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  1.     /**************************************************
  2.      * credit.c                                       *
  3.      *                                                *
  4.      * CS50                                           *
  5.      *                                                *
  6.      * Prompts the user for a credit card number and  *
  7.      *                                                *
  8.      * determines the type of the card.               *
  9.      *************************************************/
  10.  
  11.     #include <cs50.h>
  12.     #include <stdio.h>
  13.     #include <math.h>
  14.     #include <string.h>
  15.  
  16.     /* enumeration containing the result of the test on the credit card number ...
  17.        frankly I do not know why I decided to do it that way ... */
  18.     enum credit_card_type {AMEX, MASTERCARD, VISA, INVALID} type;
  19.  
  20.     // used to obtain a string equivalent of an enumerated value
  21.     char card_type[][26] = {"AMEX", "MASTERCARD", "VISA", "INVALID"};
  22.  
  23.     void is_it_legit(long long int credit_card_number, int credit_card_number_length);
  24.     void what_kinda_credit_card(long long int credit_card_number, int credit_card_number_length);
  25.  
  26.     int
  27.     main(void)
  28.     {
  29.             // used to determine the length of the credit card number
  30.             char temp_credit_card_number[17];
  31.  
  32.             // prompt the user for a credit card number
  33.             printf("Enter a credit card number: ");
  34.             long long int credit_card_number = GetLongLong();
  35.  
  36.             // "credit_card_number" converted to a string in order to determine its length
  37.             snprintf(temp_credit_card_number, 17, "%lld", credit_card_number);
  38.             int credit_card_number_length = strlen(temp_credit_card_number);
  39.  
  40.             what_kinda_credit_card(credit_card_number, credit_card_number_length);
  41.  
  42.             return 0;
  43.     }
  44.  
  45.     /*
  46.      * Test the legitimacy of the credit card number.
  47.      */
  48.     void
  49.     is_it_legit(long long int credit_card_number, int credit_card_number_length)
  50.     {
  51.             long long int every_other_digit;
  52.             long long int left_out_digit;
  53.             int products_digits_sum = 0;
  54.             int left_out_digits_sum = 0;
  55.  
  56.             /* isolate every other digits starting with the number's second-to-last digit,
  57.                multiply it by 2, and then add those products' digits together */
  58.             for (int i = credit_card_number_length - 2; i > -1; i -= 2 ) {
  59.                     every_other_digit = ((long long int) (credit_card_number / pow(10, i))) % 10;
  60.                     products_digits_sum += ((every_other_digit * 2) / 10) + ((every_other_digit * 2) % 10);
  61.             }
  62.  
  63.             // isolate the digits which weren't multiplied by 2, and then add these digits together
  64.             for (int i = credit_card_number_length - 1; i > -1; i -= 2) {
  65.                     left_out_digit = ((long long int) (credit_card_number / pow(10, i))) % 10;
  66.                     left_out_digits_sum += left_out_digit;
  67.             }
  68.  
  69.             int final_sum = products_digits_sum + left_out_digits_sum;
  70.  
  71.             // test weather the last digit of "final_sum" is 0 or not
  72.             if (final_sum % 10 == 0)
  73.                     // print type of credit card
  74.                     printf("%s\n", card_type[type]);
  75.             else
  76.                     // print invalid message
  77.                     printf("%s\n", card_type[3]);
  78.     }
  79.  
  80.     /*
  81.      * Determine the type of the credit card.
  82.      */
  83.     void
  84.     what_kinda_credit_card(long long int credit_card_number, int credit_card_number_length)
  85.     {
  86.             // these variables store the first or two first digits of a credit card number.
  87.             int visa_first_digit_13 = (credit_card_number / pow(10, 12));
  88.             int visa_first_digit_16 = (credit_card_number / pow(10, 15));
  89.             int amex_two_first_digits = (credit_card_number / pow(10, 13));
  90.             int mastercard_two_first_digits = (credit_card_number / pow(10, 14));
  91.  
  92.             switch (credit_card_number_length) {
  93.                 case 13:
  94.                         // VISA ?
  95.                         if (visa_first_digit_13 == 4) {
  96.                                 /* the integer value "2" is assigned to the variable "result".
  97.                                    In the enumeration "credit_card_type" the integer constant 2
  98.                                    is named VISA */
  99.                                 type = 2;
  100.                                 is_it_legit(credit_card_number, credit_card_number_length);
  101.                         } else
  102.                                 // print invalid message
  103.                                 printf("%s\n", card_type[3]);
  104.                         break;
  105.  
  106.                 case 15:
  107.                         // AMEX ?
  108.                         if ((amex_two_first_digits == 34) || (amex_two_first_digits == 37)) {
  109.                                 type = 0;
  110.                                 is_it_legit(credit_card_number, credit_card_number_length);
  111.                         } else
  112.                                 printf("%s\n", card_type[3]);
  113.                         break;
  114.  
  115.                 case 16:
  116.                         // VISA ?
  117.                         if (visa_first_digit_16 == 4) {
  118.                                 type = 2;
  119.                                 is_it_legit(credit_card_number, credit_card_number_length);
  120.                               // MASTERCARD ?
  121.                         } else if ((mastercard_two_first_digits == 51)
  122.                                    || (mastercard_two_first_digits == 52)
  123.                                    || (mastercard_two_first_digits == 53)
  124.                                    || (mastercard_two_first_digits == 54)
  125.                                    || (mastercard_two_first_digits == 55)) {
  126.                                 type = 1;
  127.                                 is_it_legit(credit_card_number, credit_card_number_length);
  128.                         } else
  129.                                 printf("%s\n", card_type[3]);
  130.                         break;
  131.  
  132.                 default:
  133.                         printf("%s\n", card_type[3]);
  134.             }
  135.     }
  136.  
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https://ideone.com/nT0xI
language:
C (gcc 8.3)
created:
14 years ago
visibility:
secret

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