The if () then Block

You can find the if () then block inside the Control Block Palette. It looks like Figure 3-4.

The if () then block is a C-block, so you can nest other blocks inside of it. But it also has a hexagon-shaped space for a Boolean block between the words if and then. This is where you put a block that responds with a True or False answer.

For example, if you want to write a program that plays a sound whenever you press the space bar, you can write that with an if () then block, as shown in Figure 3-5.

The program in Figure 3-5 contains a forever loop that runs when you click the Green Flag. The loop checks over and over to see whether you’ve pressed the space bar. If so, it plays the “meow” sound. If you never press the space bar, the meow sound never plays.

What if you want the program to do something else when the space bar isn’t being pressed? This is where the if () then, else block, which is covered in the next section, comes into play.

The if () then, else Block

The if () then, else block works much the same as an if () then block. The if is () then, else block has a second C-block within it that you can add blocks to that run when the blocks in the if section don’t run.

Figure 3-6 shows a program that waits for you to press the space bar. If you don’t press the space bar, Scratch turns clockwise 15 degrees.

You can do so many interesting things with if () then, else blocks by nesting other if () then and if () then, else blocks within them. For example, if you want to play the meow sound when the space bar is pressed, and play a drum when the up-arrow key is pressed, you can nest another if () then inside of the else C-block, as shown in Figure 3-7.

Things start to get pretty wild (and maybe a little confusing) when you have multiple levels of nesting inside of if () then and if () then, else blocks. But with practice, you can create some pretty cool things! Figure 3-8 shows how you can use nested if () then, else blocks to create a small drum set on your keyboard.

The program in Figure 3-8 checks whether you’re pressing any of the arrow keys and plays a different drum for each of them.

Boolean Blocks

Any block that can be either true or false is called a Boolean block. The key pressed block is an example of a Boolean block. The key is either pressed (true) or not pressed (false). There’s no “sort of” when it comes to Boolean blocks.

Scratch’s Boolean blocks are kept in two block palettes: the Sensing Block Palette and the Operators Block Palette. You can identify Boolean blocks because they’re always hexagon-shaped.

Figure 3-9 shows all the hexagon-shaped blocks in Scratch.

There are only 11 hexagon-shaped (Boolean) blocks, but all of them, with the exception of the mouse down? block, contain many different possibilities for determining true or false values in your programs.

You can use only Boolean blocks inside Control blocks that have hexagon spaces. Here’s what each Boolean block does:

• touching ()? block. The touching ()? block is true if the sprite is touching whatever you select from its drop-down menu. You can select any other sprite in the program, as well as mouse-pointer and edge. When you select edge, the touching ()? block is true if the sprite is touching the edge of the Stage. When you select mouse-pointer the touching ()? block is true when the mouse pointer is touching the sprite.
• touching color ()?. The touching color ()? block is true if the sprite is touching any object with the color you provide in its color menu.
• color () touching color ()?. The color () touching color ()? block is true if the first color (which is within the sprite) is touching the second color, which may be part of the background or part of another sprite.
• key () pressed?. The key () pressed? block is true when the selected key is pressed.
• mouse down?. The mouse down? block is true when the mouse button is pressed.
• () < (). The () < () block has two spaces for values. It’s true when the first value is less than the second one. For example, if you put 10 in the first box and 30 in the second box, then the less than block is true. If you put 30 in the first box and 10 in the second box, the less than block is false.
• () = (). The () = () block is true if the value on the left is the same as the value on the right.
• () > (). The () > () block is true if the value on the left is greater than the value on the right.
• () and (). The () and () block compares the results of two Boolean Blocks and is true if both of the Boolean blocks it contains are true.
• () or (). The () or () block compares the results of two Boolean blocks and is true if one or both of the Boolean blocks is true.
• not (). The not () block reverses whatever Boolean blocks it contains.

Figure 3-10 shows an example use of the not () block. Is the result of the not () block true or false?

If you said that the result of the not () block in Figure 3-10 is true, you’re right! Because “apples” is not equal to “oranges,” the not () block reports back with a true.

