Page 146 - Learn To Program With Scratch

In many programming situations, however, you may want to alter this
sequential flow of program execution. If you were writing an application to
tutor children in basic arithmetic, you’d want to execute certain blocks to
reward correct answers and a completely different set of blocks for wrong
answers (to reveal the right answer or offer another chance, for example).
Your script can decide what to do next by comparing the student’s input
with the correct answer. This is the basis of all decision-making tasks.
In this chapter, we’ll explore the decision-making commands available
in Scratch and write several programs that use these commands to test
inputs and perform different actions.
First, I’ll introduce you to Scratch’s comparison operators and show
how you can use them to compare numbers, letters, and strings. Then, I’ll
introduce the if and if/else blocks and explain their key role in decision
making. You’ll also learn how to test multiple conditions using nested if and
if/else blocks and write a menu-driven program to put these blocks into
action. After that, I’ll introduce logical operators as an alternative way to
test multiple conditions. In the last section, we’ll write several interesting
programs based on all of the concepts you’ve learned so far.

comparison operators

You make decisions every day, and each decision normally leads you to per-
form certain actions. You may think, for example, “If that car is less than
$2,000, I’ll buy it.” You then ask about the car’s price and decide whether
or not you want to buy it.
You can make decisions in Scratch, too. Using comparison operators,
you can compare the values of two variables or expressions to determine
whether one is greater than, less than, or equal to the other. Comparison
operators are also called relational operators because they test the relation-
ship between two values. The three relational operators supported in
Scratch are shown in Table 6-1.

Table 6-1: Relational Operators in Scratch
Operator Meaning Example

greater than
Is price greater than 2,000?

less than
Is price less than 2,000?

equal to
Is price equal to 2,000?

124 Chapter 6
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