Need Help

**Joes108** · 10-22-2002

Hi I need help starting this program
The program check to see if a credit card is valid or not
the number is 16 digits long
EX: Card Number 1234567890123456

Code:

1. Double the value of alternating digits of the credit card number
1  2   3   4    5   6   7    8   9    0   1   2    3   4     5   6
   x2       x2       x2       x2        x2      x2       x2       x2
--------------------------------------------------------------------
1  4   3   8    5   12   7  16  9   0   1   4    3    8    5   12

2. Add all the digits
1 + 4 + 3 + 8 + 5 + 12 + 7 + 16 + 9 + 0 + 1 + 4 + 3 + 8 + 5 + 12 =98

3. Divide the sum by 10
98/10=9.8

If the number is evenly divisible then it is valid
If it is not evenly divisible than it is invalud

thanks for the Help Joe

**Cshot** · 10-22-2002

Post some code attempts and we'll be glad to help.

**Joes108** · 10-22-2002

I want to enter it in as a string then convert it to a integer but how do I do that

**Magos** · 10-22-2002

Traverse the string, then add every element in it to the total sum. Note that '0' == 48, '1' == 49, '2' == 50, etc...

Code:

#define FirstDigit 48

int SumNumbersInString(char* String)
{
   int Sum = 0;
   if(String != NULL)
   {
      for(int i=0; i<strlen(String); i++)
      {
         Sum += (String[i] - FirstDigit);
      }
   }
   return Sum;
}

**ammar** · 10-22-2002

Do you mean a char * or an ansi string?

**Cshot** · 10-22-2002

Never mind

Magos did the char * way in case you're wondering.

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