Standalone Comments

Standalone comments are comments that you add to your script but that aren’t connected to any blocks or code in particular. Figure 3-11 shows an example of a standalone comment that we added to the keyboard drum set program.

Standalone comments are great for describing the whole program, as in Figure 3-11. You can also use them to give instructions on how to use the program, or how to play a game, or to remind yourself about improvements that you want to make to a program in the future.

To make a comment, right-click in the Scripts Area and select Add Comment. A new yellow note appears. You can drag a comment anywhere in the Scripts Area, and you can resize it by dragging its lower-right corner.

If you add several comments to your program, your Scripts Area can start to look cluttered. Fortunately, you can make comments smaller by clicking on the arrow in the top-left corner of the comment. Clicking the arrow on a comment “collapses” it so that it takes up less space, but you need to click the arrow again if you want to read it.

Figure 3-12 shows what a collapsed comment looks like.

Block Comments

You can also attach comments to blocks in the Scripts Area. When a comment is connected to a block, it’s called a block comment.

Block comments are useful for explaining what a particular block or group of blocks does. For example, Figure 3-13 shows the drum set program with block comments added to show at a glance what each part does.

Looping Forever

The forever loop is the simplest type of loop. To use it, just drag it to your Scripts Area and put some blocks inside it to make those blocks loop forever until you stop it. There are several ways to stop a forever loop:

• Press the Stop Sign next to the Green Flag.
• Close the browser window where the program is running, or simply go to a different web page.
• Turn off your computer.
• Use the stop () block.

Stopping Loops with the stop () Block

The stop () block, shown in Figure 3-14, stops the running of a script, or of multiple scripts.

Notice that the stop () block has a flat edge on the bottom. Because its purpose is to end the script, it doesn’t make any sense to put anything after it. So, the flat edge indicates that it’s not possible to attach other blocks to its bottom.

The drop-down menu in a stop () block lets you choose whether the stop () block stops all the scripts for the current sprite, just part of it, or all the scripts running in the current sprite.

Creating a Counting Loop

Counting loops repeat the code within them a certain number of times. They’re created by using the repeat () block. Figure 3-15 shows what a repeat () block looks like.

Notice that the repeat () block has an oval in it. You can put a number into the oval, or you can put any of the oval-shaped blocks inside it. Figure 3-16 shows the use of an oval-shaped block inside of a repeat () block.

The program in Figure 3-16 causes Scratch the Cat to ask the users how many times they want the loop to run. It then uses that number to loop the program that number of times! Pretty cool, eh?

Looping Until a Condition Is Met

The final type of loop is called a conditional loop. It’s created using the repeat until () block, which is shown in Figure 3-17.

The repeat until () block uses a Boolean block inside of it to determine whether it should stop looping.

Figure 3-18 shows a program that creates a loop that makes Scratch the Cat move and say “Going for a walk!” until she runs into the edge of the Stage.

Waiting

When loops run too fast, or when you want something to wait for something else to happen in your program, you can use the wait blocks. Scratch has two wait blocks:

• wait () secs. Waits a specified number of seconds (or a fraction of a second).
• wait until (). Waits until a condition is true.

Both wait blocks cause the running of the current script to sit there and do nothing for a while. We might hate waiting, but Scratch doesn’t mind a bit. Situations when you might want a script to wait include the following:

• To create a pause between movements
• To have a sprite in a game wait until a key is pressed before proceeding with what it was doing
• To pause a script whenever a “pause” button is clicked in a music-player script

Can you think of other times when you might want to pause a script?

Setting the Stage

Our scene begins in a peaceful stream in the forest. But, all we have now is a polar bear in a snowstorm. Follow these steps to set the backdrop.

1. Click the first icon in the row of icons under New Backdrop in the lower-left corner of the Project Editor.

   The Backdrop Library opens.

2. Click Outdoors in the left menu of the Backdrop Library and find the backdrop named Water and Rocks.

3. Click the Water and Rocks backdrop and click the OK button to set it as a backdrop for the current project.

   Your Stage should now look like Figure 3-20.

