Suppose you type a 14-digit credit card number into a Web site, but mistype one of the digits or inadvertently interchange two adjacent digits. The Web site will perform a validation check that always detects the first type of error and nearly always detects the second type of error. The validation check is as follows:
1) Starting with the leftmost digit, double it and then double every other digit after it. However, if any of the doubled digits is a two-digit number, subtract 9 from it. Then sum these new digits. For instance, if the credit card number is 58667936100244, then the digits considered are 5,6,7,3,1,0,4, their new replacements are 1,3,5,6,2,0,8, and the sum of the replacements is 25.
2) Sum together the remaining seven digits from the credit card number. That is, the digits in the odd-numbered positions. With the credit card number above, we obtain 8 + 6 + 9 + 6 + 0 + 2 + 4 = 35.
3) Add together the two sums. If the result is a multiple of 10, then accept the credit card number. Otherwise, reject it. We accept the credit card number above since 25 + 35 = 60, a multiple of 10. Write a program that performs data validation on a credit card number. One possible outcome (sample data): Enter a credit card number: 58667936100244 The number is valid.

Explanation:

Using Python 3

def validate_card(card_number):

   even_positioned_digits = card_number[::2]

   odd_positioned_digits = card_number[1::2]

   sum_even_digits = 0

   for digit in even_positioned_digits:

       if int(digit) * 2 > 9:

           sum_even_digits += (int(digit) * 2) - 9

       else:

           sum_even_digits += int(digit)

   sum_odd_digits = sum(int(x) for x in odd_positioned_digits)

   total_sum = sum_even_digits + sum_odd_digits

   return total_sum % 10 == 0

if __name__ == '__main__':

   card_number = input('Enter a credit card number >>> ')

   if validate_card(card_number):

       print('The number is valid.')

   else:

       print('Invalid card number entered.')

a  social news website  is an  internet  website  that features  user-posted stories. such stories are ranked based on popularity, as voted on by other users of the site or by website administrators. users typically comment online on the news posts and these comments may also be ranked in popularity. since their emergence with the birth of  web 2.0, social news sites have been used to link many types of information, including news, humor, support, and discussion. all such websites allow the users to submit content and each site differs in how the content is moderated. on the  slashdot  and  fark  websites,  administrators  decide which articles are selected for the front page. on  reddit  and  digg, the articles that get the most votes from the community of users will make it to the front page. many social news websites also feature an online comment system, where users discuss the issues raised in an article. some of these sites have also applied their voting system to the comments, so that the most popular comments are displayed first. some social news websites also have a  social networking  function, in that users can set up a user profile and follow other users' online activity on the website.