pythonpython-3.xisbn

Python Program not working as expected

I tried to create a python-based ISBN13 Validity Calculator, which is supposed to work according to the following snip of code. It takes in an argument of type string and prints the result.

I don't understand where I got wrong here, In this program, common sense tells me num will give a 3 or a 1 alternatively.

def isbncheck(isbn):
    num = int(0)
    for i in range(0,12):
        if num % 2 is 1:
            num = num + (int(isbn[i])*3)
            print(isbn[i] + " * 3 + at i=" + str(i))
        elif num % 2 is 0:
            num = num + (int(isbn[i]))
            print(isbn[i] + " * 1 + at i=" + str(i))
        else:
            print("Unhandled Exception.")

    print("_"*7)
    print(num)
    num = num%10
    print(num)
    print(isbn[12])
    print(int(isbn[12]))
    if num != int(isbn[12]):
        print("This is not valid")
    else:
        print("It is Valid.")

print(isbncheck("9788177000252"))

I expected the output to alternate factors of 1 and 3. Instead, I got this:

9 * 1 + at i=0
7 * 3 + at i=1
8 * 1 + at i=2
8 * 1 + at i=3
1 * 1 + at i=4
7 * 3 + at i=5
7 * 1 + at i=6
0 * 3 + at i=7
0 * 3 + at i=8
0 * 3 + at i=9
2 * 3 + at i=10
5 * 3 + at i=11
_______
96
6
2
2
This is not valid
None

How do you work around it? Other than not expanding the loop?

Solution

  • Your problem is here:

    for i in range(0,12):
    if num % 2 is 1:

    num is your accumulated sum. You have to check the index, not the sum so far.

    for i in range(len(isbn)):
    digit = int(isbn[i])
    if i % 2 == 1:
        num += 3 * digit

    Also note some minor improvements: