1.7 Scr0370: the repeat loop in scratch 2.0  (Page 6/7)

The program named ForLoop02

This program is an upgrade of the program named ForLoop01 . The behavior of this program is the same as the earlier program with the followingexceptions:

• A slider variable named numberLoops is placed in the upper left corner of the Stage as shown in Image H .
• When the user clicks the green flag, orthogonal axes are drawn through the origin of the Stage and the cat is placed at the origin.
• When the user clicks the cat, the number of times the code in the repeat block is executed is determined by the current value of the slider variable.
• As the code in the repeat block is repetitively executed, the path of the cat is traced out on the Stage as a series of short straightline segments, producing the circular path shown in Image I .

Figure

Image h. program forloop02 output after clicking the green flag.

Figure

Image i. program forloop02 output after clicking the cat.

The program code

The code for this program is shown in Image J .

Figure

Image j. program code for the program named forloop02.

Create a variable

The code in the left panel in Image J creates a variable with a slider named numberLoops and causes it to be displayed on the Stage as shown in Image I .

The initialization code

Although there is quite a lot of code that is executed when the green flag is clicked, there is nothing new there. All of that code has been explained in thisor earlier modules. For review, the green pen blocks and the blue go to blocks are used together to draw the Cartesian coordinates. Note that the pen is down when the initialization code finishes executing. As a result, a line willbe drawn whenever the cat sprite moves.

The repeat block

The only thing that is really new to this program is the use of the variable block named numberLoops to specify the number of times that the code in the repeat block will be executed each time the cat is clicked. Otherwise the code in the repeat block in Image J is the same as the code in the repeat block shown in Image F .

The operational difference

The operational difference is that each time the repeat block in Image F is executed, the code inside the repeat block will be executed 24 times. Each time the repeat block in Image J is executed, the number of times that the code inside the repeat block will be executed is specified by thecurrent position of the slider in Image I . This, in effect converts the static program named ForLoop01 into an interactive program in which the user has some control over the program behavior at runtime.

As mentioned earlier, because the pen is down, a line is drawn each time the cat sprite moves. That is what draws the circular path shown in Image I .

Online versions of these programs are available

Copies of these two programs have been posted online for your review (see Resources for the URL) . If you don't find the programs using that URL, search the Scratch site for the usernamed dbal.