Need Feedback : Credit (Pset1- CS50x) - c

Question

#include <stdio.h>
#include <cs50.h>
#include <math.h>
int main(void) {
long int number;
int count = 0; // starting the count counter
do {
number = get_long("Please enter your 13-16 digit credit card number: \n"); //getting input from the user
} while (number <= 0);
int d1 = number % 10; //separating digit number 1
int d2 = ((number % 100) / 10); //separating digit number 2
int d3 = ((number % 1000) / 100); //separating digit number 3
int d4 = ((number % 10000) / 1000); //separating digit number 4
int d5 = ((number % 100000) / 10000); //separating digit number 5
int d6 = ((number % 1000000) / 100000); //separating digit number 6
int d7 = ((number % 10000000) / 1000000); //separating digit number 7
int d8 = ((number % 100000000) / 10000000); //separating digit number 8
int d9 = ((number % 1000000000) / 100000000); //separating digit number 9
int d10 = ((number % 10000000000) / 1000000000); //separating digit number 10
int d11 = ((number % 100000000000) / 10000000000); //separating digit number 11
int d12 = ((number % 1000000000000) / 100000000000); //separating digit number 12
int d13 = ((number % 10000000000000) / 1000000000000); //separating digit number 13
int d14 = ((number % 100000000000000) / 10000000000000); //separating digit number 14
int d15 = ((number % 1000000000000000) / 100000000000000); //separating digit number 15
int d16 = ((number % 10000000000000000) / 1000000000000000); //separating digit number 16
int last13 = number / 1000000000000; //finding the last one digit of 13 digit VISA
int last15 = number / 10000000000000; //finding the last two digit of 15 digit AMEX
int last16v = number / 1000000000000000; //finding the last one digit of 16 digit VISA
int last16m = number / 100000000000000; //finding the last two digit of 16 digit MASTERCARD
int a2 = (d2 * 2);
int a4 = (d4 * 2);
int a6 = (d6 * 2);
int a8 = (d8 * 2);
int a10 = (d10 * 2);
int a12 = (d12 * 2);
int a14 = (d14 * 2);
int a16 = (d16 * 2);
if (a2 >= 10) { // separating the digits of the multiple
int a11a = (a2 % 100) / 10;
int a12a = (a2 % 10);
a2 = a11a + a12a;
}
if (a4 >= 10) { // separating the digits of the multiple
int a31 = (a4 % 100) / 10;
int a32 = (a4 % 10);
a4 = a31 + a32;
}
if (a6 >= 10) { // separating the digits of the multiple
int a51 = (a6 % 100) / 10;
int a52 = (a6 % 10);
a6 = a51 + a52;
}
if (a8 >= 10) { // separating the digits of the multiple
int a71 = (a8 % 100) / 10;
int a72 = (a8 % 10);
a8 = a71 + a72;
}
if (a10 >= 10) { // separating the digits of the multiple
int a91 = (a10 % 100) / 10;
int a92 = (a10 % 10);
a10 = a91 + a92;
}
if (a12 >= 10) { // separating the digits of the multiple
int a111 = (a12 % 100) / 10;
int a112 = (a12 % 10);
a12 = a111 + a112;
}
if (a14 >= 10) { // separating the digits of the multiple
int a131 = (a14 % 100) / 10;
int a132 = (a14 % 10);
a14 = a131 + a132;
}
if (a16 >= 10) { // separating the digits of the multiple
int a151 = (a16 % 100) / 10;
int a152 = (a16 % 10);
a16 = a151 + a152;
}
int b = d1 + d3 + d5 + d7 + d9 + d11 + d13 + d15;
int c = a2 + a4 + a6 + a8 + a10 + a12 + a14 + a16;
int l = b + c;
int j = a2 + a4 + a6 + a8 + a10 + a12;
int k = d1 + d3 + d5 + d7 + d9 + d11 + d13;
int m = j + k;
int e = d1 + d3 + d5 + d7 + d9 + d11 + d13 + d15;
int x = a2 + a4 + a6 + a8 + a10 + a12 + a14;
int n = x + e;
printf("%i", n);
if (l % 10 == 0) { //checking if the checksum is valid
for (int i = number; number != 0; i++) { // counting the credit card number
number /= 10;
count++;
}
if (count == 16) {
if (last16m == 51) { // checking the last two digit of credit card
printf("MASTERCARD\n"); // printing the result based on last two digits
} else
if (last16m == 52) { // checking the last two digit of credit card
printf("MASTERCARD\n"); // printing the result based on last two digits
} else
if (last16m == 53) { // checking the last two digit of credit card
printf("MASTERCARD\n"); // printing the result based on last two digits
} else
if (last16m == 54) { // checking the last two digit of credit card
printf("MASTERCARD\n"); //// printing the result based on last two digits
} else
if (last16m == 55) { // checking the last two digit of credit card
printf("MASTERCARD\n"); // printing the result based on last two digits
} else
if (last16v == 4) { // checking the last two digit of credit card
printf("VISA\n"); // printing the result based on last two digits
} else {
printf("INVALID\n");
}
if (count != 16) {
printf("INVALID\n");
}
} else {
printf("INVALID\n");
}
}
if (m % 10 == 0) { //checking if the checksum is valid
for (int i = number; number != 0; i++) { // counting the credit card number
number /= 10;
count++;
}
if (count == 13) {
if (last13 == 4) { // checking the last one digit of credit card
printf("VISA\n"); // printing the result based on last one digit
} else {
printf("INVALID\n");
}
} else {
printf("INVALID\n");
}
}
if (n % 10 == 0) { //checking if the checksum is valid
for (int i = number; number != 0; i++) { // counting the credit card number
number /= 10;
count++;
}
if (count == 15) {
if (last15 == 34) { // checking the last two digit of credit card
printf("AMEX\n"); // printing the result based on last two digits
} else
if (last15 == 37) { // checking the last two digit of credit card
printf("AMEX\n"); // printing the result based on last two digits
} else {
printf("INVALID\n");
}
if (count != 15) {
printf("INVALID\n");
}
} else {
printf("INVALID\n");
}
}
}
I tried using 4003600000000014 and it gives the correct output. Taking 4222222222222, it returns INVALID. It also doesn't works for AMEX numbers (15 digits). It doesn't responds on giving 4111111111111113 as input (it should return 0). Please tell me what's wrong, I am a newbie to programming. This is the result it gives on checking it through check50 https://submit.cs50.io/check50/44370b403c33f9b47d462e3278458901599d16e1.

