exercism fetch go isbn-verifier

Isbn Verifier

Check if a given ISBN-10 is valid.


Given an unknown string the program should check if the provided string is a valid ISBN-10. Putting this into place requires some thinking about preprocessing/parsing of the string prior to calculating the check digit for the ISBN.

The program should allow for ISBN-10 without the separating dashes to be verified as well.


Let's take a random ISBN-10 number, say 3-598-21508-8 for this. The first digit block indicates the group where the ISBN belongs. Groups can consist of shared languages, geographic regions or countries. The leading '3' signals this ISBN is from a german speaking country. The following number block is to identify the publisher. Since this is a three digit publisher number there is a 5 digit title number for this book. The last digit in the ISBN is the check digit which is used to detect read errors.

The first 9 digits in the ISBN have to be between 0 and 9. The check digit can additionally be an 'X' to allow 10 to be a valid check digit as well.

A valid ISBN-10 is calculated with this formula (x1 * 10 + x2 * 9 + x3 * 8 + x4 * 7 + x5 * 6 + x6 * 5 + x7 * 4 + x8 * 3 + x9 * 2 + x10 * 1) mod 11 == 0 So for our example ISBN this means: (3 * 10 + 5 * 9 + 9 * 8 + 8 * 7 + 2 * 6 + 1 * 5 + 5 * 4 + 0 * 3 + 8 * 2 + 8 * 1) mod 11 = 0

Which proves that the ISBN is valid.


Converting from string to number can be tricky in certain languages. It's getting even trickier since the check-digit of an ISBN-10 can be 'X'.

Bonus tasks

To run the tests run the command go test from within the exercise directory.

If the test suite contains benchmarks, you can run these with the -bench flag:

go test -bench .

Keep in mind that each reviewer will run benchmarks on a different machine, with different specs, so the results from these benchmark tests may vary.

Further information

For more detailed information about the Go track, including how to get help if you're having trouble, please visit the exercism.io Go language page.


Converting a string into a number and some basic processing utilizing a relatable real world example. https://en.wikipedia.org/wiki/International_Standard_Book_Number#ISBN-10_check_digit_calculation

Submitting Incomplete Solutions

It's possible to submit an incomplete solution so you can see how others have completed the exercise.