L3 — Loops & Shapes | Teacher Slides

Lesson 3 of 6

Loops & Shapes

Using for loops to draw geometric shapes with Edison

Year 9 · Robotics with Edisons

Learning Objectives

1Explain what a for loop is and how range(n) controls the number of repetitions

2Calculate the correct turn angle for any regular polygon using 360 ÷ sides

3Write and test an EdPy program that makes Edison draw a chosen shape using a for loop

Prior Knowledge: Lesson 2 — Ed.Drive(), Ed.TurnRight(), named variables, downloading to Edison.

Prep: Have markers/pens and paper ready for Edison to draw on (or use tape to mark the floor). Students who finish early can try the spiral extension.

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Key Vocabulary

Iteration and loop terminology

🔄

Iteration

Repeating a block of code. Each repeat is called one iteration. Loops are the programming construct that implements iteration.

📐

For Loop

A loop that repeats a fixed number of times. In Python: for i in range(4): — the indented code runs 4 times.

🔢

range(n)

Generates a sequence of numbers from 0 to n−1. range(4) gives 0,1,2,3 — 4 numbers total, so the loop body runs 4 times.

↔️

Indentation

Python uses 4 spaces of indentation to show what code belongs inside the loop. Incorrect indentation causes errors. EdPy (Python) is strict about this.

Analogy: "Imagine writing 'stir the pot' on a recipe 10 times vs writing 'stir 10 times'. The loop is the second version — less writing, same result."

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How For Loops Work

Trace through the execution step by step

The Code

sides = 4
side_length = 20
turn_angle = 90
speed = Ed.SPEED_3

for i in range(sides):
    Ed.Drive(Ed.FORWARD, speed, side_length)
    Ed.TurnRight(speed, turn_angle)

The indented lines run on EVERY iteration. Lines outside the loop run once, before or after.

Execution Trace

i =	Action
0	Drive forward 20cm → Turn right 90°
1	Drive forward 20cm → Turn right 90°
2	Drive forward 20cm → Turn right 90°
3	Drive forward 20cm → Turn right 90°
Loop ends — square complete ✓

Key question: "i starts at 0, not 1. Does that matter for our shape?" — No, because we only care about the number of iterations, not the value of i.

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Turn Angle Formula

The maths behind drawing any regular polygon

Turn Angle = 360 ÷ Number of Sides

Shape Reference

Shape	Sides	360 ÷ sides	Turn Angle
⬛ Square	4	360 ÷ 4	90°
🔺 Triangle	3	360 ÷ 3	120°
⬠ Pentagon	5	360 ÷ 5	72°
⬡ Hexagon	6	360 ÷ 6	60°
⭐ Octagon	8	360 ÷ 8	45°

Generalised Code

import Ed
Ed.EdisonVersion = Ed.V3
Ed.DistanceUnits = Ed.CM
Ed.Tempo = Ed.TEMPO_MEDIUM

sides = 6        # change this!
side_length = 20
turn_angle = 360 // sides  # integer division
speed = Ed.SPEED_3

for i in range(sides):
    Ed.Drive(Ed.FORWARD, speed, side_length)
    Ed.TurnRight(speed, turn_angle)

Ed.PlayBeep()  # signal completion

// vs /: // is integer (whole number) division. Edison needs whole degrees — 360 / 5 = 72.0 but 360 // 5 = 72.

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Student Task

Code a shape — must NOT be a square

Direct students to: student.html → L3 Loops & Shapes. Steps 1–3.

1Step 1: Write the square loop example in EdPy. Screenshot and answer: what's the difference between a loop and 4 copied lines?

2Step 2: Study the angle table. Write the generalised code using 360 // sides. Screenshot showing the calculation.

3Step 3: Choose any shape that is NOT a square. Code and test it. A triangle, hexagon, and octagon are all valid.

🥉 Bronze

Working loop that draws a non-square shape. Uses range(sides) and 360 // sides.

🥈 Silver

+ Generalised: changing one variable (sides) should change the whole shape. + Ed.PlayBeep() at end.

🥇 Gold

Spiral: add a variable that increases side_length each iteration using side_length += 2 inside the loop.

Physical tip: Tape a marker to the underside of Edison and place it on paper to see the shape drawn. Alternatively, set up a clear path on the floor and watch the turn angles.

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Plenary

Consolidate and preview Lesson 4

Exit Ticket — Mini-Whiteboard / Show of Hands

"I want to draw a regular 12-sided shape (dodecagon). What value do I give sides and what will turn_angle be?"

✅ Today we covered

For loops and range(n)
Python indentation rules
Turn angle formula: 360 ÷ sides
Integer division with //
Drawing shapes with Edison

⏭ Next Lesson

Lesson 4: Flowcharts & Obstacle Detection

We will design algorithms using flowcharts, then program Edison to detect and respond to obstacles using IR sensors and if/elif/else.

Exit ticket answer: sides = 12, turn_angle = 360 // 12 = 30°.