HP 40gs HP 39gs_40gs_Mastering The Graphing Calculator_English_E_F2224-90010.p - Page 278

Still referring to the code on the previous page, you will see that it refers to PageNum. The sketches in the calculator's SKETCH view are numbered 1, 2, 3...etc. Sketch number 1 is always present but after that only 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 PageNum is the pointer to the sketch you want and the actual sketch page itself is called Page. Thus the two lines after ELSE are telling the program to store the PLOT view into the first page of the SKETCH view using the command PLOTÆ. This command stores the PLOT view into whatever graphics variable you specify. In this case into Page. The PLOT view must exist before this can be done or the program will crash. This is the reason for setting up the flag G discussed earlier - by doing that we ensure that this section of code only runs if something has been plotted. If you run the program and then later change to the SKETCH view you will be able to see this 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 ENTER key to terminate. The first line before the loop begins assigns initial values to the variables M (the gradient) and C (the yintercept). The DO...UNTIL loop which follows (partly in the next section of code) loops through the code within it until the ENTER key is pressed. Within the loop, the previously stored SKETCH view is transferred from storage to the display using DISPLAY. The DISPLAY command means "transfer to the display screen". The equation of the current line is then displayed in the top left corner using the DISPXY command which allows you to write text onto the screen. Two versions are needed to avoid an expression like "y=2x+ -1" and write instead "y=2x-1". 278