Answer 1

There are multiple problems in the code:
the type for number should be unsigned long long to ensure it is large enough to handle the 16 digits. unsigned long may be large enough on some platforms, such as 64-bit linux and OS/X, but not on others such as all versions of Windows.
the computations are lengthy and cumbersome, it would be much more readable to use an array of digit values.
it is unclear whether you compute the count of digits once or multiple times. The loop modifies number and does not initialize count explicitly (and the variable i is useless). If run multiple times, it works by coincidence as number will already be 0 from the first run. You should use a temporary variable with the proper type and compute count just once.
the tests for different card number lengths may have false positives: for example if m % 10 == 0 and the card number does not have 13 digits, you would print INVALID incorrectly. The test should be first on the number of digits, then on the checksum.
it is very confusing to refer to the initial digits as the last digits.
Here is a simpler version using an array:
#include <stdio.h>
#include <stdlib.h>
// emulate cs50 get_long_long()
static long long get_long_long(const char *prompt) {
for (;;) {
long long n;
int c;
printf("%s", prompt);
if (scanf("%llu", &n) == 1)
return n;
printf("Invalid input, try again\n");
// flush the rest of the input line
while ((c = getchar()) != EOF && c != '\n')
continue;
if (c == EOF)
exit(1);
}
}
int main(void) {
long long int number, num;
int digit[16] = { 0 };
int count;
for (;;) { //getting input from the user
number = get_long_long("Please enter your 13-16 digit credit card number: \n");
if (number > 0)
break;
}
// only accept numbers with 13 to 16 digits
if (number < 1000000000000 || number > 9999999999999999) {
printf("INVALID\n");
return 1;
}
for (count = 0, num = number; num != 0; count++) {
digit[count] = num % 10;
num = num / 10;
}
// compute checksum
int checksum = 0;
for (int i = 0; i < 16; i += 2) {
int a1 = digit[i];
int a2 = digit[i + 1] * 2;
if (a2 >= 10)
a2 -= 9;
checksum += a1 + a2;
}
//printf("checksum: %d, ", checksum);
int first = digit[count - 1];
int first2 = 10 * digit[count - 1] + digit[count - 2];
if (checksum % 10 == 0) {
if (count == 16) {
if (first2 >= 51 && first2 <= 55) {
printf("MASTERCARD\n");
return 0;
} else
if (first == 4) {
printf("VISA\n");
return 0;
}
} else
if (count == 13) {
if (first == 4) {
printf("VISA\n");
return 0;
}
} else
if (count == 15) {
if (first2 == 34 || first2 == 37) {
printf("AMEX\n");
return 0;
}
}
}
printf("INVALID\n");
return 1;
}