credit.c

#include <cs50.h>
#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <string.h>

int main(void)
{
    long long n = 0;
    do
    {
        n = get_long_long("credit card number: ");
    }
    while(n < 0);
    
    int digit = 0;
    {
        // if int k = n; it will be Floating point exception, cause An integer in C cannot be bigger than 2147483647 (10 digits).
        long long k = n;
        while (k > 0)
        {
            k = k / 10;
            digit++;
        }
    }
    
    int sum_odd = 0;
    for (int i = 0; i <= digit; i = i + 2)
    {
        // pow(a, b)返回的是一个浮点数，不能进行取模运算；所以应加上(int)
        sum_odd = sum_odd + (n / (long long)pow(10, i )) % 10;
    }
    
    int sum_even = 0;
    for (int j = 1; j <= digit; j = j + 2)
    {
        int a = 2 * ((n / (long long)pow(10, j)) % 10);
        sum_even = sum_even + a % 10 + a / 10;
    }
    
    int sum = (sum_odd + sum_even) % 10;
    int start = n / (long long)pow(10, digit - 2);
    
    if (sum == 0)
    {
        if (digit == 15 && (start == 34 || start == 37))
        {
            printf("AMEX\n");
        }
        else if((start == 51 || start == 52 || start == 53 || start == 54 || start == 55) && digit == 16)
        {
            printf("MASTERCARD\n");
        }
        else if ((digit == 13 || digit == 16) && (start / 10 == 4))
        {
            printf("VISA\n");
        }
        else
        {
            printf("INVALID\n");
        }
    }
    else
    {
        printf("INVALID\n");
    }
}