CS50 Problem Set 1 (Credit) 2020 help needed - c

Question

I am trying to prompt the user for a credit card number and determine whether it is a real credit card number or not, and if so what type of credit card number.
I thought I'd finally got it however when doing check50 the following two inputs produce no output:
1234567890
4111111111111113
They should be giving INVALID but I can't figure out why they aren't giving any output.
This is my code:
#include <stdio.h>
#include <cs50.h>
#include <math.h>
int main(void)
{
long Card_Number;
int Digit_Number = 0, Current_Digit = 0, Even_x2_Product = 0, Even_Digits = 0, Odd_Digits = 0,
Total_Digit_Sum = 0;
bool is_even = false;
// Prompt User for Credit Card Number
do
{
Card_Number = get_long("Card Number: ");
}
while (Card_Number < 0);
// Check First Digits of Number
int Digits_MstrCrd = Card_Number / pow(10, 14);
int Digits_Visa_16 = Card_Number / pow(10, 15);
int Digits_AmEx = Card_Number / pow(10, 13);
int Digits_Visa_13 = Card_Number / (pow(10, 12));
// Loop to determine identity of each digit
while (Card_Number != 0)
{
// Get Last Digit of Number
Current_Digit = (Card_Number % 10);
// Increase Digit Number by 1
Digit_Number += 1;
// Check if Current Digit is at Odd or Even Position in Card Number
if (is_even == true)
{
// Multiply Digit by 2
Even_x2_Product = Current_Digit * 2;
// Add Digits of Multiplication Product
while (Even_x2_Product != 0)
{
Even_Digits += Even_x2_Product % 10;
Even_x2_Product /= 10;
}
// Tell Program Next Digit is Odd
is_even = false;
}
else
{
// Add Odd Digits
Odd_Digits += Current_Digit;
// Tell Program Next Number is Even
is_even = true;
}
// Remove Last Digit and Repeat
Card_Number /= 10;
}
// Add Odd and Even Digits Together
Total_Digit_Sum = Even_Digits + Odd_Digits;
// Loop to Check if Card Number is Valid
if (Total_Digit_Sum % 10 == 0)
{
// Check Mastercard
if (Digit_Number == 16)
{
if (Digits_MstrCrd <= 55 && Digits_MstrCrd >= 51)
{
printf("MASTERCARD\n");
}
// Check Visa 16
else if (Digits_Visa_16 == 4)
{
printf("VISA\n");
}
else
{
printf("INVALID\n");
}
}
// Check American Express
else if (Digit_Number == 15)
{
if (Digits_AmEx == 34 || Digits_AmEx == 37)
{
printf("AMEX\n");
}
else
{
printf("INVALID\n");
}
}
// Check Visa 13
else if (Digit_Number == 13)
{
if (Digits_Visa_13 == 4)
{
printf("VISA\n");
}
else
{
printf("INVALID\n");
}
}
else
{
printf("INVALID\n");
}
}
}

Answer 1

What does the program print if Total_Digit_Sum % 10 is not equal to 0? It has no else; there are no commands after the closing } of the block.

