Worksheet

F6.4 Debugging logic errors

Robust programs · GCSE · OCR J277 2.3.2, AQA 8525 3.2.11, Edexcel 1CP2 6.1.2 · about 15 min

What this lesson is about

Finding the mistake that gives no message: watching, printing, stepping and common bugs.

• Step 1: know what should happen
• Step 2: find where it first goes wrong
• Step 3: form a theory and test it
• Common logic errors
• Explaining it out loud

Questions 5 marks in all

1. [1 mark]What is the first step in finding a logic error?

   1. AKnow exactly what the program should do
   2. BDelete the last line
   3. CRewrite the program
   4. DAdd more loops

2. [1 mark]A loop meant to run four times uses range(3). What kind of bug is this?

   1. AAn off-by-one error
   2. BA syntax error
   3. CA runtime error
   4. DA type error

3. [1 mark]Why change only one thing at a time when debugging?

   1. ASo you know which change fixed or broke the program
   2. BPython only allows one change
   3. CIt is faster to type
   4. DTo keep the program short

4. [1 mark]Which help find where a logic error first appears?

   Tick every answer that is true.

   1. APrinting variables inside a loop
   2. BStepping through with a debugger
   3. CWatching the robot's trail
   4. DAdding more comments

5. [1 mark]What does this program print?

   total = 0
   for x in [4, 6]:
       total = 0
       total = total + x
   print(total)

The task: fix the square

This program should drive a square of 25 cm sides clockwise, turning right at each corner, and finish back at the start, facing the way it began. It has three logic errors. Find them with the trail, prints or Debug, and fix them one at a time.

# the two lines every program starts with: the commands, then the robot
from bugbot import *
connect()

side = 25
for i in range(3):
    side = 20
    forward(60, distance=side)
    turn_left(30, angle=90)

Plan your program here, then type it in and press Run.

Do it on the robot
www.bugbotlab.com/learn/f6-4-debugging-logic-errors/
The simulator checks it and tells you when it passes. Nothing to install, no account.

Challenges

1. Write a program with an off-by-one error in it for a partner, and time how long they take to find it with Debug.
2. The parking sensor from F2.8 stops too close to the wall at high speed. Is that a logic error? How would you find the best stopping distance?
3. Add a print that shows a variable's value and its type. When would the type be the clue?