Page 152 - Learn To Program With Scratch

starship is capable of firing missiles. When the starship gets hit by a certain
amount of enemy fire, you’d set the canFire flag to 0 to indicate that the
attack system has become dysfunctional; at that point, pressing the space-
bar won’t fire any more missiles.
Although you can name your flags anything you want, I recommend
using names that reflect their true/false nature. Table 6-3 shows some
examples of flags you might use in the space shooter game.

Table 6-3: Some Examples of Using Flags

Example Meaning and Possible Course of Action
Game has not started yet . Ignore all keyboard inputs .

Game has started . Start processing user input .

Game is not over yet . Show remaining time .

Game is over . Hide the remaining time display .

The starship is not hit by enemy’s fire . Alarm sound is off .

The starship has been hit by a missile . Play the alarm sound .

Now that you know how to use the if block and flags, let’s talk about
another conditional block, one that will let you execute one block of code
when a certain condition is true and another if that condition is false.

The if/else Block
Imagine that you are creating a game to teach basic math to elementary stu-
dents. The game presents an addition problem and then asks the student to
enter an answer. The student should receive one point for a correct answer
and lose one point for an incorrect answer. You can perform this task using
two if statements:

If the answer is correct, add one point to score
If the answer is incorrect, subtract one point from score
You could also simplify this logic—and make the code more efficient—
by combining the two if statements into one if/else statement as follows:

If the answer is correct
add one point to score
Else
subtract one point from score

130 Chapter 6
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