1.5 Scr0350: relational operators in scratch 2.0  (Page 2/6)

Operator categories

There are several different kinds of operators. The easiest way to study them is to divide them into categories such as the following

• arithmetic
• relational
• logical
• bitwise
• assignment

An earlier module explained arithmetic operators. This module will explain relational operators. Future modules will explain the other kinds of operators.

A previous module introduced you to the selection structure. This module will expand on that concept.

Preview

In this module, I will present and explain a Scratch program named Relational01 . This program illustrates the use of the following relational operators :

The program creates five variables with the names listed below along with a button and displays them on the Stage as shown in Image 1 .

• LeftOperand - a slider
• RightOperand - a slider
• LessThan
• Equals
• GreaterThan

Figure

Image 1. project relational01 at startup.

Image 1 is what you should see on the left side of your screen when you navigate to the project (see Resources ) and click the large green flag. Note the values showing in each of the orange boxes.I will have more to say about them later.

Image 2 shows the result of moving the thumb on the top slider one click to the left (move to -1) and then clicking the button at the bottom. Once again, note the values showing in the orange boxes, which I will have more to say about later.

Figure

Image 2. project relational01 output.

Image 3 shows a reduced view of the programming interface immediately after having clicked the green flag. Although it is not easy to see, thevariables on the stage match Image 1 .

Figure

Image 3. reduced screen shot of program relational01 programming interface.

Program operation

When the user clicks the green flag, the values of the slider variables shown in Image 1 are set to 0. Expressions containing relational operators are evaluated to set the values of the bottom three variables to theboolean values of true or false.

The user slides the two sliders to change the values of LeftOperand and RightOperand .

When the user clicks the button at the bottom of Image 1 , three separate event handlers on the button test the left operand against the right operand for less than , equal to , and greater than and display string values describing the results in the three variables having the corresponding names in Image 1 and Image 2 . Those three event handlers are shown as the bottom three scripts in the programming panel of Image 3 .

Discussion and sample code

A button and five variables

As you can immediately below the stage in Image 3 , a button was added to this program. If you examine the toolbox and the stage in Image 3 , you will see that five variables were created for the program and that all fiveof the variables were displayed in the Stage. (The checkboxes for all five variables were checked causing them to appear in the Stage.) A better view of the Stage is provided in Image 1 .