Form 3 ICT

Writing an Algorithm in Plain English

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A friend from another country has never played Snakes and Ladders. You are not allowed to show them, only tell them, in words, exactly what to do. Could you write instructions so clear that they could play the whole game without asking a single question?

A colourful Snakes and Ladders board on a red cloth, with dice, a dice cup and small counters beside it.
A Snakes and Ladders board with dice and counters. The game has simple rules, which makes it perfect for practising writing an algorithm.

Image source: File:Snakes and Ladders board.jpg (Wikimedia Commons)

In this post:

  • What a narrative algorithm is
  • Breaking a big task into smaller subtasks
  • Writing clear steps, with Snakes and Ladders as the example
  • Playing a game that follows an algorithm one step at a time
  • Sorting subtasks, spotting unclear steps and writing your own

Narrative algorithms

An algorithm is a set of step-by-step instructions for doing a task. A narrative algorithm is one written in ordinary sentences, like a story told in order, usually with numbered steps. (In the next posts you will draw algorithms as flowcharts instead.)

A good algorithm is:

  • Clear: each step means one thing only. “Move a bit” is unclear; “move forward 3 squares” is clear.
  • In the right order: you cannot move your counter before you roll the die.
  • Complete: it covers every situation that can happen (what if you land on a snake?).
  • Finite: it has a clear start and a clear end, so it does not go on forever.

Step 1: divide the task into subtasks

Big tasks are easier if you first split them into smaller jobs, called subtasks, and then write the steps for each one. For example, getting ready for school could be split into: get washed, get dressed, eat breakfast and pack your bag. Each subtask has its own steps.

Try it: which main task does each subtask belong to?

Drag (or tap) each subtask into the main task it belongs to.

Boil the water
Make a music playlist
Choose a date and place
Check whether someone has reached the last square
Roll the die
Mix in the cocoa and sugar
Collect money for snacks
Climb a ladder when you land at its foot

Plan a class lime

Make a cup of cocoa

Play Snakes and Ladders

Step 2: write the steps

Here is a narrative algorithm for Snakes and Ladders on a 30-square board. Notice how it covers the start, every turn, the ladders and snakes, the winner and the order of turns:

  1. Both players place their counters just before square 1. The youngest player goes first.
  2. The player whose turn it is rolls the die.
  3. Move the counter forward by the number rolled. If that would take it past square 30, it stays where it is.
  4. If the counter lands at the foot of a ladder, move it up to the top of the ladder.
  5. If the counter lands on a snake’s head, move it down to the end of the snake’s tail.
  6. If the counter is on square 30, that player wins and the game ends.
  7. If the player rolled a 6, they roll again (go back to step 2). Otherwise, it is the other player’s turn (go back to step 2).
Look at steps 4, 5, 6 and 7. Each starts with If. They are decisions: something happens only when a condition is true. And step 7 sends you back to step 2, which is a loop. You will draw decisions and loops in flowcharts later in this course.

Activity: play by the algorithm

Try it: Snakes and Ladders, one step at a time

Click Do the next step to carry out the algorithm one step at a time. The step being carried out is highlighted. Watch how the game follows the steps exactly, just like a computer would.

The algorithm

  1. Both counters start before square 1. Blue goes first.
  2. Roll the die.
  3. Move forward that many squares (stay put if it would go past 30).
  4. Foot of a ladder? Climb up.
  5. Snake’s head? Slide down.
  6. On square 30? You win, and the game ends.
  7. Rolled a 6? Roll again. If not, the other player’s turn. Go back to step 2.

Activity: spot the unclear step

Try it: fix the algorithm

Click the best answer. You get feedback straight away; only your first click counts towards your score.

1. “Move your counter some squares.” What is wrong with this step?

2. An algorithm says: “1. Move your counter. 2. Roll the die.” What is the problem?

3. An algorithm for the game never says what happens when someone reaches square 30. What is missing?

4. Which step is clearest?

Watch: what is an algorithm?

This short video explains what an algorithm is, with everyday examples.

Video: Computer Science Basics: Algorithms – LearnFree – https://www.youtube.com/watch?v=kM9ASKAni_s

Your turn: write a narrative algorithm

Choose one task and write a numbered narrative algorithm for it. First list the subtasks, then write the steps for each. Swap with a partner: can they follow your steps exactly, without asking a question?

  • How to play noughts and crosses (tic-tac-toe)
  • How to make a cheese sandwich
  • How to find the larger of two numbers
  • How to top up a phone with credit

Self-check quiz

Click the best answer. You get feedback straight away; only your first click counts towards your score.

1. An algorithm is…

2. A narrative algorithm is written…

3. Splitting “get ready for school” into wash, dress, eat and pack is called…

4. Why must an algorithm be finite?

5. In the Snakes and Ladders algorithm, which step is a decision?

6. Which word makes the order of narrative steps clear?

Related posts on islandclass.org

For later, in Forms 4–5 (CSEC IT): these go deeper and are for when you reach CSEC Information Technology.

Next: Flowchart Symbols

Syllabus link: Forms 1–3 ICT curriculum, Module 5, Objective 5.8 – develop a narrative solution to a given task.

Attributions to media used in this post:

Updates

2026/10/21

- Fixed the activity code so its buttons keep working.

© 2026 Vedesh Kungebeharry. All rights reserved.

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