Check Digits (Cambridge (CIE) IGCSE Computer Science)
Revision Note
Written by: Robert Hampton
Reviewed by: James Woodhouse
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]
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