HP 39gs Master Manual page 278

Still referring to the code on the previous page, you will see that it refers to . The sketches in the
PageNum
calculator's view are numbered 1, 2, 3...etc. Sketch number 1 is always present but after that only
SKETCH
sketches that have been created are available and the program will crash if you try to access one that does
not exist.
The aplet variable is the pointer to the sketch you want and the actual sketch page itself is called
PageNum
. Thus the two lines after are telling the program to store the view into the first page of the
Page ELSE PLOT
view using the command . This command stores the view into whatever graphics
SKETCH PLOTÆ PLOT
variable you specify. In this case into .
Page
The view must exist before this can be done or the program will crash. This is the reason for setting up
PLOT
the flag discussed earlier – by doing that we ensure that this section of code only runs if something has
G
been plotted. If you run the program and then later change to the view you will be able to see this
SKETCH
stored image. Finally, the user is presented with two messages which tell them what to do.
The next section contains the code which performs the work in the aplet by setting up a loop which repeats
until the user presses the key to terminate.
ENTER
The first line before the loop begins assigns initial values to the variables (the gradient) and (the y-
M C
intercept). The loop which follows (partly in the next section of code) loops through the code
DO...UNTIL
within it until the key is pressed.
ENTER
Within the loop, the previously stored view is transferred from storage to the display using
SKETCH
. The command means "transfer to the display screen". The equation of the current
DISPLAY DISPLAY
line is then displayed in the top left corner using the command which allows you to write text onto
DISPXY
the screen. Two versions are needed to avoid an expression like "y=2x+ -1" and write instead "y=2x-1".
278