4529
6A student says: "A check digit only checks that the number has the right length"		FAn incorrect digit error: one digit was typed wrongly, so the check digit no longer matches

Definitions

check digit

Fill in the Blanks

Complete each line of code by filling in the missing parts.

IF ______(entered) <> 10 THEN OUTPUT "Digit missing or extra" ENDIF

IF calculated __ checkDigit THEN OUTPUT "Wrong digit entered" ENDIF

IF LENGTH(entered) _ 10 THEN OUTPUT "Omitted digit" ENDIF

IF LENGTH(entered) _ 10 THEN OUTPUT "Extra digit typed" ENDIF

IF calculated <> checkDigit ____ OUTPUT "Digits swapped: transposition" ENDIF

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A number ← "4719"

B typed ← "4179"

C // digits 2 and 3 are swapped

D OUTPUT "Omitted digit error"

Question 2

A number ← "4719"

B typed ← "479"

C OUTPUT "Extra digit error"

Question 3

A // A check digit fixes typos

B correct ← "4719"

C typed ← "4729"

D OUTPUT "Wrong digit error"

Question 4

A correct ← "4719"

B typed ← "47195"

C IF LENGTH(typed) < 4 THEN

D OUTPUT "Extra digit"

E ENDIF

Question 5

A correct ← "4719"

B typed ← "47195"

C IF LENGTH(typed) < 4 THEN

D OUTPUT "Omitted digit"

E ENDIF

Answer Key

Typing errors that a check digit detects

Section A: Quick Questions

B — Transposition error

A — Incorrect digit

B — Omitted digit

A — Extra digit

A — Two digits swapped by mistake

Section B: 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.

Section C: Definitions

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.

Section D: Fill in the Blanks

IF LENGTH(entered) <> 10 THEN
   OUTPUT "Digit missing or extra"
ENDIF

IF calculated <> checkDigit THEN
   OUTPUT "Wrong digit entered"
ENDIF

IF LENGTH(entered) < 10 THEN
   OUTPUT "Omitted digit"
ENDIF

IF LENGTH(entered) > 10 THEN
   OUTPUT "Extra digit typed"
ENDIF

IF calculated <> checkDigit THEN
   OUTPUT "Digits swapped: transposition"
ENDIF

Section E: Spot the Error

D — 4719 became 4179, so two digits were swapped. That is a transposition error, not an omitted digit. The output should be "Transposition error".

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.

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.

C — An extra digit makes the typed number longer than the correct one. The test should be LENGTH(typed) > 4, not < 4.

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.