Variables, lists and debugging
Bugs are normal. Finding them quickly is a method, not a talent.
Part of your national curriculum
- Algorithms and programming: Use two or more programming languages, at least one of which is textual, to solve a variety of computational problems
- Algorithms and programming: Use a variety of data structures, including lists, tables or arrays, and design modular programs that use procedures or functions
- Algorithms and programming: Understand simple algorithms, expressed using sequence, selection and iteration, and correct errors in algorithms
- Algorithms and programming: Design and develop modular programs that use procedures or functions, and test and correct them systematically
Lesson overview
What you'll learn in this lesson
Test and debug a program systematically
Key learning points
- • Storing data
- • Systematic testing
This lesson at a glance
- 30 minutes
- 10 parts to scroll through
- 3 quick checks
- Marked quiz at the end
- Gentle pace: short sittings with pauses
Words to know
Scroll down — the lesson carries on below
Watch & discover
Part 1 of 10
Watch first
Variables, lists and debugging
An English teaching video on this topic from LearnFree. Watch it, then scroll down for the lesson.
In a nutshell
Test and debug a program systematically
Explore the idea
Part 2 of 10
Learn
Storing data
A variable holds one value; a list holds many, accessed by index. Meaningful names save more time than clever code.
Explore the idea
Part 3 of 10
Learn
Systematic testing
Test normal, boundary and invalid inputs. When something fails, change one thing at a time and print values to see what the program actually holds.
Quick check
Part 4 of 10
Quick check
A list stores…
Quick check
Part 5 of 10
Quick check
A boundary test uses…
Quick check
Part 6 of 10
Quick check
When debugging you should change…
Challenge round
Part 7 of 10
Game · Sort it
Which of these are true?
Drag each card into the right column. Tap a card first if dragging is fiddly.
True
Not true
Challenge round
Part 8 of 10
Game · Fill the gaps
Finish the sentences
Choose the word that belongs in each gap.
When something fails, change one thing at a time and print values to see what the ____ actually holds.
Mastery quiz
Part 9 of 10
Marked quiz
End of lesson quiz: Variables, lists and debugging
4 questions, marked with the reasoning shown. No timer.
1. Why use a list rather than 30 separate variables for 30 test scores?
2. A list stores…
3. A boundary test uses…
4. When debugging you should change…
Lesson round-up
Part 10 of 10
Lesson round-up
Ready when you are
Quiz score
Not sat
Games
Not played
Points this lesson
0
Best run
0 in a row
Luna: 0 out of 3 on the practice checks. Only if you feel up to it — one more?
Ask LunaDo the work
Your written work for this lesson
This is the part you'd do in your book at school. Pick the pieces below, write here or on paper, and photograph anything you draw or work out.
How a supermarket checkout knows the price
When a barcode is scanned, the checkout does not store the price itself. It sends the barcode number to a database and reads back a record: product name, price, whether it needs an age check. The record is the single place the price lives, so changing one row changes every till in the shop at once.
Problems come from bad data, not bad scanners. If two products share a barcode, or a price update reaches the shop overnight but the till holds an old copy in its cache, customers are charged the wrong amount. Shops check this with test transactions: scan a known item, expect a known price, record what actually happened.
Good systems also log every scan. If a customer complains, the log shows exactly which record was used and when.
Written for Good Learning Co.
Reading questions
- Write in your own words what the checkout sends and what it receives back.
- Explain one way bad data could cause a wrong price, and how logging helps find it.
- Design a test table with four tests for a new checkout, with expected and actual results.
Reading and notes
ReadingRead the lesson text again and note the key terms and how each one is used, in your own words — as you would for revision.
Half a page of notes
Extended writing
WritingExplain a real system that uses this idea — how it works, where it can go wrong, and how you would make it safer or better.
250–350 words
Algorithm on paper
DiagramWrite your steps out as pseudocode or a flowchart before you type anything, then photograph it.
1 flowchart or 10 lines
Code write-up
WritingCopy your code in, then explain underneath what each part does and what you had to fix.
Code + 5 sentences
Test table
Working outMake a test table: what you tried, what you expected, what actually happened.
4–6 tests
Saved as you type.
Part 1 of 10 · Watch & discover
How this is going
- Not looked at yet
Use two or more programming languages, at least one of which is textual, to solve a variety of computational problems
- Not looked at yet
Use a variety of data structures, including lists, tables or arrays, and design modular programs that use procedures or functions
- Not looked at yet
Understand simple algorithms, expressed using sequence, selection and iteration, and correct errors in algorithms
- Not looked at yet
Design and develop modular programs that use procedures or functions, and test and correct them systematically