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I have been working on this code for a while and I can't seem to get to work right. I have been trying to make a credit card validator program in c for a class assignment. Any help would be welcome.

```
#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
void checkCard( long x[16], long *s );
int main ()
{
//define variables for the data
long *s;
long x[16], i;
malloc(sizeof(x[16]));
//ask user to enter a credit card number as a long int. Number must have between 13-16 digits. It must start with 4 for Visa cards,
//5 for Master cards, 37 for American Express cards, or 6 for Discover cards
//store the data that the user entered
//call to array containing the credit card check function
//print for the user whether the credit card number is valid or invalid
printf( "Enter a credit card number as a long integer: \n" );
scanf( "%ld", x );
checkCard( x, s );
return 0;
}
void checkCard( long x[16], long *s )
{
long S1, S2, Sum;
//Step 1: Double every second digit from right to left. If doubling of a digit results in a two-digit number, add up the two digits to get a single digit number.
x[14] = x[14] * 2;
if ( x[14] > 9 ) {
x[14] = x[14] - 9;
}
x[12] = x[12] * 2;
if ( x[12] > 9 ) {
x[12] = x[12] - 9;
}
x[10] = x[10] * 2;
if ( x[10] > 9 ) {
x[10] = x[10] - 9;
}
x[8] = x[8] * 2;
if ( x[8] > 9 ) {
x[8] = x[8] - 9;
}
x[6] = x[6] * 2;
if ( x[6] > 9 ) {
x[6] = x[6] - 9;
}
x[4] = x[4] * 2;
if ( x[4] > 9 ) {
x[4] = x[4] - 9;
}
x[2] = x[2] * 2;
if ( x[2] > 9 ) {
x[2] = x[2] - 9;
}
x[0] = x[0] * 2;
if ( x[0] > 9 ) {
x[0] = x[0] - 9;
}
//Step 2: Now add all single-digit numbers from Step 1.
S1 = x[14] + x[12] + x[10] + x[8] + x[6] + x[4] + x[2] + x[0];
//Step 3: Add all digits in the odd places from right to left in the card number.
S2 = x[15] + x[13] + x[11] + x[9] + x[7] + x[5] + x[3] + x[1];
//Step 4: Sum the results from Step 2 and Step 3.
Sum = S1 + S2 - 48;
//Step 5: If the result from Step 4 is divisible by 10, the card number is valid; otherwise, it is invalid.
if ( Sum % 10 == 0 ) {
printf( "%ld is valid.\n", *x );
} else {
printf( "%ld is invalid.\n", *x );
}
exit(main());
}
```