2.5 Scr0150: relational operators  (Page 3/6)

The program code

Image 2 shows a full-size view of the center panel in Image 1 . This panel contains the program code associated with the button.

Figure

Image 2. the center programming panel for relational01.

Initialize the variables

The top script in Image 2 initializes the values of all the variables to zero when the user clicks thegreen flag. You have seen code like this several times before in this series of modules, so I won't discuss it further.

When the button is clicked ...

Each of the bottom three scripts in Image 2 are executed when the user clicks the button in the Stage area in Image 1 . These three scripts are very similar. However, they use three different relational operators to compare the values of the left and right operands and they set the values inthree different output variables accordingly.

I will walk you through the construction of one of the scripts with a detailed explanation of each step in the process. You should be able toextend that explanation to the other two scripts.

An if-else block

I'm going to walk you through the process of creating one of the bottom three scripts shown in Image 2 .

Image 3 shows the result of

• Selecting the button sprite in the lower-right panel of Image 1 .
• Selecting Control in the upper-left panel.
• Dragging the block labeled when button clicked into the center panel.
• Dragging an if-else block from the lower-left panel into the center panel.
• Connecting the if-else block to the block that is labeled when button clicked .

By the way, in case I forgot to tell you before, you can set the name of the sprite to button by clicking in the dark gray area in the upper-right of Image 3 and typing the name of the sprite.

Figure

Image 3. starting to use an if-else block.

Need to put a conditional clause in the pocket

Note the depressed pocket immediately to the right of the word if on the if-else block. We must drop another block of the correct shape into this pocket. The code in that block must evaluate to either true orfalse.

If that code evaluates to true, the code that we will insert into the mouth of the block immediately below the word if will be executed.

If the code that we drop into the pocket evaluates to false, the code that we insert into the mouth of the block immediately below the word else will be executed.

Where are the blocks with the correct shape?

There are two buttons (possibly more) that we can click in the upper left in Image 3 that will expose blocks with shapes matching the pocket immediately to the right of the word if in Image 3 :

• Sensing
• Operators

We used a block having the correct shape from the Sensing group in the earlier module titled Scr0130: Sequence, Selection, and Loop . An image of the blocks exposed by the Sensing button is shown in Image 12 in that earlier module.

Will use blocks from the Operators group

In this module, we will use three of the blocks that are exposed by clicking the green Operators button as shown in Image 4 .

Figure

Image 4. green blocks exposed by clicking the operators button.