Check Digits (Cambridge (CIE) O Level Computer Science): Revision Note

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