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artemkorsakov's solution

to ISBN Verifier in the Go Track

Published at Feb 26 2019 · 0 comments
Test suite


This exercise has changed since this solution was written.

The ISBN-10 verification process is used to validate book identification numbers. These normally contain dashes and look like: 3-598-21508-8


The ISBN-10 format is 9 digits (0 to 9) plus one check character (either a digit or an X only). In the case the check character is an X, this represents the value '10'. These may be communicated with or without hyphens, and can be checked for their validity by the following formula:

(x1 * 10 + x2 * 9 + x3 * 8 + x4 * 7 + x5 * 6 + x6 * 5 + x7 * 4 + x8 * 3 + x9 * 2 + x10 * 1) mod 11 == 0

If the result is 0, then it is a valid ISBN-10, otherwise it is invalid.


Let's take the ISBN-10 3-598-21508-8. We plug it in to the formula, and get:

(3 * 10 + 5 * 9 + 9 * 8 + 8 * 7 + 2 * 6 + 1 * 5 + 5 * 4 + 0 * 3 + 8 * 2 + 8 * 1) mod 11 == 0

Since the result is 0, this proves that our ISBN is valid.


Given a 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 be able to verify ISBN-10 both with and without separating dashes.


Converting from strings to numbers can be tricky in certain languages. Now, it's even trickier since the check digit of an ISBN-10 may be 'X' (representing '10'). For instance 3-598-21507-X is a valid ISBN-10.

Bonus tasks

  • Generate a valid ISBN-13 from the input ISBN-10 (and maybe verify it again with a derived verifier).

  • Generate valid ISBN, maybe even from a given starting ISBN.

Running the tests

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 and --benchmem flags:

go test -v --bench . --benchmem

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.


package isbn

// Source: exercism/problem-specifications
// Commit: 3134243 isbn-verifier: Crafted input to catch more incorrect algorithms (#1255)
// Problem Specifications Version: 2.7.0

var testCases = []struct {
	isbn        string
	expected    bool
	description string
	{"3-598-21508-8", true, "valid isbn number"},
	{"3-598-21508-9", false, "invalid isbn check digit"},
	{"3-598-21507-X", true, "valid isbn number with a check digit of 10"},
	{"3-598-21507-A", false, "check digit is a character other than X"},
	{"3-598-P1581-X", false, "invalid character in isbn"},
	{"3-598-2X507-9", false, "X is only valid as a check digit"},
	{"3598215088", true, "valid isbn without separating dashes"},
	{"359821507X", true, "isbn without separating dashes and X as check digit"},
	{"359821507", false, "isbn without check digit and dashes"},
	{"3598215078X", false, "too long isbn and no dashes"},
	{"00", false, "too short isbn"},
	{"3-598-21507", false, "isbn without check digit"},
	{"3-598-21515-X", false, "check digit of X should not be used for 0"},
	{"", false, "empty isbn"},
	{"134456729", false, "input is 9 characters"},
	{"3132P34035", false, "invalid characters are not ignored"},
	{"98245726788", false, "input is too long but contains a valid isbn"},


package isbn

import (

func TestIsValidISBN(t *testing.T) {
	for _, test := range testCases {
		observed := IsValidISBN(test.isbn)
		if observed == test.expected {
			t.Logf("PASS: %s", test.description)
		} else {
			t.Errorf("FAIL: %s\nIsValidISBN(%q)\nExpected: %t, Actual: %t",
				test.description, test.isbn, test.expected, observed)
package isbn

import (

var reg, _ = regexp.Compile("^[0-9\\-]{9,}X?$")

// IsValidISBN determines whether the number is valid ISBN.
func IsValidISBN(input string) bool {
	if !reg.MatchString(input) {
		return false
	candidate := strings.Replace(input, "-", "", -1)
	if len(candidate) != 10 {
		return false
	sum := 0
	var num int
	for i, c := range candidate {
		if c == 'X' {
			num = 10
		} else {
			num = int(c) - '0'
		sum += (10 - i) * num
	return sum%11 == 0

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