Credit.c

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

int main(void)
{

    long number1 = get_long("Number: ");
    long number2 = number1;

    long length = 0;
    while (number1 > 0)
    {
        number1 = number1 / 10;
        length++;
    }

    long long j;
    long number[length];
    for (j = 0; j < length; j++)
    {
        number[j] = number2 % 10;
        number2 = number2 / 10;
    }

    long long length1 = 0;
    for (j = 1; j < length; j = j + 2)
    {
        if (number[j] >= 0)
        {
            length1++;
        }
    }

    long cs[length1];
    long long jj = 0;
    for (j = 1; j < length; j = j + 2)
    {
        cs[jj] = number[j] * 2;
        jj++;
    }

    int checksum1 = 0;
    for (j = 0; j < length1; j++)
    {
        if (cs[j] >= 10)
        {
            checksum1 = checksum1 + cs[j] % 10;
            checksum1 = checksum1 + cs[j] / 10;
        }
        else
        {
            checksum1 = checksum1 + cs[j];
        }
    }

    for (j = 0; j < length; j = j + 2)
    {
        checksum1 = checksum1 + number[j];
    }

    checksum1 = checksum1 % 10;

    if (checksum1 != 0)
    {
        printf("INVALID\n");
    }
    else if (checksum1 == 0)
    {
        if (length == 13)
        {
            if (number[12] == 4)
            {
                printf("VISA\n");
            }
            else
            {
                printf("INVALID\n");
            }
        }
        else if (length == 15)
        {
            if (number[14] == 3 && number[13] == 4)
            {
                printf("AMEX\n");
            }
            else if (number[14] == 3 && number[13] == 7)
            {
                printf("AMEX\n");
            }
            else
            {
                printf("INVALID\n");
            }
        }
        else if (length == 16)
        {
            if (number[15] == 4)
            {
                printf("VISA\n");
            }
            else if (number[15] == 5 && number[14] == 1)
            {
                printf("MASTERCARD\n");
            }
            else if (number[15] == 5 && number[14] == 2)
            {
                printf("MASTERCARD\n");
            }
            else if (number[15] == 5 && number[14] == 3)
            {
                printf("MASTERCARD\n");
            }
            else if (number[15] == 5 && number[14] == 4)
            {
                printf("MASTERCARD\n");
            }
            else if (number[15] == 5 && number[14] == 5)
            {
                printf("MASTERCARD\n");
            }
            else
            {
                printf("INVALID\n");
            }
        }
        else
        {
            printf("INVALID\n");
        }
    }

}