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  1. /**************************************************
  2.  * new_credit.c                                   *
  3.  *                                                *
  4.  * CS50                                           *
  5.  *                                                *
  6.  * Prompts the user for a credit card number,     *
  7.  *                                                *
  8.  * determines the type of the card                *
  9.  *                                                *
  10.  * (Visa, American Express, Mastercard)           *
  11.  *                                                *
  12.  * and tests its validity.                        *
  13.  *************************************************/
  14.  
  15. #include <stdio.h>
  16. #include <stdlib.h>
  17. #include <stdbool.h>
  18. #include <math.h>
  19.  
  20. #define BUFFER_SIZE 256
  21.  
  22. char*           get_input(void);
  23. bool            is_number(char *alleged_number);
  24. long long int*  string_to_long_long(char *string);
  25. int             count_digits(long long int number);
  26. void            credit_card_type(long long int credit_card_number, int credit_card_number_length);
  27. bool            is_credit_card_valid(long long int credit_card_number, int credit_card_number_length);
  28.  
  29. int
  30. main(void)
  31. {
  32.         long long int *credit_card_number = string_to_long_long(get_input());
  33.         int credit_card_number_length = count_digits(*credit_card_number);
  34.  
  35.         credit_card_type(*credit_card_number, credit_card_number_length);
  36.  
  37.         return 0;
  38. }
  39.  
  40. /*
  41.  * Gets a credit card number from the user. 
  42.  */
  43. char*
  44. get_input(void)
  45. {
  46.         char temp_credit_card_number[BUFFER_SIZE];
  47.  
  48.         // prompt the user for a credit card number
  49.         printf("Enter a credit card number: ");
  50.         // copy from standard input to temp_credit_card_number[]
  51.         while (fgets(temp_credit_card_number, BUFFER_SIZE, stdin) != NULL) {
  52.                 // add the trailling '\0' at the end of the string
  53.                 for (int i = 0; i < BUFFER_SIZE; i++) {
  54.                         if (temp_credit_card_number[i] == '\n') {
  55.                                 temp_credit_card_number[i] = '\0';
  56.                                 break;
  57.                         }
  58.                 }
  59.                 // check user input
  60.                 if ( !(is_number(temp_credit_card_number)) ) {
  61.                         printf("Retry: ");
  62.                 } else {
  63.                         break;
  64.                 }
  65.         }
  66.  
  67.         // get_input() returns a pointer
  68.         char *value = temp_credit_card_number;
  69.  
  70.         return value;
  71. }
  72.  
  73. /*
  74.  * Checks that the user input is a number. 
  75.  */
  76. bool
  77. is_number(char *alleged_number)
  78. {
  79.         long long int number;
  80.         char *invalid;
  81.  
  82.         number = strtoll(alleged_number, &invalid, 10);
  83.         if (*invalid != '\0') {
  84.                 return false;
  85.         }
  86.  
  87.         return true;
  88. }
  89.  
  90. /*
  91.  * Converts a string to a long long integer.
  92.  */
  93. long long int*
  94. string_to_long_long(char *string)
  95. {
  96.         char *invalid;
  97.         long long int number = strtoll(string, &invalid, 10);
  98.         long long int *value = &number;
  99.  
  100.         return value;
  101. }
  102.  
  103. /*
  104.  * Counts the number of digits of a number.
  105.  */
  106. int
  107. count_digits(long long int number)
  108. {
  109.         int i;
  110.  
  111.         for (i = 0; number != 0 ; i++) {
  112.                 number /= 10;
  113.         }
  114.  
  115.         return i;
  116. }
  117.  
  118. /*
  119.  * Determines the type of a credit card, 
  120.  * (Visa, American Express, Mastercard)
  121.  */
  122. void
  123. credit_card_type(long long int credit_card_number, int credit_card_number_length)
  124. {
  125.         // these variables store the first or two first digits of a credit card number.
  126.         int visa_first_digit_13 = (credit_card_number / pow(10, 12));
  127.         int visa_first_digit_16 = (credit_card_number / pow(10, 15));
  128.         int amex_two_first_digits = (credit_card_number / pow(10, 13));
  129.         int mastercard_two_first_digits = (credit_card_number / pow(10, 14));
  130.  
