# Checking a string for a valid ISBN number

### Checking a string for a valid ISBN number

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Continuing the series of "things you may have done in college" this next one is all about array manipulation. Again it's one I've heard of being used in a whiteboard/tech interview situation (so worth brushing up on!). The scenario here is just the creation of a method which:

1. Accepts a string
2. Checks to see if that string is actually a valid 10 digit ISBN number format  (valid format - an actually valid ISBN no. is one which has been issued to a published book)

The only other stipulation is to do it in the first instance without using the built-in .NET string methods. So it would be quicker to use String.Split('-') etc... but the idea here is to test the basic knowledge, so the first effort here uses just standard array walking loops. I may add a more modern implementation later, but as  previously stated I would love if these post generated some comments, and perhaps other implementation styles in those comments.

I took the "business logic", i.e. the 10 digit ISBN rules from here: http://www.isbn.org/standards/home/isbn/international/html/usm4.htm

This is the first hastily written version:

`` private static bool IsTenDigitISBN(string isbnString) { if (string.IsNullOrWhiteSpace(isbnString)) { return false; }  char[] isbnArray = isbnString.ToArray();  if (isbnArray[0] == '-' || isbnArray[isbnArray.Length - 1] == '-') { return false; }  if (isbnArray.Length != 10 && isbnArray.Length != 13) { return false; }  int dashCount = 0; int sum = 0; int digit=0;  //now replace the '-' chars in pos 0-9 for (int i = 0; i < isbnArray.Length-1; i++) { if (isbnArray[i] == '-') { dashCount++; isbnArray[i]=isbnArray[i--+1]; for (int x = i + 1; x < isbnArray.Length-1; x++) { isbnArray[x] = isbnArray[x + 1]; } } } if (dashCount != 0 && dashCount != 3) { return false; }  //if array is 13 long, we must have 3 dashes, 10 long must have no dashes if ((isbnArray.Length == 13 && dashCount !=3) && (isbnArray.Length == 10 && dashCount !=0)) { return false; }  for (int i=0; i < 9; i++) { if (ToDigit(isbnArray[i], ref digit)) { int multiplier = 10 - i; int val=digit*multiplier; sum += val; } else { return false; } }  //now add the check digit (or 10 if it is an 'X' character) if (isbnArray[9] == 'X' || isbnArray[9] == 'x') { sum += 10; } else { if (ToDigit(isbnArray[9], ref digit)) { sum += digit; } else { return false; } }  //Sum of digits + check digit should have a remainder of 0 when divided by 11 int check = sum % 11;  if (check == 0) { return true; } else { return false; } }``

As you can see there is one other small method called within the check method - the ToDigit() method which converts a char to an int (if the char is one of '0' - '9'). It is shown here:

`` /// <summary> /// An "atoi()" style function for single characters /// </summary> /// <param name="ch">character which should represent a digit</param> /// <param name="digit">pass by ref int used to return the value</param> /// <returns></returns> private static bool ToDigit(char ch, ref int digit) { char[] digits = { '0', '1', '2', '3', '4', '5', '6', '7', '8', '9' };`` bool found=false;  for (int i = 0; i < digits.Length; i++) { if (ch==digits[i]) { digit=i; found=true; } }  if (found) { return true; }``  return false; }``

So that's it - comments welcome. Questions which could be asked around this include the standard "which unit tests would you write". Here's some examples of strings to pass:

Valid - return true:

IsTenDigitISBN("90-70002-34-5"));
IsTenDigitISBN("0-8436-1072-7"));
IsTenDigitISBN("1590593006"));

Invalid - return false:

IsTenDigitISBN("-"));
IsTenDigitISBN("--1234556789--"));
IsTenDigitISBN("123-45-567-8-9"));

Other questions to ask then would be perhaps:

• How to improve it/rewrite it using .NET string methods.
• How to check for a 13 digit ISBN also - in the same method?
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