Avatar of murdho

murdho's solution

to ISBN Verifier in the Clojure Track

Published at Jul 13 2018 · 0 comments
Test suite

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.## Source

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.


(ns isbn-verifier-test
  (:require [clojure.test :refer [deftest is]]
            [isbn-verifier :refer [isbn?]]))

(deftest valid-isbn-number
  (is (= true (isbn? "3-598-21508-8"))))

(deftest invalid-isbn-check-digit
  (is (= false (isbn? "3-598-21508-9"))))

(deftest valid-isbn-number-with-a-check-digit-of-10
  (is (= true (isbn? "3-598-21507-X"))))

(deftest check-digit-is-a-character-other-than-X
  (is (= false (isbn? "3-598-21507-A"))))

(deftest invalid-character-in-isbn
  (is (= false (isbn? "3-598-2K507-0"))))

(deftest X-is-only-valid-as-a-check-digit
  (is (= false (isbn? "3-598-2X507-9"))))

(deftest valid-isbn-without-separating-dashes
  (is (= true (isbn? "3598215088"))))

(deftest isbn-without-separating-dashes-and-X-as-check-digit
  (is (= true (isbn? "359821507X"))))

(deftest isbn-without-check-digit-and-dashes
  (is (= false (isbn? "359821507"))))

(deftest too-long-isbn-and-no-dashes
  (is (= false (isbn? "3598215078X"))))

(deftest too-short-isbn
  (is (= false (isbn? "00"))))

(deftest isbn-without-check-digit
  (is (= false (isbn? "3-598-21507"))))

(deftest too-long-isbn
  (is (= false (isbn? "3-598-21507-XX"))))

(deftest check-digit-of-X-should-not-be-used-for-0
  (is (= false (isbn? "3-598-21515-X"))))
(ns isbn-verifier
  (:require [clojure.string :as s]))

; inspired by @kaukas's solution
(def digit-map
  (-> "0123456789"
      (zipmap (range 10))
      (assoc \X 10)))

(def pattern-isbn-10 #"^\d{1}-?\d{3}-?\d{5}-?[\dX]$")

(defn isbn-format? [isbn]
  (if (re-matches pattern-isbn-10 isbn) true false))

(defn clean-digits [isbn]
  (->> isbn
       (map digit-map)
       (filter identity)))

(defn calc-mod [coll]
  (mod (reduce + coll) 11))

(defn apply-element-formula [coll]
  (map-indexed #(* (inc %1) %2) coll))

(defn isbn? [isbn]
    (isbn-format? isbn)
    (zero? (-> isbn

Community comments

Find this solution interesting? Ask the author a question to learn more.

What can you learn from this solution?

A huge amount can be learned from reading other people’s code. This is why we wanted to give exercism users the option of making their solutions public.

Here are some questions to help you reflect on this solution and learn the most from it.

  • What compromises have been made?
  • Are there new concepts here that you could read more about to improve your understanding?