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guys i have this assignment about luhns algorithm and i cant seem to know where to start (i am very weak in functions and arrays) , if anyone can help me out with the start of the i would be very thankful then later on i will be disscussing my doubts. thank you in advance.

Credit cards use a system of blocked numbers similar to the ISBN. The first digit of a
credit card number is the Major Industry Identifier (MII) and identifies which group
issued the card. The next block of numbers is the issue identifier. Including the MII, the
Issue Identifier is 6 digits long. The account number begins with the 7th digit and ends
with the next-to-last digit. The final digit is the check digit.
The process used to calculate the check digit is the Luhn algorithm1
(mod 10
algorithm), named after IBM scientist Hans Peter Luhn. The algorithm is in the public
domain and is used to validate Canadian social insurance numbers.
It is designed to protect against accidental errors, such as single transpositions of
adjacent digits although it won’t detect transposition of two-digit sequences. This
algorithm works as follows:
• Double all even-positioned digits when counting from right to left.
• Determine the sum of the undoubled digits from the products (Step 1) and each of
the unaffected (odd-positioned) digits in the original number.
• Verify the account number by determining if the sum from step 2 is a multiple of 10.
For example consider the task of validating the check digit for 5314772685932112.

To be a valid account number, this sum must be evenly divisible by 10. If the check
digit were 7, the result would be congruent to (0 mod 10); but because the check digit
is 2, the sum is not divisible by 10. Therefore, this account number is not valid.
Task
Using the information given, write a program that takes a 16-digit credit card number
as input, computes the checksum, and interprets whether the credit card number is
valid or not.

Create the following three functions (apart from the main function) in your program:
readCC:
Read in the 16-digit credit card number, and return it to main.
The number should be read in as a string and converted to be
stored in an integer array, which is returned from the function.
The function should also validate that the ISBN entered has the
correct number of digits before proceeding with checking its
validity. If it does not have the correct number of digits, provide
an appropriate error message and exit the program.

isValid:
Take the credit card number as input, and check its validity,
outputting an appropriate message to the user. This function
calls the function runLuhn, which returns a checksum. If the
value returned is zero, the number is valid, if non zero, it is not
valid.

runLuhn:

Take the credit card number as input and calculate the checksum
using Luhn’s algorithm, returning its value (i.e. the remainder
obtained when the sum value calculated is divided by 10).
The main function should run the function readCC, followed by the function isValid.
The input for the function read_CC is:
XXXXXXXXXXXXXXXX
where the X represents a digit in the number. Your program can assume numbers with
16 digits.
Your program should use the following three programming structures:
1. functions
2. strings and arrays
3. pass-by-reference

TESTING THE PROGRAM
Your program should work properly on the following data:
MC 5105-1051-0510-5100 Valid
MC 5111-1111-1111-1118 Valid
MC 5555-5555-5555-4444 Valid
VISA 4012-8888-8888-1881 Valid
VISA 4408-0412-3456-7890 Invalid
VISA 4417-1234-5678-9112 Invalid

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Last Post by joe97
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You have here very complete algorithm, but you should show effort of implementing it, even if you do not get everything right. After we can assist you in debugging or point weak points to concentrate in.

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Alright here's a rough code of how i started. some where some mistake has happened but not sure where and i wasnt sure how could i declare the functions im asked to use (readCC, isValid and runLuhn)

#include <stdio.h>

int isvalid(long num);
int sumofdoubleevenplace(long num);
int getdigit(int num);
int sumofoddplace(long num);
int prefixmatched(long num,int d);
int getsize(long d);
int getprefix(long num,int k);

main(){
  long cardnum=0;
  printf("Enter credit card number ");
  scanf("%ld",&cardnum);
  if(isvalid(cardnum)==1)
    printf("Valid card number\n");
else
    printf("Invalid card number\n ");
return 0;
}

int isvalid(long num){
if(((sumofoddplace(num)+sumofdoubleevenplace(num))%10==0)
   && (getsize(num)<=16 && getsize(num)>=13)
   && (prefixmatched(num,4)==1 || prefixmatched(num,5)==1 ||
       prefixmatched(num,6)==1 || prefixmatched(num,37==1)))
    return 1;
else
    return 0;
}

int sumofdoubleevenplace(long num){
int numdigits=getsize(num)-1;
int sum=0,i;
num/=10;
for(i=0;i<numdigits;i+=2){
    sum+=getdigit((int)(2*(num % 10)));
    num/=100;
}
return sum;
}
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