Check Digits (Cambridge International IGCSE Computer Science)

Spec point — 2.2.3 · "3 Describe how a check digit is used to detect errors in data entry and identify examples of when a check digit is used, including international standard book numbers (ISBN) and bar codes"

Check Digits

What is a check digit?

• A check digit is the last digit included in a code or sequence, used to detect errors in numeric data entry

• Examples of errors that a check digit can help to identify are:

  • Incorrect digits entered
  • Omitted or extra digits
  • Phonetic errors

• Added to the end of a numerical sequence they ensure validity of the data

• Calculated using standardised algorithms to ensure widespread compatibility

• Examples of where check digits can be used include:

  • ISBN book numbers
  • Barcodes

ISBN book numbers

• Each book has a unique ISBN number that **identifies **the book
• A standard ISBN number may be ten digits, for example, 965-448-765-9
• The check digit value is the** final digit** (9 in this example).
• This number is chosen specifically so that when the algorithm is completed the result is a whole number (an integer) with no remainder parts
• A check digit algorithm is performed on the ISBN number and if the result is a whole number, then the ISBN is valid

Barcodes

• Barcodes consist of black and white lines which can be scanned using barcode scanners
• Barcode scanners shine a laser on the black and white lines which reflect light into the scanner
• The scanner reads the distance between these lines as numbers and can identify the item
• Barcodes also use a set of digits to uniquely identify each item
• The final digit on a barcode is usually the check digit, this can be used to validate and authenticate an item

Worked Example Check digit algorithms are used to determine whether an error has occurred in transmitted data.

State the names of two examples of a check digit algorithm.

[2]

Answer

• ISBN [1]
• Barcode [1]