Computer Science
Grade 4
20 min
Debugging Challenge: Solving a Complex Bug
Students will be challenged to solve a complex bug in a program.
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1
Introduction & Learning Objectives
Learning Objectives
Identify a bug that is caused by more than one error in a program.
Use a step-by-step process to trace the flow of a program and find incorrect values.
Predict a program's output and compare it to the actual output to find a bug.
Isolate a problem by testing small parts of the code individually.
Fix multiple related errors to solve a complex bug.
Explain how changing one part of the code can affect another.
Have you ever tried to build a LEGO set, but one piece was in the wrong spot AND another piece was missing, making the whole thing wobbly? LEGO That's what a complex bug in code is like!
In this lesson, we'll become super code detectives! We will learn how to find and fix tricky bugs that are caused by more than one problem working toge...
2
Key Concepts & Vocabulary
TermDefinitionExample
Complex BugA problem in a program caused by two or more smaller errors working together, making it extra tricky to find.A game's score doesn't go up because the code uses a minus sign instead of a plus sign, AND the score variable is reset to zero in the wrong place.
Logical ErrorA bug where the code runs without crashing, but it does the wrong thing. The computer follows the instructions perfectly, but the instructions are wrong.You tell a robot to take 5 steps forward to reach a door, but the door is actually 10 steps away. The robot follows your command but doesn't reach the door.
Code TracingReading through your code one line at a time, pretending you are the computer, to see exactly what it's doing.Using your finger to follow each line of a r...
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Core Syntax & Patterns
The Detective's Method
1. Predict. 2. Run. 3. Compare.
First, PREDICT what your code should do. Second, RUN the program and see what it actually does. Third, COMPARE the result to your prediction. The difference between them is your clue!
Check the Variables
Print or display your variables at different steps.
The best way to see what your program is thinking is to check the state of its variables. By printing a variable's value before and after a function runs, you can see if it changed correctly.
One Change at a Time
Change only one thing in your code, then test it again.
When you find a bug, don't try to fix everything at once. Make one small change that you think will help, and then run the program to see what happens. This helps you know exactly wh...
4 more steps in this tutorial
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Challenging
A function `findLarger(num1, num2)` should return the larger of two numbers. It works for `findLarger(3, 8)` and `findLarger(10, 2)`. However, `findLarger(7, 7)` does not return anything. What is the most likely logical mistake in the algorithm?
A.The function can't handle the number 7.
B.The code checks `if num1 > num2` and `if num2 > num1`, but it forgets to handle the case where `num1 == num2`.
C.The function is subtracting the numbers instead of comparing them.
D.The computer needs to be restarted.
Challenging
In your drawing program, every 4th shape you draw is a triangle, even when you choose to draw a circle. The first 3 shapes are correct. Where would you look for the source of this bug?
A.In the code that draws circles.
B.In the code that checks the mouse position.
C.In a part of the code that uses a counter variable and checks if it is the 4th time through a loop.
D.In the code that lets you choose a color.
Challenging
You wrote code to make a character walk, and it works. Your friend wrote code to make clouds move, and it works. When you combine the code, the character stops walking. You both used a variable named `speed`. What is the most likely cause?
A.The character is afraid of clouds.
B.Computers can't move two things at once.
C.Your friend's code for the cloud `speed` is interfering with your `speed` variable for the character.
D.The screen is not big enough for a character and clouds.
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