# Question about Pset1 - credit

I had work on the credit for 3 hours, and I had reach to most of the function - and I checked that the number received from: Luhn (a) last digit(c) and digit count (cc) are right by using 4003600000000014, which a = 20, c = 0 and cc = 16

However, for some reasons that I keep getting INVALID as answer, can anyone found out why and let me know? Thanks a lot QvQ

``````# include <stdio.h>
``````

# include <cs50.h>

int main (void){ long n = 0; do{ n = get_long ("Please Enter Your Credit Card Number:"); } while (n < 0);

``````int n1 = n % 100/10*2;
int n2 = n % 10000/1000*2;
int n3 = n % 1000000/100000*2;
int n4 = n % 100000000/10000000*2;
int n5 = n % 10000000000/1000000000*2;
int n6 = n % 1000000000000/100000000000*2;
int n7 = n % 100000000000000/100000000000000*2;
int n8 = n % 10000000000000000/1000000000000000*2;

// Make sure each number is 1 digit

int f1 = n1 % 100/10;
int f1a = n1 % 10;
int f2 = n2 % 100/10;
int f2a = n2 % 10;
int f3 = n3 % 100/10;
int f3a = n3 % 10;
int f4 = n4 % 100/10;
int f4a = n4 % 10;
int f5 = n5 % 100/10;
int f5a = n5 % 10;
int f6 = n6 % 100/10;
int f6a = n6 % 10;
int f7 = n7 % 100/10;
int f7a = n7 % 10;
int f8 = n8 % 100/10;
int f8a = n8 % 10;

int t1 = n % 10;
int t2 = n % 1000/100;
int t3 = n % 100000/10000;
int t4 = n % 10000000/1000000;
int t5 = n % 1000000000/1000000000;
int t6 = n % 100000000000/100000000000;
int t7 = n % 10000000000000/1000000000000;
int t8 = n % 1000000000000000/100000000000000;

int q = f1+f1a+f2+f2a+f3+f3a+f4+f4a+f5+f5a+f6+f6a+f7+f7a+f8+f8a;
int a = q + t1 + t2 + t3 + t4 + t5 + t6 + t7 + t8;
int c = a % 10;
int cc = 0;//count digit
int nn = n;//count rest of n
do{
nn = nn/10;
cc ++;
}
while (nn > 0);
//checksum 1
if (c == 0){
if (cc == 15){
int t = n % 100000000000000/1000000000000;
//checksum AMEx
if (t == 34 || t == 37){
printf("AMEX\n");
}
else{
printf("INVALID\n");
}
}
//checksum Visa
else if  (cc == 13){
int v1 = n % 1000000000000/100000000000;
if (v1 == 4){
printf("VISA\n");
}
else{
printf("INVALID\n");
}
}
// checksum visa and master
else if (cc == 16){
int x = n % 1000000000000000/100000000000000;
int y = n % 1000000000000000/10000000000000;
if (y == 4){
printf("VISA\n");
}
else{
if (x >= 51 && x <=55){
printf("MASTERCARD\n");
}
else {
printf("INVALID\n");
}
}
}
}
else {
printf("INVALID\n");
}

``````

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