Answer 2

I'll show you in your program (with embedded multiline comments) all the objecttionable things I've seen by simple inspection (I've had to modify it a little, as you don't provide an implementation of get_long(char *prompt) function, and some other files you also don't provide. Later I give you a better solution, that doesn't have the problem of the integer limit, as it uses strings to calculate the checksum.
At the end there's a reference to a github repository where all versions of the solution are considered (including a DFA ---Deterministic Finite Automaton--- probably the fastest solution to the problem)
/* sorry, I need to comment this, as you have not provided this
* file. */
//#include <cs50.h>
/* you don't need math.h if you are using only integers */
//#include <math.h>
/* what is needed is stdbool.h, to use booleans in C */
#include <stdbool.h>
#include <stdio.h> /* and stdio, of course */
unsigned long long get_long(char *prmpt)
{
unsigned long long result;
fprintf(stderr, "%s> ", prmpt);
/* this loop is not protected against EOF, so you will have
* to interrupt the program if you reach the end of file
* here. */
while (scanf("%llu", &result) != 1)
fprintf(stderr, "?? >");
return result;
}
int main(void)
{
/* you need a 64bit number, so better use a long long here
* 32bit integers range only up to 4294967296, which is too
* short to use in your problem.
* on dividing your card number by 100000000000000 you'll
* allways get 0.
*/
long Card_Number;
int Digit_Number = 0, Current_Digit = 0, Even_x2_Product = 0, Even_Digits = 0, Odd_Digits = 0,
Total_Digit_Sum = 0;
bool is_even = false;
// Prompt User for Credit Card Number
do
{
Card_Number = get_long("Card Number: ");
}
while (Card_Number < 0);
// Check First Digits of Number
/* don't use pow(3) to produce a constant to divide
* in floating point by a power of ten. It allways
* produces inexact results, ad 1/10 cannot be represented
* as a finite number of digits in base 2. Just use
* 100000000000000LL, instead.
* In order to get the ttype of card, it is better to compare
* the number, as in
* // number is 15digits, at least
* if (Card_number >= 1000000000000000ULL) {
* Digit_number = 15;
* } else if (Card_number >= 10000000000000ULL) {
* Digit number = 14;
* } else if (Card_number >= 1000000000000ULL) {
* Digit_number = 13;
* ...
*/
int Digits_MstrCrd = Card_Number / pow(10, 14);
int Digits_Visa_16 = Card_Number / pow(10, 15);
int Digits_AmEx = Card_Number / pow(10, 13);
int Digits_Visa_13 = Card_Number / (pow(10, 12));
// Loop to determine identity of each digit
while (Card_Number != 0)
{
// Get Last Digit of Number
Current_Digit = (Card_Number % 10);
// Increase Digit Number by 1
/* why do you increment the digit by one, the digit value
* is just that, the remainder of the integer division.
*/
Digit_Number += 1;
// Check if Current Digit is at Odd or Even Position in Card Number
/* better use if (is_even) as is_even is already a
* boolean */
if (is_even == true)
{
// Multiply Digit by 2
Even_x2_Product = Current_Digit * 2;
// Add Digits of Multiplication Product
/* Even_x2_Product cannot be higher that 18,
* so why not just check if it is greater than 10
* and then subtract 10 and add 1 (or better,
* just subtract 9), as in:
if (Even_x2_Product >= 10)
Even_x2_product -= 9;
*/
while (Even_x2_Product != 0)
{
Even_Digits += Even_x2_Product % 10;
Even_x2_Product /= 10;
}
// Tell Program Next Digit is Odd
/* Shouldn't we add this result somewhere,
* mod 10 ??? Like in:
accumulated_checksum += Even_x2_Product;
Note: you do in the odd part.
*/
is_even = false;
}
else
{
/* I suggest you to add all digits together.
* As in:
accumulated_checksum += Current_digit;
*/
// Add Odd Digits
Odd_Digits += Current_Digit;
// Tell Program Next Number is Even
is_even = true;
}
/* if we have added two digits (the accumulated_checksum
* and the calculated one, no possibility of having more
* than 18 as the sum is possible, so check if the result
* is 10 or more, and subtract 10 to eliminate the carry.
if (accumulated_checksum >= 10)
accumulated_checksum -= 10;
*/
// Remove Last Digit and Repeat
Card_Number /= 10;
}
/* you can use only one sum. Both are digits... and if you
* have made the checks suggested above, it is already a number
* modulo 10. */
// Add Odd and Even Digits Together
/* this is not necessary */
Total_Digit_Sum = Even_Digits + Odd_Digits;
// Loop to Check if Card Number is Valid
/* you don't need to calculate the modulo 10 here, as you
* have eliminated all the higher digits in the last loop.
*/
if (Total_Digit_Sum % 10 == 0)
if (Total_Digit_Sum % 10 == 0)
{
// Check Mastercard
/* this is not the number of digits you have, this is the
* integer result of the division by a huge number...
* most of the times this will be zero, but it never be
* 16, with the numbers you are giving for the cards. */
if (Digit_Number == 16)
{
if (Digits_MstrCrd <= 55 && Digits_MstrCrd >= 51)
{
printf("MASTERCARD\n");
}
// Check Visa 16
else if (Digits_Visa_16 == 4)
{
printf("VISA\n");
}
else
{
printf("INVALID\n");
}
}
// Check American Express
/* also this is not true, by the same reason above. */
else if (Digit_Number == 15)
{
if (Digits_AmEx == 34 || Digits_AmEx == 37)
{
printf("AMEX\n");
}
else
{
printf("INVALID\n");
}
}
// Check Visa 13
/* same as above */
else if (Digit_Number == 13)
{
if (Digits_Visa_13 == 4)
{
printf("VISA\n");
}
else
{
printf("INVALID\n");
}
}
else
{
/* so you always end here */
printf("INVALID\n");
}
}
}
There's no need to convert the string of digits into a number... this will make your processing more complicated, and you will need to swith to long long numbers to use it on the longest card numbers.
I have developed this routine:
#include <ctype.h>
#include <stdio.h>
#include <string.h>
#include "main.h"
#include "proc.h"
int process(const char *str)
{
int l = strlen(str);
const char *p = str + l;
int res = 0;
enum {
ODD_DIGIT,
EVEN_DIGIT,
} pos = ODD_DIGIT;
DEB("processing: [%s]\n", str);
while (--p >= str) {
if (!isdigit(*p)) {
WARN("%s\n", str);
WARN("%*s^: is not a digit\n", (int)(p-str), "");
return -1;
}
int dig = *p - '0';
switch (pos) {
case ODD_DIGIT: pos = EVEN_DIGIT;
DEB("Add dig(%d) to res(%d)\n", dig, res);
res += dig; break;
case EVEN_DIGIT: pos = ODD_DIGIT;
DEB("Add double(dig(%d)) to res(%d)\n", dig, res);
dig <<= 1;
if (dig >= 10)
dig -= 9;
res += dig; break;
}
if (res >= 10)
res -= 10;
DEB("res <= %d\n", res);
}
DEB("Returning => %d\n", res);
if ((flags & FLAG_QUIET) == 0) {
printf("%s: %d\n", str, res);
}
return res;
}
that uses a string of digits, and processes it from right to left (beginning on the end of the string) It is part of this code, published in github and that you can download the complete program from here. You'll find there the version published here, if you checkout the version tagged as SO_60424279, and in the branch master you'll get a table driven DFA implementation that should run faster than this one.
To compile, just execute
make
in the directory you extracted the source.

