Page 157 - Learn To Program With Scratch

The and Operator

The and operator takes two expressions as parameters. If both expressions
are true, the and operator returns true; otherwise, it returns false. The
truth table for and, which lists the output of the operator for all possible
combinations of inputs, is shown in Table 6-5.

Table 6-5: Truth Table for the and Operator

X Y
true true true
true false false
false true false
false false false

As an example of using the and operator, let’s say we’re creating a game in
which the player gets 200 bonus points when the score reaches 100 in the
first level. The game level is tracked by a variable named level, and the score
is tracked using a variable named score. Figure 6-12 shows how these condi-
tions can be tested using nested if blocks u or with the and operator v.

 
Figure 6-12: Checking multiple conditions using nested if blocks and the and operator

In both cases, the bonus points are added only when both condi-
tions are true. As you can see, the and operator provides a more concise
way for performing the same test. The command(s) inside the if block in
Figure 6-12 v will be executed only if level equals 1 and score equals 100. If
either condition is false, the entire test evaluates to false, and the change
score by 200 block will not be executed.

The or Operator
The or operator also takes two expressions as parameters. If either expres-
sion is true, the or operator returns true. It returns false only when the two
expressions are both false. The truth table for the or operator is given in
Table 6-6.

Making Decisions 135

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