Validating credit cards - c

Question

I'm very new to C language much less programming in general and am working through the cs50 edx course. I encountered this problem in week 1 where I'm supposed to validate credit cards given their characteristics (they're in the comments).
After hours worth of research and trials, I managed to solve it though I think it's way too long and I am 90% sure it can be shortened with loops. I added the long version that works and an iteration of it which uses loops that I just can't get to work. Any advice would help and be very much appreciated!
#include <stdio.h>
#include <cs50.h>
int main(void)
{
//user input for credit card number
long creditCard = get_long("Credit Card: ");
//initialize array for finding digits
int digit[16];
int oddDigits[8];
//separate all digits as variables in array digits[]
digit[0] = ((creditCard / 1000000000000000) % 10);
digit[1] = ((creditCard / 100000000000000) % 10);
digit[2] = ((creditCard / 10000000000000) % 10);
digit[3] = ((creditCard / 1000000000000) % 10);
digit[4] = ((creditCard / 100000000000) % 10);
digit[5] = ((creditCard / 10000000000) % 10);
digit[6] = ((creditCard / 1000000000) % 10);
digit[7] = ((creditCard / 100000000) % 10);
digit[8] = ((creditCard / 10000000) % 10);
digit[9] = ((creditCard / 1000000) % 10);
digit[10] = ((creditCard / 100000) % 10);
digit[11] = ((creditCard / 10000) % 10);
digit[12] = ((creditCard / 1000) % 10);
digit[13] = ((creditCard / 100) % 10);
digit[14] = ((creditCard / 10) % 10);
digit[15] = (creditCard % 10);
//double the value of every other digit as oddDigits[]
oddDigits[0] = digit[0] * 2;
oddDigits[1] = digit[2] * 2;
oddDigits[2] = digit[4] * 2;
oddDigits[3] = digit[6] * 2;
oddDigits[4] = digit[8] * 2;
oddDigits[5] = digit[10] * 2;
oddDigits[6] = digit[12] * 2;
oddDigits[7] = digit[14] * 2;
//the first part in finding the sum of every other digit
for (int o = 0; o < 8; o++) {
if (oddDigits[o] > 9) {
oddDigits[o] -= 9;
}
}
//part 2 of finding the sum of every other digit
int sum = 0;
for (int s = 0; s < 8; s++) {
sum += oddDigits[s];
}
//sum of everything else
for (int s2 = 1; s2 < 16; s2 += 2) {
sum += digit[s2];
}
//check if last digit is 0
int check = sum % 10;
//last part of validation
if (check != 0)
printf("INVALID\n");
//type of card
else {
if (digit[0] == 0 && digit[1] == 3 && (digit[2] == 4 || digit[2] == 7)) //15 digits starting with either 34 or 37
{
printf("AMAX\n");
}
else if (digit[0] == 5 && (digit[1] == 1 || digit[1] == 2 || digit[1] == 3 || digit[1] == 4 || digit[1] == 5)) //16 digits starting with either 51 - 54
{
printf("MASTERCARD\n");
}
else if (digit[0] == 4) //16 digits starting with 4
{
printf("VISA\n");
}
else if (digit[0] == 0 && digit[1] == 0 && digit[2] == 0 && digit[3] == 4) //13 digits starting with 4
{
printf("VISA\n");
}
else
{
printf("INVALID\n");
}
}
}
The only part I changed is where I try to get every digit of the number using for loops and the part where I try to double every other digit... I did a test where after the loop it will print all the digits separately and it always prints 1 no matter what and I'm wondering what I'm doing wrong here...
//initialize array for finding digits
int digit[16];
int oddDigits[8];
//separate all digits as variables in array digits[]
for (int d = 0; d < 16; d++) {
for (long i = 1000000000000000; i > 9; i /= 10) {
digit[d] = ((creditCard / i) % 10);
}
}
//double the value of every other digit as oddDigits[]
for (int n = 0; n < 8; n++) {
for (int t = 0; t < 15; t += 2) {
oddDigits[n] = digit[t]*2;
}
}

Answer 1

While you are separating all digits as variables in array digits[], the for loop runs till.
digit[14] = ((creditCard / 10) % 10);
to fix this change it as
for (long i = 1000000000000000; i > 0; i /= 10)
{
digit[d] = ((creditCard / i) % 10);
}
ie change 9 to 0. in this way, the last iteration of the loop will run
digit[15] = ((creditCard/1) % 10);
which is same as
digit[15] = (creditCard % 10);
now see, the outer loop is running 16 times. the inner loop is also running 16 times and the inner loop is running only once per iteration of the outer loop. So one of the loops is redundant. remove the outer loop. keep the d variable as 0 and increment it by 1 after dividing the digit. i.e.
int d = 0;
//separate all digits as variables in array digits[]
for (long i = 1000000000000000; i > 9; i /= 10)
{
digit[d] = ((creditCard / i) % 10);
d++;
}
You can also do it like this
long i = 1000000000000000;
for (int d = 0; d < 16; d++)
{
digit[d] = ((creditCard / i) % 10);
i /= 10;
}
Either one will work.
Now this part //double the value of every other digit as oddDigits[]
It can also be done with 1 loop. In your code, the variable t is just double of the variable n in each step. So, you don't need the t variable too. (just replace t by 2*n)
//double the value of every other digit as oddDigits[]
for (int n = 0; n < 8; n++)
{
oddDigits[n] = digit[2*n]*2;
}