Why is the last element not appending to my array of digits? - c

Question

Hi I am new to "C" I have a question on some output I got.
The number we are working with as data is: [4003600000000014].
I have every element appended to this array except for one I am not sure how to get the last number to print.
I am curious about the number 6422180 I am not sure what it is can someone explain.
// This is an example of Luhn's algorithm.
#include <stdio.h>
#include <ctype.h>
#define MAX_CRED_LEN 16
// Function prototypes.
void payment_method();
int main(void)
{
payment_method();
return 0;
}
long long int get_credit_card_num()
/*
Gets the users credit card number.
*/
{
long long int d;
do
{
printf("\nPlease enter in your credit card number: ");
if (scanf("%lld", &d) == 1)
{
printf("\nYour credit card number: (%lld)\n%s", d,
"------------------------\n\n");
continue;
}
else
{
printf("\n\tINVALID INPUT CHARACTER ENTERED PLEASE TRY AGAIN!\n%s",
"\t*************************************************");
fflush(stdin); // Clear input buffer of any characters!
get_credit_card_num(); // Start the function again to prompt user.
}
} while (d < 0);
return d;
}
int num_of_ccdigits()
/*
Counts the amount of digits the user entered. Then compares the number of
digits to a specific length offered by cardholders they are 13, 15, and 16
digits long.
*/
{
long long int cc_num = get_credit_card_num();
int digits = 0, arr[MAX_CRED_LEN];
// Count how many digits the user entered then compare the length.
while (cc_num != 0)
{
cc_num /= 10;
arr[digits] = cc_num % 10;
digits++;
}
// Display the number of digits that the user entered.
printf("\nNumber of digits: [%i]", digits);
int a = 0, i;
for (i = 0; i < digits; i++)
{
printf("\n[%d]:\tYOUR DIGIT ARRAY: %i", a, arr[i - 1]);
++a;
}
printf("\n\nARRAY WITHOUT LINE BREAKS: %i", arr);
// Check to see if this is a valid credit card number or a supported one.
if ((digits != 13) && (digits != 15) && (digits != 16))
{
printf("\n\tMUST BE AT LEAST 13, 15, AS HIGH AS 16 DIGITS.\n%s",
"\t**********************************************\n");
// if no 13, 15, or 16 digits long restart loop.
num_of_ccdigits();
}
return digits; // return the amount of digits in the credit card number.
}
void payment_method()
{
int num_of_digits = num_of_ccdigits();
}

Answer 1

I see a few issues here. For your first question, the code is indexing out of bounds of arr, causing undefined behavior:
for (i = 0; i < digits; i++)
{
printf("\n[%d]:\tYOUR DIGIT ARRAY: %i", a, arr[i - 1]);
++a;
}
On the first iteration, i = 0 and we proceed to index into arr[0 - 1] or arr[-1]. Whoops. This might produce a garbage value or cause a crash (or anything else).
By the way, a is redundant here--just use i to get the index (having to resort to a to get the index smells like something is off). The arr[i - 1] trick might have been done to compensate for another bug:
while (cc_num != 0)
{
cc_num /= 10;
arr[digits] = cc_num % 10;
digits++;
}
The problem here is that cc_num /= 10; says "move on to the next digit". But this skips the first digit completely. This should be:
while (cc_num != 0)
{
arr[digits] = cc_num % 10;
digits++;
cc_num /= 10;
}
Another problem which can be identified by turning on warnings is
ARRAY WITHOUT LINE BREAKS: -1747166688
Here, the compiler tells us the problem:
$ clang-7 -o main main.c
main.c:67:49: warning: format specifies type 'int' but the
argument has type 'int *' [-Wformat]
printf("\n\nARRAY WITHOUT LINE BREAKS: %i", arr);
~~ ^~~
1 warning generated.
To dump the array's values, use a loop and print each element individually.
Other remarks:
Invalid user entries cause an infinite loop.
Good job breaking things into functions, but try to avoid side effects in functions, especially printing. For example num_of_ccdigits does quite a lot more than simply getting the number of credit card digits. It also does validation and I/O. If you remove the print statements and make it responsible for purely counting the digits in a number, you can rename it count_digits(long long int num) and make it reusable on any long long int. Then, handle printing and validation elsewhere, say, print_digits(long long int num) and validate_cc_digit_length(long long int num). Check out single responsibility principle.
I'm a bit confused by the purpose of the function
void payment_method()
{
int num_of_digits = num_of_ccdigits();
}
Function names are typically actions, describing what the function does, rather than nouns, which are typically variables. Consider something like get_payment_method().
Also, since num_of_ccdigits calls itself recursively on bad data, it can potentially cause the stack to overflow. Using a while loop is a better way to handle bad input that can go on unbounded.
For initialization like:
int digits = 0, arr[MAX_CRED_LEN];
prefer:
int digits = 0;
int arr[MAX_CRED_LEN];
which is easier to read.