Placing the Frog

It’s time to bring in the star of our game—the frog. Follow these steps to add a frog sprite:

1. Click the first icon in the New Sprite menu to open the Sprite Library.

2. Locate the Frog sprite, click it, and click OK.

   The frog is added to the center of the Stage.

3. Drag the frog to position it on a nice fly-catching rock in the stream, like that one toward the left.

That’s all for the frog! All the action in this game takes place through the hippo-fly sprite and through a sprite that you create for the frog’s tongue.

Adding the Hippo-Fly

Now it’s time for the hippo-fly. Follow these steps to add him to the Stage.

1. Open the Sprite Library and locate the flying hippo in the Animals category.
2. Click the hippo and click OK to add it to the Stage.
3. Click the Shrink tool in the toolbar and use it to make the hippo smaller. (A smaller hippo-fly will make the game more of a challenge.)

That’s all there is to it! You now have the two main characters on set and ready to go, as shown in Figure 3-21. Next up, you add some action!

Scripting the Hippo-Fly

The hippo-fly’s only job in this game is to fly around as fast as he can, in random directions. Here’s how to make him do that:

1. Click on the hippo in the Sprite Pane.
2. Drag the when green flag clicked block to the Scripts Area.
3. Drag a forever loop to the Scripts Area and snap it to the when green flag clicked block.
4. Inside the forever loop, drag a move () steps block.
5. Change the value in the move () steps block to 30.
6. Drag a turn clockwise () degrees block to the Scripts Area and snap it to the bottom of the move () steps block.
7. Click into the Operators Block Palette and drag a pick random () to () block into the turn clockwise () degrees block.
8. Set the values in the pick random () to () block to 1 and 10.
9. Drag an if on edge, bounce block to the Scripts Area and snap it to the turn clockwise () degrees block.

Your finished script for the hippo-fly should look like Figure 3-22. Check your work carefully before moving on.

Making the Tongue

You may have noticed that the frog’s tongue is always sticking out. It wouldn’t be much of a game if you programmed it to just wait until the hippo-fly happens to run into the tongue. To add an element of skill to the game, you’ll give the frog an extra-long tongue that shows up only when you press the space bar.

Follow these steps to create the tongue and to make it appear when the space bar is pressed.

1. Click the icon that looks like a paintbrush in the New Sprite toolbar.

   The Paint Editor opens.

2. Change the color for the brush to red by clicking red in the Color Palette at the bottom of the Paint Editor.
3. Change the width of the paintbrush to something around the same width as the frog’s existing tongue, by sliding the Brush Size Bar.

4. Start at the edge of the canvas and draw a curvy tongue, as shown in Figure 3-23.

   The tongue appears on the Stage as you draw it.

5. Click and drag the tongue on the Stage to move it into position so that it covers up the existing tongue on the frog.

   You may need to redraw or tweak your new tongue so that it fits.

6. Click the i icon on the tongue sprite to open the Sprite Info Pane.

7. Change the name of the sprite to Frog Tongue, and uncheck the box next to the word Show.

   The tongue disappears from the Stage.

8. Switch to the Scripts Area by clicking the Scripts tab at the top of the Project Editor.
9. With the tongue selected in the Sprite Pane, find the when key () pressed block in the Events Block Palette and drag it to the Scripts Area.
10. Make sure that space is selected in the when key () pressed block.
11. Drag the show block from the Looks Block Palette and snap it to the when () key pressed block.
12. Drag an if () then, else block to the Scripts Area and snap it to the bottom of the show block.
13. Drag the touching () block from the Sensing Block Palette and drag it into the if () then, else block.
14. Change the value in the touching () block to the name of the hippo sprite.
15. Drag a say () block from the Looks Block Palette to inside the if part of the if () then, else block.
16. Drag another say () block from the Looks Block Palette to inside the else part of the if () then, else block.
17. Change the words that the frog says when he catches or misses the fly to whatever you want him to say.

18. Drag a hide block from the Looks Block Palette to the bottom of the if () then, else block.

    This makes the tongue go away after the fly is caught or not caught.

The finished frog tongue script should look like Figure 3-24.

Now it’s time to try it out! Click the Green Flag and see if everything works as it should.