Question

(Business: check ISBN-13)

ISBN-13 is a new standard for indentifying books. It uses 13 digits d1d2d3d4d5d6d7d8d910d11d12d13 . The last digit d13 is a checksum, which is calculated from the other digits using the following formula:

10 - (d1 + 3*d2 + d3 + 3*d4 + d5 + 3*d6 + d7 + 3*d8 + d9 + 3*d10 + d11 + 3*d12) % 10

If the checksum is 10, replace it with 0. Your program should read the input as a string. Display “incorrect input” if the input is incorrect.

Sample Run 1

Enter the first 12 digits of an ISBN-13 as a string: 978013213080

The ISBN-13 number is 9780132130806

Sample Run 2

Enter the first 12 digits of an ISBN-13 as a string: 978013213079

The ISBN-13 number is 9780132130790.

In Python.

Answer 1

Here is the completed code for this problem. Comments are included, go through it, learn how things work and let me know if you have any doubts or if you need anything to change. If you are satisfied with the solution, please rate the answer. Thanks

#code

#method to validate the input
#returns true if isbn is a 12 digit number
def isValidInput(isbn):
   #if length is not 12, returns False
    if len(isbn)!=12:
        return False
    #defining a string of digits
    digits='0123456789'
    #loops through each character in isbn
    for i in isbn:
        #if i is not a digit, returns False
        if not i in digits:
            return False
    return True #all checks passed

#method to find the checksum, given the validated isbn
def findChecksum(isbn):
    #converting isbn into a list of digits
    digits=[int(i) for i in isbn]
    #variable to store the sum of digits
    temp=0
    #looping through digits
    for i in range(len(digits)):
        #if i is an even index,simply adding digit at index i to temp
        if i%2==0:
            temp+=digits[i]
        #otherwise, multiplying current element by 3 and adding
        else:
            temp+=3*digits[i]
    #finding result of temp modulo 10
    mod=temp%10
    #finding checksum by subtracting from 10
    checksum=10-mod
    #returning 0 if checksum is 10.
    if checksum==10:
        return 0
   else:
        #returning checksum as it is
        return checksum

if __name__ == '__main__':
    #getting input, validating, printing results
    isbn=input('Enter the first 12 digits of an ISBN-13 as a string: ')
    if isValidInput(isbn):
        checksum=findChecksum(isbn)
        isbn_13=isbn+str(checksum)
        print('The ISBN-13 number is',isbn_13)
    else:
        print('incorrect input')

#output (two runs)

Enter the first 12 digits of an ISBN-13 as a string: 978013213080

The ISBN-13 number is 9780132130806



Enter the first 12 digits of an ISBN-13 as a string: 978013213079

The ISBN-13 number is 9780132130790