Answer 3

#include <stdio.h>
#include<cs50.h>
#include <math.h>
int main(void)
{
long x = get_long("enter the credit card number");
int digit = 0, sum = 0;
//digit is used for odd and even checker.
long y = x;
//checksum card digits
while (y != 0)
{
int sumeven = 0, sumodd = 0;
int rem = y % 10;
digit++;
if (digit % 2 == 0) //if digit is even
{
int multiply = rem * 2;
if (multiply == 0)
{
sumeven += multiply;
}
else
{
while (multiply != 0) //adding all digits after
{
sumeven += multiply % 10;
multiply /= 10; //minus last digit of multiply
}
}
}
else //if digit is odd
{
sumodd += rem;
}
y /= 10; //minus last digit from y
sum += sumeven + sumodd;
}
//check for valid credit card
if (digit != 13 && digit != 15 && digit != 16) //for first if
{
printf("INVALID\n");
}
else if (sum % 10 == 0)
{
if (digit == 16) //if digit is 16
{
if (x / 100000000000000 >= 51 && x / 100000000000000 <= 55)
{
printf("MASTERCARD\n");
}
else if (x / 1000000000000000 == 4)
{
printf("VISA\n");
}
else //if digit is not 16
{
printf("INVALID\n");
}
}
else if (digit == 15)
{
if (x / 10000000000000 == 34 || x / 10000000000000 == 37)
{
printf("AMEX\n");
}
else
{
printf("INVALID\n");
}
}
else if (digit == 13)
{
if (x / 1000000000000 == 4)
{
printf("VISA\n");
}
else
{
printf("INVALID\n");
}
}
}
else
{
printf("INVALID\n");
}
}`