Another algorithm that I found online it’s a method to check if a string is a valid ISBN10 or not.
Firstly let’s find out what a valid ISBN 10 is.
ISBN is the acronym for International Standard Book Number. This 10 or 13-digit number (in our case 10) identifies a specific book, an edition of a book, or a book-like product (such as an audiobook). Since 1970 each published book has a unique ISBN. ( Find out more here: link)
ISBN-10 identifiers are ten digits long. The first nine characters are digits
0-9. The last digit can be
X, to indicate a value of 10.
An ISBN-10 number is valid if the sum of the digits multiplied by their position modulo 11 equals zero.
I decided to work in Ruby this time, and now if we know what we need to create, let’s start by building some tests to validate our work.
I created some tests about the sum of the digits and also if the last digit is X to make sure and change it into a 10.
Now let’s get to solving it.
Before going into anything let’s return false if we receive as input a string with more than 10 digits.
return false if isbn.size != 10
Now let’s create to main variable total and output.
total = 0
output = true
We are going to use those later.
To check each digit I will iterate over then (converted into an array firstly) and do some checks. I will use
each_with_index as we’re going to need the position of each digit.
The checks we need to do ar the following:
- If the index of the number is equal to 9 (as it will be the last one) and is the digit is “X” to make the assignment to the number variable the value of 10.
- If the digit is any other number to assign the output with a false value. Otherwise to add to the total variable the number, converted to an integer and multiplied by its index + 1.
Finally, we need to return the output variable and make sure the total modulo 11 equals zero.
What I learned from here is that Tests are very important. Without them, I didn’t check if the digit was an integer and 0%11=0. The method will return true if we pass as input 10 letters.