MYP Design Decomposition, Patterns and Abstraction Notes

Computational Thinking: Four Moves You Can Practise

1. Computational thinking is how you get from "I want a bin that tells me when it is full" to a list of instructions a micro:bit can actually run.
2. The four moves are decomposition, pattern recognition, abstraction and algorithmic thinking.
3. You loop back through them rather than working straight down the list, because cutting a problem up usually shows you a pattern you had not noticed.
4. None of the first three moves needs a computer in front of you; a pencil and the back of a page is enough.
5. In MYP design this thinking mostly shows up in Inquiring and analysing and Developing ideas, long before a line of code gets typed.

Decomposition: Cut Until Each Piece Is One Job

1. Decomposition means splitting one large problem into smaller problems you can solve one at a time.
2. A school locker booking app sounds like a single thing until you cut it into five: store the lockers, check which are free, take a booking, show a confirmation, release everything at the end of term.
3. The test for whether you have cut far enough is simple, because you should be able to picture roughly what the code for one piece looks like.
4. Cut too little and you are still staring at a problem with no obvious first line.
5. Cut too far and you end up with forty fragments and no sense of how they join, so stop when a piece does exactly one job.
6. Pieces that do one job also split neatly between people, so a group project can run three ways at once.

Patterns Are the Work You Only Do Once

1. Pattern recognition is spotting that two parts of your problem have the same shape.
2. In the locker app, "is this locker free" and "is this locker mine" both take a locker number and hand back true or false.
3. Once you see that shape you write one checking routine and feed it a different rule, instead of writing two routines that drift apart later.
4. Patterns also cross projects, since a countdown timer, a step counter and a quiz score all hold one number that changes and gets displayed.
5. A repeated pattern is your signal to write a function, which is covered in the functions and modular design article.

Abstraction: Deciding What to Ignore

1. Abstraction is throwing away the detail that does not change the outcome.
2. The London Underground map is an abstraction, because it drops real distances and street layout and keeps only order and interchanges, which is all a passenger uses.
3. In code, a variable called distance holding the number 42 ignores whether the thing 42 centimetres away is a wall, a hand or a cat.
4. You have to understand a problem well to abstract it well, since you can only drop a detail safely once you know it does not affect the answer.
5. Abstract too hard and you lose something you needed, like a reversing sensor that ignores height and drives straight into a low bollard.

Algorithmic Thinking Puts the Pieces in Order

1. Algorithmic thinking is arranging your pieces into a sequence of steps with no gaps and no room for interpretation.
2. Order carries meaning, so you read the moisture sensor before you compare it, and compare it before you switch the pump on.
3. Every step has to be something the machine can do, so "work out if the soil is dry" becomes "if the moisture reading is below 400, treat the soil as dry".
4. Choices and repeats belong in the sequence too, such as "check every 30 minutes" or "only water if it has not watered in the last 6 hours".
5. Once the order is settled you have something worth drawing as a flowchart, which the next article covers.

Worked Example: A Bag Tag That Reminds You What to Pack

1. Year 8 students keep turning up without PE kit, so the brief is a small tag on the bag that shows what today needs.
2. Decompose it into five pieces: know what day it is, store a timetable, work out what that day needs, tell the student, let the timetable be edited.
3. Spot the pattern, since "what does Monday need" and "what does Thursday need" are the same lookup with a different day, so you write it once and call it five times.
4. Abstract away the subject names, because the tag only needs short item codes like PE, ART and SWIM to fit a 5 character display.
5. Sequence it as: on button press, read the day, look up that day's list, show each code for 2 seconds, then sleep.
6. That whole plan fits on one page of your design folder and is evidence for Developing ideas before any hardware is switched on.