Good Learning Co.
SavannahYear 8
← Computing
ComputingCoding: from blocks to Python20 min★★ difficulty

Algorithms: sequence, selection and loops

An algorithm is a precise set of steps. Every program, from a game to a search engine, is built from three simple ideas.

Part of your national curriculum
  • Algorithms and programming: Understand simple algorithms, expressed using sequence, selection and iteration, and correct errors in algorithms

Lesson overview

What you'll learn in this lesson

Understand simple algorithms, expressed using sequence, selection and iteration, and correct errors in algorithms

Key learning points

  • • Sequence
  • • Selection and iteration
  • • Try it in blocks
  • • Debugging

This lesson at a glance

  • 20 minutes
  • 19 parts to scroll through
  • 4 quick checks
  • Marked quiz at the end
  • Gentle pace: short sittings with pauses

Words to know

simplealgorithmsexpressedsequenceselection

Scroll down — the lesson carries on below

Part 1 of 19 · Intro5%
1

Introduction

Part 1 of 19

Algorithms: sequence, selection and loops illustrationVisual introduction

Picture this

Algorithms: sequence, selection and loops

An algorithm is a precise set of steps. Every program, from a game to a search engine, is built from three simple ideas.

In a nutshell

Understand simple algorithms, expressed using sequence, selection and iteration, and correct errors in algorithms

2

What you already know

Part 2 of 19

Before we start

What you already know

You've followed step-by-step instructions, like a recipe.

3

Key words

Part 3 of 19

Key words

Words you'll need today

algorithm
A precise, ordered set of steps that solves a problem.
sequence
Steps carried out one after another, in order.
selection
Choosing what to do next with an 'if' (for example, if it's raining, take a coat).
iteration
Repeating steps, also called a loop.
4

Learning cycle

Part 4 of 19

Learning cycle 1 of 2

Part 1: The three building blocks

A short piece of teaching, then a check to make sure it has landed.

5

Explore the idea

Part 5 of 19

Learn

Sequence

Order matters. 'Put on socks, then shoes' works; 'shoes, then socks' doesn't. A computer follows steps exactly as written, so every step must be in the right place.

6

Explore the idea

Part 6 of 19

Learn

Selection and iteration

Selection uses a condition: IF the path is clear, move forward, ELSE turn. Iteration repeats: 'repeat 4 times: move, turn right' draws a square in two lines instead of eight.

7

Quick check

Part 7 of 19

Quick check

What is an algorithm?

8

Quick check

Part 8 of 19

Quick check

'IF it's raining, take a coat' is an example of:

9

Reset break

Part 9 of 19

Pause

That's sitting 1 of 2 done

Stretch, get a drink, look out of the window. There is no timer and nothing is counting down — your place is saved, so you can come back in five minutes or tomorrow.

Stop here for now
10

Learning cycle

Part 10 of 19

Learning cycle 2 of 2

Part 2: Finding bugs

A short piece of teaching, then a check to make sure it has landed.

11

Explore the idea

Part 11 of 19

Learn

Try it in blocks

In the Block Lab below the lessons, guide Luna to the goal using Forward, Turn and Repeat blocks. Watch how a Repeat block makes the program shorter.

12

Explore the idea

Part 12 of 19

Learn

Debugging

A bug is a mistake in an algorithm. To find it, run the steps one at a time and compare what happens with what you expected. The first place they differ is where the bug is.

13

Quick check

Part 13 of 19

Quick check

'Repeat 4 times: move, turn right' uses:

14

Quick check

Part 14 of 19

Quick check

The best way to find a bug is to:

15

Common mix-ups

Part 15 of 19

Common mix-ups

Things people often get wrong

  • Computers don't guess what you meant. They do exactly what you wrote.
  • A loop isn't just for saving typing. It also makes programs easier to change.
16

Challenge round

Part 16 of 19

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

17

Challenge round

Part 17 of 19

Game · Fill the gaps

Finish the sentences

Choose the word that belongs in each gap.

____ uses a condition: IF the path is clear, move forward, ELSE turn.

18

Mastery quiz

Part 18 of 19

Marked quiz

End of lesson quiz: Algorithms: sequence, selection and loops

4 questions, marked with the reasoning shown. No timer.

  1. 1. Which is the correct order to make toast?

  2. 2. A loop that runs 'move, turn right' 4 times draws a:

  3. 3. A bug is:

  4. 4. An algorithm repeats 'move forward' until a wall is reached, then turns right. Which programming ideas does it use?

19

Lesson round-up

Part 19 of 19

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 4 on the practice checks. Only if you feel up to it — one more?

Ask Luna

Do 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.

Article

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

  1. Write in your own words what the checkout sends and what it receives back.
  2. Explain one way bad data could cause a wrong price, and how logging helps find it.
  3. Design a test table with four tests for a new checkout, with expected and actual results.

  • Reading and notes

    Reading

    Read 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

    Writing

    Explain 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

    Diagram

    Write your steps out as pseudocode or a flowchart before you type anything, then photograph it.

    1 flowchart or 10 lines

  • Code write-up

    Writing

    Copy your code in, then explain underneath what each part does and what you had to fix.

    Code + 5 sentences

  • Test table

    Working out

    Make a test table: what you tried, what you expected, what actually happened.

    4–6 tests

Saved as you type.

Part 1 of 19 · Introduction

Playground

Block Lab

Guide Luna to the goal

Hint: Try a Repeat block.

Your program is empty. Add some blocks.

0 blocks

How this is going

  • Not looked at yet

    Understand simple algorithms, expressed using sequence, selection and iteration, and correct errors in algorithms

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