2.3 Scr0130: sequence, selection, and loop  (Page 5/9)

Program code for the LeftBeachball

The code that I wrote (by dragging blocks into the center pane) for the sprite named LeftBeachball is shown in Image 6 .

Figure

Image 6. program code for the left beach ball.

You are already familiar with the tan block with the green flag shown in Image 6 because you learned about it in a previous module. However, theblue block in Image 6 has not been used prior to this module.

Adding the blue go to block to the program

The blue block shown in Image 6 was added to the program module by:

• Clicking the dark blue button labeled Motion in the upper left of Image 4 . (See Image 13 for all of the programming blocks belonging to the Motion group.)
• Dragging the blue block shown in Image 6 from the left panel to the center panel and clicking it into place under the tan block.
• Typing the literal values -200 and 0 into the two white boxes on the blue block.

Coordinate values

If you move the mouse pointer around in the user interface, the coordinates of the mouse pointer are displayed at the rightend of the gray area immediately below the Stage as shown in Image 4 . The x coordinates range from -240 at the left to +240 at the right. The ycoordinates range from +180 at the top to -180 at the bottom.

Behavior of the LeftBeachball

The behavior of the program module shown in Image 6 can be interpreted as follows: When the user clicks the green flag, cause the sprite named LeftBeachball to move to a location with an x (horizontal) coordinate value of -200 and a y (vertical) coordinate value of 0. Since the origin is at the center of the Stage, this causes the beach ball to move to theleft of the origin on the horizontal axis.

Some experimentation was required

Because I didn't know the diameter of the beach ball, I had to experiment to determine how far to move it to the left of the origin to locate it at the leftside of the Stage as shown in Image 4 . I settled on a value of -200 for the x coordinate and a value of 0 for the y coordinate.

Positioning the other beach ball

In the interest of brevity, I won't show the code required to position the beach ball on the right side of the Stage. I did exactly the same thing for that beach ballexcept that I specified the value of the x coordinate to be 200 instead of - 200. This causes the beach ball named RightBeachball to move to the right side of the Stage when the user clicks the green flag.

The basketball

Initializing the position of the basketball

Image 7 shows a portion of the center pane after clicking on the sprite named Basketball .

Figure

Image 7. initializing the position and orientation of the basketball.

You will note that the tan block and the uppermost blue block in Image 7 arethe same as in Image 6 except that the x coordinate value is set to 0. This causes the basketball to move to the origin when the user clicks the greenflag.

Initializing the orientation of the basketball

However, Image 7 contains a block that is not contained in Image 6 . The bottom blue block in Image 7 is used to set the orientation of the basketball. (By orientation, I mean the direction that the basketball is facing.)