Debugging - JSS 2 note

DEBUGGING

Subject: Digital Technologies

Class: JSS 2

Term: Second Term

Week: 10

Topic: Debugging

Duration: 80 Minutes (Double Period)

Learning Objectives

By the end of the lesson, students should be able to:

1. Define debugging.
2. Explain the importance of debugging in programming.
3. Identify common types of programming errors.
4. Describe the steps involved in debugging a program.
5. Identify and correct simple errors in a Scratch program.

1. Meaning of Debugging

Debugging is the process of finding, identifying and correcting errors in a computer program so that it works as intended.

An error in a computer program is commonly called a bug. A bug may prevent a program from running, cause it to produce an incorrect result or make it behave differently from what the programmer expected. Debugging is an important part of programming because even carefully written programs may contain errors.

Example

Suppose a Scratch program is designed to move a sprite 10 steps when the green flag is clicked. If the sprite does not move, the programmer should inspect the blocks to discover the cause. Possible causes include:

• The movement block is missing.
• The blocks are connected incorrectly.
• The program has not been started.
• The movement instruction is attached to the wrong event.

2. Importance of Debugging

Debugging is important for the following reasons:

1. Corrects errors: It helps programmers find and fix mistakes.
2. Improves reliability: It helps programs behave as expected.
3. Improves accuracy: It helps ensure that calculations and results are correct.
4. Saves time: Finding errors early can prevent bigger problems later.
5. Improves user experience: Correctly working programs are easier and more pleasant to use.
6. Supports learning: Debugging helps programmers understand how instructions affect a program.

3. Common Types of Programming Errors

Type of Error	Meaning	Example
Syntax Error	An error caused by breaking the rules of a programming language.	Writing an instruction incorrectly in a text-based programming language.
Logic Error	An error in the reasoning or instructions that causes an incorrect result.	A game reduces the player's score when it should increase it.
Runtime Error	An error that occurs while a program is running.	A program attempts an operation that it cannot complete during execution.

Note: In Scratch, blocks help reduce many syntax errors because students connect graphical instructions instead of typing the complete code. However, Scratch projects can still contain logic errors, incorrect block arrangements and unexpected behaviour.

4. Steps Involved in Debugging

Step 1: Identify the Problem

Observe the program and determine what is wrong. For example, a sprite may move in the wrong direction or the score may not increase.

Step 2: Reproduce the Error

Run the program again and observe the problem carefully. Check the conditions under which the error occurs.

Step 3: Inspect the Instructions

Examine the program's blocks or code. Check whether any instruction is missing, misplaced or incorrect.

Step 4: Correct the Error

Change the incorrect instruction or block arrangement.

Step 5: Test the Program Again

Run the program to see whether the correction has solved the problem.

Step 6: Repeat if Necessary

If the error remains, continue investigating and testing until the program behaves as intended.

Remember: Debugging is a process of observing, investigating, correcting and testing. Do not assume that an error has been fixed until the program has been tested again.

5. Practical Examples of Debugging

Example 1: Incorrect Sprite Movement

Expected behaviour: The sprite should move 10 steps when the green flag is clicked. Problem: The sprite does not move. Possible correction: Ensure that the green-flag event block is connected to the movement block.

When green flag clicked
Move 10 steps

In Scratch, select the Events block for the green flag and connect it to the Motion block for moving 10 steps.

Example 2: Incorrect Score

Expected behaviour: The player's score should increase by one when an object is caught. Problem: The score decreases instead. Cause: The program uses a block that changes the score by -1. Correction: Replace it with a block that changes the score by 1.

When the object touches the player
Change Score by 1

The example describes the intended logic. In Scratch, the touching condition and score-changing instruction must be assembled using the appropriate blocks.

Example 3: Repeated Action Does Not Stop

Expected behaviour: A sprite should repeat an action five times and then stop. Problem: The sprite continues repeating the action indefinitely. Possible cause: A forever loop is used instead of a repeat (5) loop. Correction: Use the repeat block when the action should occur a fixed number of times.

6. Practical Activity: Debug a Scratch Program

Objective: Students will identify and correct simple errors in a Scratch program.

Activity A: Fix the Movement Program

1. Open Scratch and create a new project.
2. Add a sprite and create a program using the green-flag event and a movement block.
3. Run the program and observe the sprite.
4. Disconnect the movement block from the event block.
5. Run the program again and observe the difference.
6. Reconnect the blocks correctly and test the program.

Activity B: Fix a Score Error

1. Create a variable named Score.
2. Set the score to zero when the green flag is clicked.
3. Use a condition to check when the player catches an object.
4. Make the score change by -1 to simulate an error.
5. Run the program and observe the incorrect result.
6. Change the instruction to increase the score by 1.
7. Run the program again to confirm that it works correctly.

Questions for the Activity

• What problem did you observe?
• Which instruction caused the problem?
• What correction did you make?
• How did you confirm that the program was working correctly?

7. Good Debugging Practices

• Read instructions carefully before changing them.
• Test a program regularly while developing it.
• Change one thing at a time where possible.
• Observe the result after each correction.
• Ask for help when an error is difficult to understand.
• Keep a working copy of an important project before making major changes.

8. Summary

• Debugging is the process of finding and correcting errors in a program.
• A bug is an error that causes a program to behave incorrectly.
• Common error types include syntax errors, logic errors and runtime errors.
• Debugging involves identifying the problem, inspecting the program, correcting the error and testing again.
• Scratch programs can be debugged by checking the arrangement and behaviour of programming blocks.

9. Evaluation Questions

1. What is debugging?
2. What is a bug in programming?
3. State three reasons debugging is important.
4. Explain the difference between a syntax error and a logic error.
5. What is a runtime error?
6. List four steps involved in debugging a program.
7. How would you correct a Scratch program whose sprite does not move?
8. Why should a program be tested after an error has been corrected?

10. Assignment

1. Define debugging and explain its importance in programming.
2. Describe three common types of programming errors and give one example of each.
3. List six steps involved in debugging a computer program.
4. Explain how to debug a Scratch program that decreases a player's score instead of increasing it.
5. State five good practices that programmers should follow when debugging.

11. Interactive Revision Quiz

Select one answer for each question and click Mark Quiz to see your score.

1. What is debugging?

Designing a computer keyboard Finding and correcting errors in a program Printing computer documents Installing a monitor

2. What is a bug in programming?

A type of computer mouse A programming error A storage device A computer game character only

3. Which error occurs when programming language rules are broken?

Syntax error Logic error Printing error Network signal

4. A program runs but produces the wrong answer. This is most likely a:

Hardware upgrade Backdrop error Logic error Screen resolution

5. When does a runtime error occur?

While a program is running Only when a computer is switched off When paper is inserted into a printer When a file is renamed correctly

6. What should a programmer do after correcting an error?

Delete the program immediately Test the program again Avoid running the program Disconnect the keyboard

7. A Scratch sprite does not move because its movement block is disconnected. What should you do?

Change the monitor Remove the sprite Connect the movement block correctly Turn off the computer

8. Which Scratch block is appropriate for repeating an action five times?

Forever Repeat (5) Stop all only Hide

9. Why is it useful to change one instruction at a time during debugging?

It makes the computer slower It prevents the program from running It makes every error disappear automatically It helps identify which correction affects the problem

10. Which sequence best describes debugging?

Identify, inspect, correct and test Print, delete, close and ignore Draw, colour, cut and paste Switch off, unplug, remove and discard