1.7 Scr0370: the repeat loop in scratch 2.0  (Page 5/7)

Can also use a variable

Note that I also created a dummy variable in Image E for illustration purposes. Because the shape of a variable block is the same as the shape of thewhite box in the repeat block, you can also drop a variable block into the box. When you do this, the value of the variable will be used to determinehow many times the code in the block will be executed. That number may be different each time the repeat block is executed.

The wait block

Image E also shows a wait block, which can be dragged into the center panel from the Control toolbox. Like the repeat block, the wait block has a white box into which you can either enter a literal value or drop a variable block.

The program will pause for a specified number of seconds (which may be a decimal fraction) each time the wait block is executed. If you enter a literal value into the box, the program will pause for the same amountof time each time the wait block is executed. If you drop a variable block into the box, the length of the pause will be the current value stored inthe variable, which may be different each time the block is executed.

The switch costume block

Finally, Image E shows a purple switch to costume block. This block is available in the purple Looks toolbox when a sprite icon is selected immediately below the stage. However, when the Stage icon is selected, theblock changes to one labeled switch to backdrop . (A discussion of backdrops will be deferred to a future module.)

As you can see, the switch to costume block has a pull-down list that allows you to select a specific costume for the block when you write theprogram. The list contains all of the costumes belonging to the selected sprite. In the case of the default cat that appears for new projects, there are two costumes in the pull-down listbecause the cat has two costumes as shown in Image D .

The program code

The code for this program is shown in Image F .

Figure

Image f. program code for the program named forloop01.

Behavior of the program

When the user clicks the green flag, the cat moves to the center of the stage (the origin of the Cartesian coordinate system) and turns to face the viewer's right. In addition, the cat switches to costume1 (see Image D ) even if that isn't necessary at this point. (Except for switching costumes, you have seen code like this in earlier modules.)

The repeat block

Then the code inside the repeat block shown in Image F is executed 24 times in succession.

During each execution (iteration) of the repeat block, the cat:

• Moves 10 steps forward.
• Waits 0.1 second.
• Switches to costume2.
• Rotates around its own origin by 15 degrees.
• Waits an additional 0.1 seconds.
• Switches back to costume 1.

This causes the cat to appear to walk (in a rather jerky fashion) in a circle (see Image G ) , ending up very close to where it started.

Figure

Image g. the cat on the stage.

Run the online version

In order to get the full impact of this program, you will either need to create the program yourself or run the online version (see Resources ) .