# artemkorsakov's solution

## to ISBN Verifier in the C# Track

Published at May 01 2019 · 0 comments
Instructions
Test suite
Solution

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

## ISBN

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.

## Example

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.

## Caveats

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.

• 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 `dotnet test` from within the exercise directory.

Initially, only the first test will be enabled. This is to encourage you to solve the exercise one step at a time. Once you get the first test passing, remove the `Skip` property from the next test and work on getting that test passing. Once none of the tests are skipped and they are all passing, you can submit your solution using `exercism submit IsbnVerifier.cs`

## Further information

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

## 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

### IsbnVerifierTest.cs

``````// This file was auto-generated based on version 2.7.0 of the canonical data.

using Xunit;

public class IsbnVerifierTest
{
[Fact]
public void Valid_isbn_number()
{
Assert.True(IsbnVerifier.IsValid("3-598-21508-8"));
}

[Fact(Skip = "Remove to run test")]
public void Invalid_isbn_check_digit()
{
Assert.False(IsbnVerifier.IsValid("3-598-21508-9"));
}

[Fact(Skip = "Remove to run test")]
public void Valid_isbn_number_with_a_check_digit_of_10()
{
Assert.True(IsbnVerifier.IsValid("3-598-21507-X"));
}

[Fact(Skip = "Remove to run test")]
public void Check_digit_is_a_character_other_than_x()
{
Assert.False(IsbnVerifier.IsValid("3-598-21507-A"));
}

[Fact(Skip = "Remove to run test")]
public void Invalid_character_in_isbn()
{
Assert.False(IsbnVerifier.IsValid("3-598-P1581-X"));
}

[Fact(Skip = "Remove to run test")]
public void X_is_only_valid_as_a_check_digit()
{
Assert.False(IsbnVerifier.IsValid("3-598-2X507-9"));
}

[Fact(Skip = "Remove to run test")]
public void Valid_isbn_without_separating_dashes()
{
Assert.True(IsbnVerifier.IsValid("3598215088"));
}

[Fact(Skip = "Remove to run test")]
public void Isbn_without_separating_dashes_and_x_as_check_digit()
{
Assert.True(IsbnVerifier.IsValid("359821507X"));
}

[Fact(Skip = "Remove to run test")]
public void Isbn_without_check_digit_and_dashes()
{
Assert.False(IsbnVerifier.IsValid("359821507"));
}

[Fact(Skip = "Remove to run test")]
public void Too_long_isbn_and_no_dashes()
{
Assert.False(IsbnVerifier.IsValid("3598215078X"));
}

[Fact(Skip = "Remove to run test")]
public void Too_short_isbn()
{
Assert.False(IsbnVerifier.IsValid("00"));
}

[Fact(Skip = "Remove to run test")]
public void Isbn_without_check_digit()
{
Assert.False(IsbnVerifier.IsValid("3-598-21507"));
}

[Fact(Skip = "Remove to run test")]
public void Check_digit_of_x_should_not_be_used_for_0()
{
Assert.False(IsbnVerifier.IsValid("3-598-21515-X"));
}

[Fact(Skip = "Remove to run test")]
public void Empty_isbn()
{
Assert.False(IsbnVerifier.IsValid(""));
}

[Fact(Skip = "Remove to run test")]
public void Input_is_9_characters()
{
Assert.False(IsbnVerifier.IsValid("134456729"));
}

[Fact(Skip = "Remove to run test")]
public void Invalid_characters_are_not_ignored()
{
Assert.False(IsbnVerifier.IsValid("3132P34035"));
}

[Fact(Skip = "Remove to run test")]
public void Input_is_too_long_but_contains_a_valid_isbn()
{
Assert.False(IsbnVerifier.IsValid("98245726788"));
}
}``````
``````﻿using System.Text.RegularExpressions;

public static class IsbnVerifier
{
private static readonly Regex isbnRegex = new Regex("^([0-9]-?){9}[0-9X]{1}\$");

public static bool IsValid(string number)
{
if (!isbnRegex.IsMatch(number))
{
return false;
}

char[] nums = number.Replace("-", "").ToCharArray();
int result = 0;
for (int i = 0; i < nums.Length; i++)
{
int num = nums[i] == 'X' ? 10 : (int)char.GetNumericValue(nums[i]);
result += (10 - i) * num;
}
return result % 11 == 0;
}
}``````