Creating HP 38G aplets - Product Development

Hewlett-Packard Journal,  June, 1996  by James A. Donnelly

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In designing an aplet for the HP 38% calculator, it's important to remember two guiding principles of the HP 38G design. First, the design recognizes that in a classroom setting there is (and should be) a large discrepancy between the amount of information entered into a graphing calculator and the amount of information produced by the calculator. We sought to minimize the calculator input required of the student and the teacher and maximize the returned information. Secondly, we sought to exclude irrelevaht possibilities. By reducing irrelevant choices you focus the user's attention on the subject of interest and avoid distractions from things that don't matter. Many choices made during the design of the HP 38G were based on this goal.

Aplets contain information and have views. The information in an aplet consists of every major piece required to produce the views: the equations, setup information, mode information, sketch or text annotations, and attached libraries or programs. In this article we'll explore a simple aplet, then look at an aplet called Polysides and examine how it was constructed.

Using Built-in Aplets

When the HP 38G is first turned on, the built-in aplets are empty. There's no equation, note, or sketch. When the user adds this information, these aplets come alive. You can save an aplet at any time, so it's easy to start one project, change in midstream to another, then come back to the first. (Because the appearance of the screens depends on the information you enter, the screens on your calculator may look different from the example screens in this article.

A simple way to illustrate the aplet concept is to explore the equation SIN(X2)/X. Select the function aplet, press SYMB, and enter the equation.

At this point, you have an aplet that's completely dedicated to your interest in the function SIN(X2)/X, and the aplet can be saved under a unique name or transmitted to another HP 38G or a computer. When the aplet is restarted on the original or another HP 38G, aH modes and scales are set just the way they were saved.

Exploring the PolySides Aplet

The PolySides aplet is designed to explore how a regular polygon can approximate a circle as the number of sides increases. PolySides can be loaded from a disk or another HP 38G in a single operation from the LIB catalog. Once loaded, PolySides appears in the aplet library.

We are now just a single keystroke away from exploying the aplet. To begin the exploration, press START. The motivation for the lesson is displayed first.

After the introduction has been read and a key pressed, the next step is to enter the radius of the circle to be approximated.

After the radius has been entered, the properties of the circle are displayed.

After the circle properties have been viewed, the note view of the aplet is displayed. The PAGE menu key switches between the pages of the note.

To see a sketch of the problem, press SKETCH.

So far we have seen a pretty complete summary of the lesson with less than a dozen keystrokes. Now we can begin to explore how may sides are needed to approximate a circle.

Our central point of departure for Polysides is VIEWS (as mentioned in the note view).

These choices let you determine what aspect of a polygon approximation to a circle you'd like to explore. To explore how the perimeter of the polygon approximates the circle, move the highlight down to Perimeter and press OK. The plottable view is presented automatically.

This is one of the HP 38Gs split views, showing the plot and numeric views of the perimeter approximation at the same time. The variable X represents the number of sides, and the equation stored in F2 returns the perimeter as a function of X. Pressing the left or right arrowkey moves the cursors for the plot and numeric views simultaneously. When the cursor is at the left edge of the plot, the table on the right shows the rapid change of the perimeter from a triangle to a nonagon.

To see the equation, just press SYMB.

The symbolic view is where aplet equations are stored. The check mark beside F2 indicates that when a plot or numeric view is displayed, F2 will be used. Another view of the equation is available by pressing SHOW.

Notice that after the introduction there is no three order of events. You can change to any of the views at any time or press HOME at any time to do a short calculation. Part of the design philosophy of aplets is to let the student explore the lesson at will, without following any particular algorithm. Once you've explored the approximations to the perimeter and area, and perhaps explored the effect on side lengths, you may decide that a polygon with 57 sides yields a fair approximation to the circumference of a unit circle.

Now you can look at a summary of a 57-sided polygon by selecting Polygon Props (for polygon properties) from the VIEWS choices.

Press OK to select this choice. Since the cursor was last on X = 57, the number of sides defaults to 57.

After the user accepts (or alters) the number of sides, the polygon data is displayed.