Page 155 - Learn To Program With Scratch

The background
of the Stage

The only sprite in
the application
(named Tutor)
Figure 6-9: User interface for the area calculator program

The user interface for this application contains the Stage’s background
image, which shows the available options (the numbers 1, 2, or 3), and the
Tutor sprite, which asks the user for a choice, performs the calculation, and
displays the result. The main script, shown in Figure 6-10, starts when the
green flag icon is clicked.

If the user entered 1, call
Rectangle and, when it
returns, move to the end.

If the user entered 2, call
Triangle and, when it
returns, move to the end.

If the user entered 3,
call Circle and, when it
returns, move to the end.
Else, say “Invalid
choice!” and then
move to the end.

Figure 6-10: The main script of the Tutor sprite

After asking the user to enter a choice, the Tutor sprite waits for the
user’s input and uses three if/else blocks to process it. If the user entered
a valid choice (that is, 1, 2, or 3), the script calls the appropriate procedure
to calculate the area of the indicated shape. Otherwise, the script invokes
the say command to inform the user that the choice entered is invalid.
The procedures for calculating the areas of the three shapes are shown
in Figure 6-11.

Making Decisions 133

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