Luhns Algorithm

Question

Hey I am doing Luhn's algorithm for an assignment for school.

A few outputs are coming out the correct way; however, some are not.

0004222222222222 is giving me a total of 44,

and

0378282246310005 is giving me a total of 48,

for a few examples.

I know my code isn't the cleanest as I am a novice but if anyone can identify where I'm getting my error I'd really appreciate

Here is my code:

cardNumber = input( "What is your card number? ")
digit = len(cardNumber)
value = 0
total = 0
while ( len( cardNumber ) == 16 and digit > 0):
    # HANDLE even digit positions
    if ( digit % 2 == 0 ):
        value = ( int( cardNumber[digit - 1]) * 2 )
        if( value > 9 ):
            double = str( value )
            value = int( double[:1] ) + int( double[-1] )
            total = total + value
            value = 0
            digit = digit - 1
        else:
            total = total + value
            value = 0
            digit = digit - 1
    # HANDLE odd digit positions
    elif ( digit % 2 != 0):
        total = total + int( cardNumber[digit - 1] )
        digit = digit - 1

Can you provide more detail as to why these output are not correct? What should they be? — Tammo Heeren, Oct 16 '16 at 22:09
there is a python implementation of the algorithm in [wikipedia](https://en.wikipedia.org/wiki/Luhn_algorithm), you can study it and try to see if you can identify the problem — Copperfield, Oct 16 '16 at 22:11
http://stackoverflow.com/questions/29197219/credit-card-number-validator-doesnt-work-correctly/29197921#29197921 — Padraic Cunningham, Oct 16 '16 at 22:17

Moses Koledoye · Accepted Answer · 2016-10-16T22:33:24.270

2

You almost got it right. Only that the last digit (or first from behind) should be considered as odd for your 16 digit card. So you should set:

digit = len(cardNumber) - 1

And then your while condition should stop at >= 0 (zeroth item inclusive); note that the len( cardNumber ) == 16 is redundant as the length of the card is constant:

while digit >= 0:

And finally your indexing of the creditcard number will no longer need a minus 1:

value = int(cardNumber[digit]) * 2
...
...
total = total + int(cardNumber[digit])

edited Oct 16 '16 at 22:33

answered Oct 16 '16 at 22:19

Moses Koledoye

77,341
8
133
139

Thanks alot for the reply! At least I know now. The assignment was due an hour and a half ago I was foolish enough to just check this now lol. Was playing Diablo for ps1 on the raspberry pi all night /facepalm – Brian Upward Oct 17 '16 at 05:26
@BrianUpward Accepting helpful answers on SO is another way of saying thank you – Moses Koledoye Oct 17 '16 at 07:00
Do I have to click so et – Brian Upward Oct 18 '16 at 05:12
Do I have to click something to accept it? I upvoted it already – Brian Upward Oct 18 '16 at 05:13
@BrianUpward It is pretty easy :) See [How do I accept an answer on stackoverflow](http://meta.stackexchange.com/questions/5234/how-does-accepting-an-answer-work) – Moses Koledoye Oct 18 '16 at 15:59

score 0 · Answer 2 · answered Oct 16 '16 at 22:57

So your code is mostly correct, the only issue is that you haven't properly defined what should be considered an "odd" and an "even" number. As you read the number from the end, "odd and even" are also relative from the end, so :

odd numbers start from the last one, and then every other one
even numbers start from the last but one, and then every other one

Example : 1234 is EOEO, 12345 is OEOEO (O means odd , E means even)

Here the fixed code (I only modified three lines, see comments):

digit = len(cardNumber)
value = 0
total = 0
while  digit > 0: # I removed the length condition
    # HANDLE even digit positions
    if ( (len(cardNumber)+1-digit) % 2 == 0 ): # <- modification here
        value = ( int( cardNumber[digit - 1]) * 2 )
        if( value > 9 ):
            double = str( value )
            value = int( double[:1] ) + int( double[-1] )
            total = total + value
            digit = digit - 1
        else:
            total = total + value
            digit = digit - 1

    # HANDLE odd digit positions
    elif ( (len(cardNumber)+1-digit) % 2 != 0): # <- modification here
        value=int( cardNumber[digit - 1] )
        total = total + int( cardNumber[digit - 1] )
        digit = digit - 1

return total

Some tests :

In : '0378282246310005' ->  Out : 60
In : '00378282246310005' ->  Out : 60
In : '0004222222222222' ->  Out : 40

Luhns Algorithm

2 Answers2