The HP-11C and HP-15C calculators are, quite simply, the best single-line display calculators ever created. If you don't absolutely need a graphing calculator, they remain the best handheld calculators ever...period. This is true despite the fact that they date back to the 1980s. These HP marvels came with a sumptuous array of built-in functions together with a simple yet powerful method for programming (which allowed you to expand way beyond its built-in features). They also sported a sleek thin design that still surpasses any of the calculators on the market today. Best-of-all, they used RPN (Reverse Polish Notation) for data entry.

RPN, for those of you unfamiliar with this odd-named duck, is the alternative to the algebraic method of data entry. The algebraic method is found on virtually all other competing calculators (most notably the ubiquitous handhelds from Texas Instruments). You know you're working with RPN if you use an Enter key instead of an Equals (=) key. Of course, the differences go beyond that simple distinction. Skipping over details, let me just say that, for anything beyond the most simple calculations, RPN is much easier to use, easier to keep track of what you've entered, and less prone to making mistakes. If you have ever been frustrated by entering a complex expression involving parentheses, for example, RPN will seem like a direct delivery from heaven.

Unfortunately, RPN lost out in the calculator wars. Even Hewlett-Packard largely abandonned it. They still make the HP-12C (a financial real estate calculator variation of the 11C). Beyond that, their calculators have either shifted to algebraic-only or a combined algebraic-RPN system. I am sure the reason for this is that, at first glance, RPN seems quirky and hard-to-learn. Admittedly, it does have a slightly steeper learning curve than algebraic entry. But once you make the minor effort to get over the hump, you are more than rewarded for your efforts. It may not seem like it at first, but RPN's "inside-out" method of working through an expression mimics the way you would do such calculations in your head—much more so than the algebraic method.

I still use my trusty HP-15C. However, I must admit that it spends most of its time in a drawer of my desk. That's because I now do almost all of my calculating on a Mac, via spreadsheets and such. And if I do want an HP-15C while working with my Mac, I simply launch a nifty emulation that's almost as good as the real thing. The only remaining advantage of the "real" 15C is its portability.

This (finally) brings me to the main point of this blog entry. Last week, I was browsing through the list of programs available for my hacked iPhone (as viewed from the Installer utility) and discovered a set of HP calculator emulations (11C, 12C, 15C and 16C). I installed the 15C. It's fantastic! After launching it, the phone becomes an HP-15C. The entire display is filled with an exact image of the calculator. To use it, you just tap the virtual buttons. The net effect is almost identical to pressing the physical buttons on an actual HP-15C. If you ever want to show off the advantages of a touchscreen interface, you'd be hard pressed to find a better example!

In a sense, I now have my HP-15C back in my pocket again. Actually, it's even better. My "new" 15C doesn't have to compete for space with my iPhone. The iPhone is the 15C. Ahhh...the joys of the iPhone...and iPhone hacks!

I only hope that, after the iPhone SDK is released in a few days, the HP-15C is one of the third-party programs that will be "officially" available, so I won't need a hack to get it.