  131.         // determines the possible type of a card then test its validity
  132.         switch (credit_card_number_length) {
  133.             case 13:
  134.                     // VISA ?
  135.                     if (visa_first_digit_13 == 4) {
  136.                             if (is_credit_card_valid(credit_card_number, credit_card_number_length))
  137.                                     printf("VISA\n");
  138.                     } else {
  139.                             printf("INVALID\n");
  140.                     }
  141.                     break;
  142.  
  143.             case 15:
  144.                     // AMEX ?
  145.                     if ((amex_two_first_digits == 34) || (amex_two_first_digits == 37)) {
  146.                             if (is_credit_card_valid(credit_card_number, credit_card_number_length))
  147.                                     printf("AMEX\n");
  148.                     } else {
  149.                             printf("INVALID\n");
  150.                     }
  151.                     break;
  152.  
  153.             case 16:
  154.                     // VISA ?
  155.                     if (visa_first_digit_16 == 4) {
  156.                             if (is_credit_card_valid(credit_card_number, credit_card_number_length))
  157.                                     printf("VISA\n");
  158.                           // MASTERCARD ?
  159.                     } else if ((mastercard_two_first_digits == 51) || 
  160.                                (mastercard_two_first_digits == 52) || 
  161.                                (mastercard_two_first_digits == 53) || 
  162.                                (mastercard_two_first_digits == 54) || 
  163.                                (mastercard_two_first_digits == 55)
  164.                                ) {
  165.                             if (is_credit_card_valid(credit_card_number, credit_card_number_length))
  166.                                     printf("MASTERCARD\n");
  167.                     } else {
  168.                             printf("INVALID\n");
  169.                     }
  170.                     break;
  171.  
  172.             default:
  173.                     printf("INVALID\n");
  174.         }
  175.  
  176.         return;
  177. }
  178.  
  179. /*
  180.  * Tets the syntactic validity of a credit card number,
  181.  * (Peter Luhn's algoritm).
  182.  */
  183. bool
  184. is_credit_card_valid(long long int credit_card_number, int credit_card_number_length)
  185. {
  186.         long long int every_other_digit; 
  187.         int sum_products_digits = 0;
  188.  
  189.         /* isolate every other digits starting with the number's second-to-last digit, 
  190.            multiply it by 2, and then add those products' digits together */
  191.         for (int i = credit_card_number_length - 2; i > -1; i -= 2 ) {
  192.                 every_other_digit = ((long long int) (credit_card_number / pow(10, i))) % 10;
  193.                 sum_products_digits += ((every_other_digit * 2) / 10) + ((every_other_digit * 2) % 10);
  194.         }
  195.  
  196.         long long int left_out_digit;
  197.         int sum_left_out_digits = 0;
  198.  
  199.         // isolate the digits which weren't multiplied by 2, and then add these digits together
  200.         for (int i = credit_card_number_length - 1; i > -1; i -= 2) {
  201.                 left_out_digit = ((long long int) (credit_card_number / pow(10, i))) % 10;
  202.                 sum_left_out_digits += left_out_digit;
  203.         }
  204.  
  205.         int final_sum = sum_products_digits + sum_left_out_digits;
  206.  
  207.         // test weather the last digit of "final_sum" is 0 or not
  208.         if (final_sum % 10 == 0) {
  209.                 return true;
  210.         } else {
  211.                 return false;
  212.         }
  213. }
Compilation error #stdin compilation error #stdout 0s 0KB
comments (?)
stdin 
Standard input is empty
compilation info 
prog.c: In function ‘get_input’:
prog.c:53: error: ‘for’ loop initial declaration used outside C99 mode
prog.c: In function ‘is_credit_card_valid’:
prog.c:191: error: ‘for’ loop initial declaration used outside C99 mode
prog.c:200: error: redefinition of ‘i’
prog.c:191: error: previous definition of ‘i’ was here
prog.c:200: error: ‘for’ loop initial declaration used outside C99 mode
stdout 
Standard output is empty
https://ideone.com/gUDcm
language:
C (gcc 8.3)
created:
14 years ago
visibility:
secret

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