66 lines
2.1 KiB
Markdown
66 lines
2.1 KiB
Markdown
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# ISBN Verifier
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Welcome to ISBN Verifier on Exercism's Elixir Track.
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If you need help running the tests or submitting your code, check out `HELP.md`.
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## Instructions
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The [ISBN-10 verification process][isbn-verification] is used to validate book identification numbers.
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These normally contain dashes and look like: `3-598-21508-8`
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## ISBN
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The ISBN-10 format is 9 digits (0 to 9) plus one check character (either a digit or an X only).
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In the case the check character is an X, this represents the value '10'.
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These may be communicated with or without hyphens, and can be checked for their validity by the following formula:
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```text
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(d₁ * 10 + d₂ * 9 + d₃ * 8 + d₄ * 7 + d₅ * 6 + d₆ * 5 + d₇ * 4 + d₈ * 3 + d₉ * 2 + d₁₀ * 1) mod 11 == 0
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```
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If the result is 0, then it is a valid ISBN-10, otherwise it is invalid.
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## Example
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Let's take the ISBN-10 `3-598-21508-8`.
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We plug it in to the formula, and get:
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```text
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(3 * 10 + 5 * 9 + 9 * 8 + 8 * 7 + 2 * 6 + 1 * 5 + 5 * 4 + 0 * 3 + 8 * 2 + 8 * 1) mod 11 == 0
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```
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Since the result is 0, this proves that our ISBN is valid.
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## Task
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Given a string the program should check if the provided string is a valid ISBN-10.
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Putting this into place requires some thinking about preprocessing/parsing of the string prior to calculating the check digit for the ISBN.
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The program should be able to verify ISBN-10 both with and without separating dashes.
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## Caveats
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Converting from strings to numbers can be tricky in certain languages.
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Now, it's even trickier since the check digit of an ISBN-10 may be 'X' (representing '10').
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For instance `3-598-21507-X` is a valid ISBN-10.
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[isbn-verification]: https://en.wikipedia.org/wiki/International_Standard_Book_Number
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## Source
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### Created by
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- @stfnsr
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### Contributed to by
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- @angelikatyborska
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- @Cohen-Carlisle
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- @devonestes
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- @herminiotorres
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- @neenjaw
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- @sotojuan
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### Based on
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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
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