Microsoft Office Tutorials and References
In Depth Information
If you like, you can eliminate the formulas in column B and create a specialized formula that
returns the last name. To do so, substitute the formula in cell B1 for the reference to cell B1 in the
formula. The result is the following array formula:
You must insert parentheses around the formula text copied from cell B1. Without the
parentheses, the formula does not evaluate correctly.
Using a megaformula to determine the
validity of a credit card number
Many people are not aware that you can determine the validity of a credit card number by using
a relatively complex algorithm to analyze the digits of the number. In addition, you can
determine the type of credit card by examining the initial digits and the length of the number. Table
20-4 shows information about four major credit cards.
Table 20-4: Information about Four Major Credit Cards
13 or 16
34 or 37
Validity, as used here, means whether the credit card number itself is a valid number as
determined by the following steps. This technique, of course, cannot determine
whether the number represents an actual credit card account.
You can test the validity of a credit card account number by processing its checksum. All account
numbers used in major credit cards use a Mod 10 check-digit algorithm. The general process is as
1. Add leading zeros to the account number to make the total number of digits equal 16.
2. Beginning with the first digit, double the value of alternate digits of the account number.
If the result is a two-digit number, add the two digits together.
3. Add the eight values generated in Step 2 to the sum of the skipped digits of the original
4. If the sum obtained in Step 3 is evenly divisible by 10, the number is a valid credit card