++grinder, an excellent point.

My laziness in not optimizing it further has been revealed.

As for counting by twos, I certainly could claim that my background gets the credit (I was a mathematics major), but I have to confess that many years ago I was the victim of a similar faux pas.

I was taking a Pascal course in college (my first structure programming language -- best language ever -- until I learned "C" a few years later).  The term project I had chosen was to create a program that generated patterns from prime numbers.  I wrote a similar is_prime subroutine which counted by ones, and a friend who looked at my code said "you realize of course...".  Whoops!