For this part of the lab, you will complete a small program to check if the input provided by the user is a palindrome. A palindrome is a string that is the same forward and backwards.

There are two main ways for testing if a string is a palindrome:

• Iterate from forward to back, and from back to forward, and check that they are the same.
• Create a copy of the string, in reverse, and check that the copy matches the original.

You will implement both; however, the challenge is working with C strings. The main portion of the program, provided to you, looks like such:

int main(int argc, char * argv[]){

  char str[1024];

  printf("Enter a string:\n");

  scanf("%1023s",str);

  if(check1(str)){
    printf("Palindrome according to check 1\n");
  }else{
    printf("NOT a palindrome according to check 1\n");
  }


  if(check2(str)){
    printf("Palindrome according to check 2\n");
  }else{
    printf("NOT a palindrome according to check 2\n");
  }

}

Note that the input is read into the str string and may be substantially shorter than the length 1024. It is also passed to the check1() and check2() functions without a length argument. You should make sure that you use strlen() somewhere.

Additionally, to complete check2() which requires making a copy of the string; don't forget that C strings must be NULL terminated. If you do not NULL terminate, strcmp() will not work.

aviv@saddleback: part1 $ ./palindrome 
Enter a string:
racecar
Palindrome according to check 1
Palindrome according to check 2
aviv@saddleback: part1 $ ./palindrome 
Enter a string:
madamimadam
Palindrome according to check 1
Palindrome according to check 2
aviv@saddleback: part1 $ ./palindrome 
Enter a string:
amanaplanacanalpanama
Palindrome according to check 1
Palindrome according to check 2
aviv@saddleback: part1 $ ./palindrome 
Enter a string:
notapalindrome
NOT a palindrome according to check 1
NOT a palindrome according to check 2
aviv@saddleback: part1 $

For this part of the lab, you will implement a checksum for credit card numbers. A checksum is a method to verify that a number meets certain properties. It is an important principle in networking and other areas, and you will implement the checksum used to check if a credit card is valid. Note that just because a number passes the checksum, it doesn't make it a truly valid credit card number, it just means that it may be used as a credit card number.

Credit card numbers are checked using Luhn's Algorithm. You can find a description on wikipedia, but it is highlighted below. It occurs in three basic steps:

• For the 16 digit card number, double ever other digit

6  4  7  4  5  6  2  3  8  9  9  7  2  7  5  6  <- number
|     |     |     |     |     |     |     |    
v     v     v     v     v     v     v     v   
12   14    10     4    16    18     4    10    <- double every other digit

• If the double of the digit is greater than 9, that is two digits in length, replace it with the sum of the digits of the product

6  4  7  4  5  6  2  3  8  9  9  7  2  7  5  6  <- number
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 
v  |  v  |  v  |  v  |  v  |  v  |  v  |  v  |
12 | 14  | 10  |  4  | 16  | 18  |  4  | 10  | 
|  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 
v  v  v  v  v  v  v  v  v  v  v  v  v  v  v  v
3  4  5  4  1  6  4  3  7  9  9  7  4  7  1  6  <- sum of digits if >9

• Take the sum of the remaining numbers

3 + 4 + 5 + 4 + 1 + 6 + 4 + 3 + 7 + 9 + 9 + 7 + 4 + 7 + 1 + 6
= 80

• If that number is a multiple of 10, then it passes the checksum.

80%10 = 0 <- PASS

For your program to work, you must take a 16 digit credit card number on the command line and print VALID or INVALID. Like so:

aviv@saddleback: part2 $ ./creditcardcheck 6474562389972756
VALID
aviv@saddleback: part2 $ ./creditcardcheck 6474562389972757
INVALID

Additionally, your program should detect three error conditions: (1) No argument provided; (2) Invalid length of credit card number; (3) Invalid numbers in credit card. Here's an example:

aviv@saddleback: part2 $ ./creditcardcheck 
ERROR: require credit card number
aviv@saddleback: part2 $ ./creditcardcheck 12345
ERROR: Invalid credit card number: Bad Length
aviv@saddleback: part2 $ ./creditcardcheck 12345a7890123456
ERROR: Invalid credit card number: Bad number 'a'

To perform these checks, consider that the command line arguments are a string, and each number is currently a character. The numeric value of that character is not the same as its integer value, but they are still ordered. For example, this will check for a valid digit:

if ( c >= '0' && c <= '9' ) 
  printf("Valid digit!\n");
else
  printf("Invalid digit!\n");

Finally, because the checksum occurs over integers and not strings, you will need to convert each individual digit into a integer. Consider creating an array for the credit card number that can store all these digits:

int ccnum[16]

You can use atoi() to perform the conversion from string to integer, but you'll need to first isolate the character of the number as its own string.

char num[2];
num[0] = '8';
num[1] = '\0'; //null terminate
int i = atoi(num); //convert to integer

aviv@saddleback: part2 $ ./creditcardcheck 6474562389972756
VALID
aviv@saddleback: part2 $ ./creditcardcheck 6474562389972757
INVALID
aviv@saddleback: part2 $ ./creditcardcheck 
ERROR: require credit card number
aviv@saddleback: part2 $ ./creditcardcheck 12345
ERROR: Invalid credit card number: Bad Length
aviv@saddleback: part2 $ ./creditcardcheck 12345a7890123456
ERROR: Invalid credit card number: Bad number 'a'