An ISBN (International Standard Book Number) can consist of 10 or 13 digits. The

Question

An ISBN (International Standard Book Number) can consist of 10 or 13 digits. The

last digit is called checksum, which is calculated using a certain formula.

Calculations: ISBN-10
The ISBN-10 consists of 10 digits: 0718079183

The last digit is calculated using the first 9 digits as:

h = +×1+ /×2+ 1×3+ 3×4+ 5×5+ 7×6+ 9×7+ ;×8+ =×9 %11

If the checksum is 10, then the 10th digit is denoted as X.

Calculations: ISBN-13
The ISBN-13 consists of 13 digits: 9780718079185

h = 10 + +3 / + 1 +3 3 + 5 +3 7 + 9 +3 ; + = +3 +B + ++ +3 +/ %10

If the checksum is 10, then put 0 as the 13th digit.

Write a program that prompts the user to enter the digits (except the last digit) of an ISBN – it can be either ISBN-10 or ISBN-13. Read the input as a string. User can enter 9 digits or 12 digits. The program should determine the checksum based on the size of the number entered. If the user enters a number that doesn’t contain either 9 or 12 digits, display a message and prompt the user to enter 9 or 12 digits.

Sample Output:
Enter either 9 or 12 digits as a string (or -1 to quit): 9787312

Invalid ISBN number.
Enter either 9 or 12 digits as a string (or -1 to quit): 978071807918 The ISBN-13 number is: 9780718079185

Enter either 9 or 12 digits as a string (or -1 to quit): 0718079183 The ISBN-10 number is: 0718079183

Enter either 9 or 12 digits as a string (or -1 to quit): 013119402 The ISBN-13 number is: 013119402X

Enter either 9 or 12 digits as a string (or -1 to quit): 978731259722 The ISBN-13 number is: 9787312597220

it should be coded in python!

Explanation / Answer

# code link in case indentation mess up: https://goo.gl/58IDLm

def get_isbn10_checkbit(isbn):
check_bit = 0
counter_bit = 10
for i in isbn:
check_bit += counter_bit*i
counter_bit -= 1
check_bit = check_bit % 11
check_bit = 11 - check_bit
if check_bit == 10:
check_bit = 'X'
return check_bit

def get_isbn13_checkbit(isbn):
check_bit = 0
odd_sum = sum(isbn[0::2])
even_sum = sum(isbn[1::2])
even_sum = even_sum*3
check_bit = (even_sum + odd_sum)%10
if check_bit != 0:
check_bit = 10 - check_bit
return check_bit

isbn = ""
while True:
isbn = raw_input("Enter either 9 or 12 digits as a string (or -1 to quit): ")
if isbn == "-1":
break
if not (len(isbn) == 9 or len(isbn) == 12):
print "Invalid ISBN number."
continue
isbn_dig = [int(i) for i in isbn]
if len(isbn_dig) == 9:
check_bit = get_isbn10_checkbit(isbn_dig)
print "The ISBN-10 number is: "
else:
check_bit = get_isbn13_checkbit(isbn_dig)
print "The ISBN-13 number is: "
print isbn + str(check_bit)

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