An example will clarify this. Suppose your number is 536,891. The digits in the odd positions are 5, 6, and 9. Their sum is 20. The digits in the even positions are 3, 8, and 1. Multiplying each by 2 gives 6, 16, and 1. The second of these (16) has two digits, so those digits are added together. This gives 6, 7, and 1. The sum of these is 14. Adding that to the 20 gives 34. The units digit is 4. The 10's complement of 4 is 6 and that is the check digit.

The final step was described in one set of notes as subtracting the total from the next higher multiple of 10. In this example that multiple would be 40 and 40 minus 34 yields the check digit of 6. I guess those notes were from pre-New Math days.

    for (i = 0; i < 10; i++)
        {
        array[i] = i;
        }

    for (i = 0; i < 10;)
        {
        array[i] = i;
        i++;
        }

Peter Cole
Duncanville, TX

Mostly because I was trying to make the point about when the ++ operation is actually performed. The original code was

    for (i = 0; i < 10; )
      {
      array[i++] = i;
      }