 # Nia11's solution

## to ISBN Verifier in the Lua Track

Published at Dec 21 2018 · 0 comments
Instructions
Test suite
Solution

# Isbn Verifier

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 `busted` from within the exercise directory.

## Further information

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

## Submitting Incomplete Solutions

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

### isbn-verifier_spec.lua

``````local valid = require('isbn-verifier').valid

describe('isbn-verifier', function()
it('valid isbn number', function()
assert.is_true(valid('3-598-21508-8'))
end)

it('invalid isbn check digit', function()
assert.is_false(valid('3-598-21508-9'))
end)

it('valid isbn number with a check digit of 10', function()
assert.is_true(valid('3-598-21507-X'))
end)

it('check digit is a character other than X', function()
assert.is_false(valid('3-598-21507-A'))
end)

it('invalid character in isbn', function()
assert.is_false(valid('3-598-2K507-0'))
end)

it('X is only valid as a check digit', function()
assert.is_false(valid('3-598-2X507-9'))
end)

it('valid isbn without separating dashes', function()
assert.is_true(valid('3598215088'))
end)

it('isbn without separating dashes and X as check digit', function()
assert.is_true(valid('359821507X'))
end)

it('isbn without check digit and dashes', function()
assert.is_false(valid('359821507'))
end)

it('too long isbn and no dashes', function()
assert.is_false(valid('3598215078X'))
end)

it('isbn without check digit', function()
assert.is_false(valid('3-598-21507'))
end)

it('too long isbn', function()
assert.is_false(valid('3-598-21507-XX'))
end)

it('check digit of X should not be used for 0', function()
assert.is_false(valid('3-598-21515-X'))
end)
end)``````
``````isbn={}

function isbn.valid(num)
local digits = {}
num:gsub('[^%dX]',''):gsub('.',function(c) table.insert(digits,c) end)
if #digits==10 then
if digits[#digits]=='X' then digits[#digits]='10' end
for k,v in pairs(digits) do
if v=='X' and k~=10 then return false end
digits[k]=tonumber(v)
end
local sum=0
for i=1,#digits do
sum = sum+digits[i]*(11-i)
end
if sum%11==0 then return true else return false end
else
return false
end
end

return isbn``````

### 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?