Page 135 - Learn To Program With Scratch

The sprite switches its costume and then calls the Recalculate proce-
dure to update the volume and surface area calculations. Note that these
scripts use the value of the current costume to determine whether the
sphere has reached its highest or lowest size, thus ensuring valid responses
to the Up and Down buttons. I’ll say more about the if block in the next
chapter, but for now, let’s discuss the sphere’s Recalculate procedure,
shown in Figure 5-29.

Figure 5-29: Recalculate procedure

First, the value of the diameter variable is set according to this formula:

diameter = 1 + 0.25 × (costume number – 1)
Since the costume number ranges from 1 to 9, the corresponding val-
ues of the diameter variable will be 1, 1.25, 1.50, ... , 2.75, 3, which is what we
intended.
The script finds the radius, r, by dividing the diameter by 2. It then cal-
culates the volume and the surface area of the sphere using the formulas
shown in Figure 5-26. The computed values will show up automatically on
the corresponding monitors on the Stage.

try it out 5-5
Open the application and run it . Study the scripts to understand how the applica-
tion works . Add a script to the Sphere sprite so it rotates and changes color as the
application runs . As another exercise, modify the original program to use a single
costume for the Sphere sprite and use the change size by block to change the
sphere’s size . The scaled image won’t look as nice, but otherwise, the application
should perform identically .

Variables 113

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