The HP-28S was a member of Hewlett-Packard's first graphic calculator family. In fact, I hesitate to call this unit a calculator at all; with its 32 kilobytes of memory and sophisticated programming language, it's more like a powerful pocket computer with a calculator keyboard.

The exterior design of this calculator ranks among the least successful of HP's calculator designs, at least in my opinion. Although I haven't had any problems with it yet, the clamshell design is subject to failure, and the flexible conductors connecting the two halves deteriorate over time or with usage. The battery compartment of this unit is simply awful; taking out the three 'N' batteries is a frustrating exercise, doing so quickly enough in order to prevent the calculator from losing its memory contents is nearly impossible.

I have no such complaints when it comes to the functional design, however. The HP-28S was a truly amazing device when it was introduced. It reminded me of that scene from Isaac Asimov's Foundationtrilogy where Hari Seldon uses an electronic device to demonstrate some of the basic conclusions of psychohistory to Gaal Dornick. Asimov may have thought that handheld devices that can do symbolic mathematics belonged to our distant Galactic future, but here I was in 1988 or so, holding one in my very hands...

Needless to say, this marvelous calculator comes with its built-in implementation of the Gamma function. Or, to be precise, its built-in factorial function in fact calculates the Gamma function of x+1 for any real x. A Gamma function program for this calculator is therefore a simple one indeed, consisting only of these three instructions:

« 1 − FACT »

This program, however short, clearly demonstrates the gross difference between the keystroke programming model of earlier programmables and the sophisticated programming language of the HP-28S. To use the program, enter it as shown, hit ENTER, then store the program in a variable ('GAMMA STO). You can now calculate the Gamma function of any value on the stack by clicking the USER key and then clicking the soft key corresponding with the GAMMAlabel.

Of course, nothing prevents you from keying in the Gamma function implementation I provided for the HP-48SX; it works fine on the HP-28S, too.

The HP-28S has an infrared port for printing. The other day, I came across a lucky find: for a grand total of ten Canadian dollars, I picked up a perfectly functional, near-mint condition HP82240A printer at a local antiques store. The printer works well both with the HP-28S and my HP-48SX calculators. I've seen similar units sell for ten times the price!