Bar code check digit

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Quick Questions

Shade in the A or B circle for each question.

1. Which digit of an EAN-13 bar code number is the check digit?

A The first digit

B The last (13th) digit

2. What does the scanner do with the check digit?

A Uses it to look up the price of the item

B Recalculates it from the other digits and compares it

3. Which weights does EAN-13 use when calculating its check digit?

A Alternating 1 and 3, the same as ISBN-13

B Counting down from 13 to 2

4. The scanner recalculates the check digit and it does not match the last digit. What happens?

A It beeps again or asks for a rescan

B It adds the item to the till using the closest matching code

5. In an EAN-13 calculation, what weight does the second digit get?

A 3

B 1

Match the Pairs

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

Left column		Right column
1weights: 1 3 1 3 1 3 1 3 1 3 1 3		ATurns the weighted total into the check digit. If the total ends in 0, the check digit is 0.
2(10 - (total MOD 10)) MOD 10		BThe first 12 digits of an EAN-13 bar code are multiplied by alternating weights of 1 and 3, the same pattern as ISBN-13.
3The scanner recalculates the check digit from the first 12 digits and compares it with the 13th digit it read.		CA match means the read passed the check, so the item should be accepted. The scanner should only beep and ask for a rescan when the digits do not match.
4If the recalculated check digit does not match, the scanner rejects the misread and beeps for the item to be scanned again.		DIF calculated = digit13 THEN accept
5weights: 3 3 3 3 3 3 3 3 3 3 3 3		EEAN-13 does not use the weight 3 for every digit. The weights alternate 1, 3, 1, 3 and so on, starting with 1.
6IF calculated = digit13 THEN rescan		FIF calculated ≠ digit13 THEN beep and rescan

Definitions

check digit

Fill in the Blanks

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

check ← (10 - (total ___ 10)) MOD 10

IF position MOD 2 = 0 THEN weight ← _

IF calculated __ lastDigit THEN OUTPUT "Scan again" ENDIF

first12 ← LEFT(code, 12) lastDigit ← MID(code, __, 1)

REPEAT INPUT code _____ checkOK = TRUE

Spot the Error

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

Question 1

A total ← weighted sum

B r ← total MOD 10

C check ← (10 - r) MOD 11

D PRINT check

Question 2

A FOR i ← 1 TO 12

B IF i MOD 2 = 1 THEN w ← 3

C ELSE w ← 3

D total ← total + w * d[i]

E NEXT i

Question 3

A code ← SCAN()

B calc ← CALC(code)

C IF calc = RIGHT(code, 1) THEN

D PRINT "Accepted"

Question 4

A check ← RIGHT(code, 1)

B calc ← CALC(LEFT(code, 12))

C IF calc <> check THEN

D OUTPUT "Accepted"

E ENDIF

Question 5

A total ← 1

B FOR i ← 1 TO 12

C total ← total + w * d[i]

D NEXT i

Answer Key

Bar code check digit

Section A: Quick Questions

B — The last (13th) digit

B — Recalculates it from the other digits and compares it

A — Alternating 1 and 3, the same as ISBN-13

A — It beeps again or asks for a rescan

A — 3

Section B: Match the Pairs

weights: 1 3 1 3 1 3 1 3 1 3 1 3 → The first 12 digits of an EAN-13 bar code are multiplied by alternating weights of 1 and 3, the same pattern as ISBN-13.

(10 - (total MOD 10)) MOD 10 → Turns the weighted total into the check digit. If the total ends in 0, the check digit is 0.

The scanner recalculates the check digit from the first 12 digits and compares it with the 13th digit it read. → IF calculated = digit13 THEN accept

If the recalculated check digit does not match, the scanner rejects the misread and beeps for the item to be scanned again. → IF calculated ≠ digit13 THEN beep and rescan

weights: 3 3 3 3 3 3 3 3 3 3 3 3 → EAN-13 does not use the weight 3 for every digit. The weights alternate 1, 3, 1, 3 and so on, starting with 1.

IF calculated = digit13 THEN rescan → A match means the read passed the check, so the item should be accepted. The scanner should only beep and ask for a rescan when the digits do not match.

Section C: Definitions

check digit — The last digit of a bar code number, worked out from the other digits. The scanner recalculates it and rejects a misread if the digits do not match, so the item must be scanned again.

Section D: Fill in the Blanks

check ← (10 - (total MOD 10)) MOD 10

IF position MOD 2 = 0 THEN weight ← 3

IF calculated <> lastDigit THEN
  OUTPUT "Scan again"
ENDIF

first12 ← LEFT(code, 12)
lastDigit ← MID(code, 13, 1)

REPEAT
  INPUT code
UNTIL checkOK = TRUE

Section E: Spot the Error

C — EAN-13 bar codes use modulus 10, not 11. The check digit is (10 - r) MOD 10. Modulus 11 is used for other check digit schemes.

B — EAN-13 uses weights 1 and 3 alternately, starting with 1 for the first digit. Odd positions get weight 1 and even positions get weight 3, so the line should set w ← 1.

B — The scanner must recalculate the check digit from the first 12 digits only. Passing the whole code includes the check digit itself. It should be CALC(LEFT(code, 12)).

D — When the recalculated digit does not match the check digit, the scan is rejected. The scanner should beep and ask for a rescan, not accept the item.

A — The running total must start at 0. Starting at 1 adds an extra amount to the weighted sum and gives the wrong check digit.