Viewing tip
I recommend that you open another copy of this module in a separate browser window and use the following links to easily find and view the imageswhile you are reading about them.
Images
- Image A . Pseudocode for the program named ForeverLoop01.
- Image B . Program code for the Scratch v1.4 version of the program named ForeverLoop01.
- Image C . Program code for the Scratch v2.0 version of the program named ForeverLoop01.
- Image D . Comparison between repeat and forever/if blocks.
- Image E . Program code for the program named ForeverLoop02.
- Image F . Pseudocode for the program named ForeverLoop02.
- Image G . Pseudocode for the program named RepeatUntil01.
- Image H . Program code for the program named RepeatUntil01.
- Image I . Final screen output from the program named RepeatUntil01.
- Image J . Blocks that can be used to create loop structures in v2.0.
General background information
In an earlier module, I told you that any programming logic problem could be solved using an appropriate combination of only three programming structures,none of which are complicated. The three structures are known generally as:
- The sequence structure.
- The selection or decision structure.
- The loop, repetition, or iteration structure.
I have explained the sequence and selection structures and some of the loop structures in earlier modules. I will continue to concentrate on the loopstructure in this module.
Preview
In this module, I will present and explain the following three Scratch programs:
- ForeverLoop01 - This program uses a forever block in conjunction with an if block to cause a sprite to move in a small circle while the space bar is pressed.
- ForeverLoop02 - This program illustrates the creation of a counter loop using a forever block in conjunction with an if block.
- RepeatUntil01 - This program uses a repeat until block to cause a sprite to run in a small circle until the user presses the spacebar.
In addition, I will provide the specifications for a student-programming project for you to demonstrate your understanding of what you learned from thethree programs listed above and from earlier modules.
Discussion and sample code
The program named ForeverLoop01
This program illustrates the use of a forever block in conjunction with an if block. When the user clicks the green flag, the program implements the pseudocode shown in Image A .
Image a. pseudocode for the program named foreverloop01.
Move the cat to the center of the stage.
Turn the cat to face to the right.while(true){//infinite loop
if(space bar is pressed){Change cat to next costume image
Move cat forward 10 steps.Wait 0.1 seconds.
Turn cat clockwise 15 degrees.}//end if statement
}//end infinite while loop
If the space bar is pressed, the cat moves around the circumference of a small circle, appearing to walk due to the visual effect produced by switchingbetween two images (known in Scratch as costumes) . As explained in a previous module,the two costumes were designed to produce the illusion of walking.
Program code for the Scratch v1.4 version of the program named ForeverLoop01
The code for the older Scratch v1.4 version of this program is shown in Image B .