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My solution to CS50 Hacker pset1 - "Bad Credit"
#include <stdio.h>
#include <cs50.h>
#include <string.h>
#include <ctype.h>
#include <math.h>
// calculates the number of digits in the card number
int getCardDigits(long long card_num)
{
int card_digits = (int)log10(card_num) + 1;
return card_digits;
}
int main(void)
{
// get card number from user
printf("Number: ");
long long card_num = GetLongLong();
// use digits function
int card_digits = getCardDigits(card_num);
// if not a valid credit card length print invalid immediately
if (card_digits != 13 && card_digits != 15 && card_digits != 16)
{
printf("INVALID\n");
return 0;
}
// storing the individual digits in an array
int numberArray[card_digits];
long long count = card_num;
int i = 0;
while (count != 0)
{
numberArray[card_digits - 1 - i] = count % 10;
count = count/10;
i++;
}
// Every other number starting from the second to last in array
int numberArray2[card_digits/2];
int k = 2;
int l = 0;
while ((card_digits - k) > -1)
{
numberArray2[l] = 2 * numberArray[card_digits - k];
k = k + 2;
l++;
}
int digit_times_two = 0;
for (int j = 0; j < card_digits/2; j++)
{
if (numberArray2[j] > 9)
{
int split_digit = 0;
digit_times_two = digit_times_two + (numberArray2[j] % 10);
split_digit = numberArray2[j]/10;
digit_times_two = digit_times_two + split_digit;
}
else
{
digit_times_two = digit_times_two + numberArray2[j];
}
}
// the remainging digits added together after put in 3rd array
int numberArray3[card_digits - (card_digits/2)];
int m = 1;
int n = 0;
while ((card_digits - m) > -1)
{
numberArray3[n] = numberArray[card_digits - m];
m = m + 2;
n++;
}
int other_digit_count = 0;
for (int j = 0; j < (card_digits - (card_digits/2)); j++)
{
other_digit_count = other_digit_count + numberArray3[j];
}
int companyId;
int companyId2;
// validating Luhn algorithm
if ((digit_times_two + other_digit_count) % 10 == 0)
{
companyId = numberArray[0];
companyId2 = numberArray[1];
}
else
{
printf("INVALID\n");
return 0;
}
// checking which card company
if (card_digits == 15 && companyId == 3 && (companyId2 == 4 || companyId2 == 7))
{
printf("AMEX\n");
}
else if (card_digits == 16 && companyId == 5 && (companyId2 >= 1 && companyId2 <= 5))
{
printf("MASTERCARD\n");
}
else if ((card_digits == 13 || card_digits == 16) && companyId == 4)
{
printf("VISA\n");
}
else
{
printf("INVALID\n");
}
return 0;
}
@CraigRodrigues

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Owner Author

commented Jun 3, 2016

STEPS:

  1. Prompt user for credit card number. (long long)
  2. Check length of the card #. If not valid print INVALID.
  3. Function: cardLength to get the number of digits in a card number using Log10 + 1.
  4. Put the digits into an array by using % and a loop and /10 etc.
  5. Loop from the back to the front of the new array of digits and take every other digit.
  6. Multiply the results of 5 by 2.
  7. Add the individual digits together (what do you do about double digits?)
  8. Add to #7 the sum of the remaining digits.
  9. If answer to #8's final digit is a zero then the card number is legit.
  10. Check the card's first digit(s) to get the card company.
  11. If #9 & #10 are true then print out what card company it is and return 0.

NOTES/THOUGHTS:

  • Had to do a bit of research for the questions I had such as how to find out the number of digits in an integer and how to put the individual digits of a number into an array. Once I found those it was just a matter of piecing the puzzle together doing down the list of steps above.
  • One loop I had initialized the counter i inside the loop itself so it actually never incremented. That took a while to see, yet it was such a simple problem!
  • I had to figure out how to reverse a loop to put the digits into it back to front instead of front to back.
  • Put "return 1" in a couple of places when the program really wanted "return 0" regardless.
  • Need to remember that using the data type int only takes the integer portion not anything after the decimal. Useful to remember for this problem set.
@CaptainDaVinci

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commented Sep 14, 2016

Have you encountered segmentation fault while compiling?

@devsarmico

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commented Feb 4, 2017

This seems to be a little complicated for me.
Week 1 on Cs50x, pset 1
I've already done: hello.c, water.c, mario.c, greedy.c
But I'm still trying to understand how to proceed with credit.c

I've read your code and It helped to understand a few things but
I'm getting lost from line 31.

@shedevralen

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commented Jul 21, 2018

I don't understand since 38th line.

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