2.3 Scr0130: sequence, selection, and loop

Table of contents

• Preface
  • Viewing tip
    • Images
• General background information
  • What is structured programming?
    • One door in and one door out
    • Nesting of structures is allowed
  • Pseudocode
  • The sequence structure
    • The action elements themselves may be structures
  • The selection structure
    • Test a condition for true or false
    • The action elements themselves may be structures
    • Sometimes no action is required on false
  • The loop structure
    • Perform the test and exit on false
    • Perform some actions and repeat the test on true
    • Each action element may be another structure
    • Need to avoid infinite loops
  • Other possible structures
• Preview
  • Screen output for the program named IfSimple01
  • Click the green flag or the basketball
• Discussion and sample code
  • Three sprites
  • Program code for the LeftBeachball
    • Adding the blue go to block to the program
    • Coordinate values
    • Behavior of the LeftBeachball
  • Positioning the other beach ball
  • The basketball
    • Initializing the position of the basketball
    • Initializing the orientation of the basketball
    • Handling mouse events on the basketball
    • Behavior of the bottom script
    • The selection block
    • Two independent decisions
  • Selection control structures available in Scratch
    • How do the two bottom blocks differ?
    • Specifying the condition on which the decision will be based
    • Two groups of programming blocks have the correct shape
    • Will use the touching block
  • Go back and examine the script
    • Conditions have been established - need actions
  • Programming blocks belonging to the Motion group
    • Turn around and face the other way
  • An online version of this program is available
• Run the program
• Student programming project
  • Orthogonal axes in Cartesian coordinates
  • Draw a straight line to the location of the next mouse click
  • A sneak peek at the solution
• Summary
• What's next?
• Resources
• Miscellaneous

Preface

This module is one in a collection of modules designed to help beginners of all ages (8 and up) learn how to create the code for computer programs. Information is provided not only for the beginners themselves butalso for their parents and teachers where appropriate.

The purpose of this module is to teach you about structured programming; the sequence structure, the selection structure, and the loop structure. The module will also teach you how to write a Scratch program that illustrates the selection structure, mouse events, and Cartesian coordinates.

The module also presents a student project by which you can demonstrate your understanding of the concepts learned by studying the module.

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 1 . The sequence structure in pseudocode.
• Image 2 . The selection structure in pseudocode.
• Image 3 . The loop structure in pseudocode.
• Image 4 . User interface for the program named IfSimple01.
• Image 5 . A paraphrased version of the Scratch code.
• Image 6 . Program code for the left beach ball
• Image 7 . Initializing the position and orientation of the basketball.
• Image 8 . All of the code that applies to the basketball.
• Image 9 . Pseudocode for a selection structure.
• Image 10 . Two independent decisions.
• Image 11 . Selection control structures available in Scratch.
• Image 12 . Programming blocks belonging to the Sensing group.
• Image 13 . Programming blocks belonging to the Motion group.
• Image 14 . Initial output from the program named IfWithVar01.
• Image 15 . Program output after having clicked twice in the Stage area.