为什么最后一个元素没有追加到我的数字数组中？

嗨，我是“ C”的新手，我对我得到的某些输出有疑问。

我们正在处理的数字为：[ 4003600000000014 ]。除了一个我不确定如何获取最后一个要打印的数字，我将每个元素附加到此数组。

我对数字6422180感到好奇，不确定是否可以解释。

// 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();

}