Validating credit card numbers in python

Question

My code output is incorrect even though I can't see why.If anyone could shed some light on my problem, I would really appreciate it.

For an assignment we are to allow the user to input a 15 or 16 digit credit card number and return whether the number given is valid.

we are using the Luhn's test to verify the number and this is where my code isn't working. the card number 4222222222222220 should return as valid, but mine isn't working and I believe its because I'm not fully understanding what the code is meant to do.

Luhn's Test: Let us say that the credit card number was made of the following digits:

d15 d14 d13 d12 d11 d10 d9 d8 d7 d6 d5 d4 d3 d2 d1 d0

The last digit d0 is the check digit in the Luhn's algorithm. The algorithm goes as follows:

Multiply all the odd digits d1, d3, … d15 by 2.

Sum the digits of each product.

Now add all the even digits d0, … d14 and the single digit products of the odd digits.

If the final sum is divisible by 10 then the credit card is valid, otherwise it is invalid.

My code is:

def len_check(x):
  length = len(str((x)))
  if (length == 15) or (length == 16):
    return True
  else:
    return False 

def is_valid(x):
  card = x
  num_list= list((str(card)))
  sum_odd = 0
  sum_even = 0
  even_count = 0
  odd_count = 0
  total_sum = 0
  length = 0
  for i in num_list:
    length += 1
  print(length)
  count = 0
  if length == 16:
    odd_count = 15
    even_count = 14
  if length == 15:
    odd_count = 13
    even_count = 14
  print(even_count)
  print(odd_count)

  while (count <= length-2):

    print('even count', even_count)
    print('D-even', num_list[even_count])
    sum_even += int(num_list[even_count])
    even_count -=2
    print('sum even', sum_even)

    print('odd count', odd_count)
    print('D-odd', num_list[odd_count])
    prod = 2 * int(num_list[odd_count])
    sum_odd += prod
    odd_count -=2
    print('sum odd', sum_odd)

    count +=2

  total_sum = sum_odd + sum_even
  print('total sum', total_sum)

  if total_sum % 10 == 0:
    return True
  else:
    return False



def main():
  cc_num = int(input('Enter at 15 or 16-digit credit card number: '))

  if not len_check(cc_num):
    print('Not a 15 or 16-digit number')
  else:
    if not is_valid(cc_num):

      print('Invalid credit card number')
    else:
      print('valid card number')



main()

You might want to highlight your actual question, it's hard to spot amidst all that. — ShadowRanger, Oct 23 '15 at 06:42
Let me try to help: what have you tried? Have you made sure that `sum_even` and `sum_odd` are the sums you expect to get? How? — alf, Oct 23 '15 at 06:44
As far as I understand what the program is asking, as in when I do it by hand, I am correct, I believe the issue lies in my understanding of how the test is supposed to work. — Rsherrill, Oct 23 '15 at 06:46
when you have a 16 digit number you're doubling the even (from right) digits not odd ones — Azad, Oct 23 '15 at 07:09

Sharad · Answer 1 · 2015-10-23T07:30:49.753

The program doesn't appear very clear on logic to me. First it is unclear as to why you would loop only till length-2; in case there are 15 digits you might miss one digit as length you will miss one even digit (loop running 7 times and there being 8 even digits from 0 through 14).

Now in case of numbers positioned at odd places you are multiplying by 2 and adding them directly. According to the algorithm you should take sum of the digits and then add. So if the number in odd place is 9 and 2*9 = 18; then you should add the digits (which gives 1 + 8) and then add it to the sum_add. That's another flaw I noticed.

One more thing; for calculating the length of the list you can use len(num_list). Also you should reverse the list using list.reverse because in case of 15 digits you will land into problem.

You can make your program much better if you use list comprehension. Something like this

num_list = num_list.reverse
even_list = [num_list[i] for i in xrange(0,len(num_list),2)]
odd_list =  [num_list[i] for i in xrange(1,len(num_list),2)]

This will segregate your even and odd numbers and you can proceed accordingly. Also to find sum of digits you can check if a number is greater than 9. If it is greater subtract 9 and you will get your sum.

You can go through this link to get an understanding of the algorithm Luhn's Algorithm
Scroll down to the luhn's algorithm. It covers an example

Validating credit card numbers in python

1 Answers1