Topic 11 Algorithms
KS3 Computing · An algorithm is a precise, step-by-step plan to solve a problem. Computational thinking turns a messy problem into one. Every algorithm is built from just three things: sequence, selection and iteration.
Programming Computational ThinkingCambridge Lower Secondary Computing 0860
Say each word, then read what it means. Every one is in the glossary too.
Read it here, or open the study page below to go one step at a time with things to try.
A bug is a mistake in a program that makes it do the wrong thing.
Debugging means finding bugs and fixing them.
Every programmer writes bugs. Good programmers find them calmly, one step at a time.
Scratch blocks only snap together where they fit. So most Scratch bugs are logic errors: the script runs, but does the wrong thing.
PRIMM is five steps for working with a program someone else wrote.
| Letter | Step | What you do |
|---|---|---|
| P | Predict | Read the code. Say what you think it will do, before you run it. |
| R | Run | Run it and watch. Was your prediction right? |
| I | Investigate | Look closely. Trace it block by block. Change one thing and see what happens. |
| M | Modify | Change the code so it does something new. |
| M | Make | Write your own program using the same idea. |
Why predict first?
A wrong prediction is useful. The place where the code surprises you is where to look for the bug.
Change only one thing, then test. Change three things at once and you will not know which one fixed it.
| Bug | What you see | Fix |
|---|---|---|
| Variable never reset | The score starts at the last game's number | Put set [score v] to (0) under when green flag clicked |
| Wrong sign | Lives go up instead of down | Use change [lives v] by (-1) |
| Missing loop | The sprite moves once, then stops | Put the blocks inside forever or repeat |
| Wrong number | Repeat 3 with turn 90 draws only three sides of a square | Use repeat (4) |
| Blocks in the wrong order | The sprite says 'Done!' before it moves | Drag the say block below the move blocks |
| Script on the wrong sprite | The wrong sprite moves | Drag the script onto the right sprite |
This script should count lives down from 3 and then say 'Game over'. It never says it. Why? Script: set lives to 3. Repeat until lives = 0: wait 1 second, change lives by 1. Say Game over.
change [lives v] by (1) adds 1.change [lives v] by (-1) and run it again.Open the study page and go one step at a time.
One question at a time. The reason shows after each answer.