Programming for Children

Lessons

Primary 6 · Science and Elementary Technology

Programming for Children

6 lessons

Lesson 4 of 6

Drawing Shapes in Scratch

30 min

Let's begin

Keza's class loved drawing polygons with the turtle. "Can a Scratch sprite draw too?" asks Keza. It can! A sprite can carry a pen, just like the turtle. Today you will use Motion, Pen and Control blocks to draw shapes and patterns.

Add the Pen extension

The Pen blocks are hidden at first. Click the Add Extension button in the bottom-left corner of Scratch and choose Pen. A new green Pen category appears:

Pen block	What it does
erase all	Clears every line and stamp from the stage.
pen down	The sprite draws a line wherever it moves.
pen up	The sprite moves without drawing.
stamp	Leaves a picture of the sprite on the stage.
set pen color to / set pen size to	Changes the colour or thickness of the line.

Motion blocks and the stage

move (10) steps moves the sprite forward; turn ↻ (15) degrees turns it clockwise; go to x: ( ) y: ( ) jumps to a place; point in direction (90) makes it face right. The centre of the stage is x: 0, y: 0. The x number goes from left (−240) to right (240), and the y number from bottom (−180) to top (180). So x: −200, y: −100 is near the bottom left.

A square in Scratch

when green flag clicked
erase all
go to x: (0) y: (0)
point in direction (90)
pen down
repeat (4)
move (100) steps
turn ↻ (90) degrees
pen up

The polygon rule from Turtle Blocks still works: turn = 360 ÷ number of sides. Change repeat (4) and 90 to repeat (6) and 60, and you get a hexagon.

Two kinds of loop

repeat (n) runs the blocks inside it n times, then the program carries on.
forever runs the blocks inside it again and again until you click the red stop sign.

In the Raspberry Pi project Lost in space, a rocket flies to the Earth with repeat (200) [move (2) steps], and a monkey spins with forever [turn ↻ (1) degrees].

Patterns with stamp

repeat (12)
stamp
turn ↻ (30) degrees

This leaves 12 copies of the sprite in a full circle, because 12 × 30 = 360, like the petals of a flower. You can also put a loop inside a loop: repeat (36) [draw a square, then turn ↻ (10) degrees] makes a beautiful spinning-squares pattern.

Common bugs

No pen down: the sprite moves but nothing is drawn.
No erase all: old drawings stay on the stage.
No go to and point in direction at the start: the shape begins in a different place each time.

Tip

Pen DOWN to draw, pen UP to travel. Start every drawing program with erase all for a clean page.

✦

Try it yourself

How many degrees should a Scratch sprite turn at each corner of an equilateral triangle?
What does the erase all block do?
How many copies of the sprite does repeat (12) [stamp, turn ↻ (30) degrees] leave?
Keza's program has no pen down block. What will happen?
Where on the stage is x: 0, y: 0?

Answers

120 (360 ÷ 3).
It clears all the lines and stamps from the stage.
12, arranged in a full circle.
The sprite will move, but it will not draw anything.
In the centre of the stage.

Quick check

Match each Pen block to what it does.

Tap one on the left, then its partner on the right.

What you learned today

Add the Pen extension to use erase all, pen down, pen up, stamp and pen colour.
Motion blocks plus a repeat loop draw polygons; turn = 360 ÷ number of sides.
repeat (n) repeats a set number of times; forever repeats until you stop the program.

Sources and credits

Paint box, by the Raspberry Pi Foundation, Raspberry Pi Projects, CC BY-SA 4.0, adapted by Ganzaa.
Lost in space, by the Raspberry Pi Foundation, Raspberry Pi Projects, CC BY-SA 4.0, adapted by Ganzaa.

The exercises (Try it yourself and the practice questions) are written by Ganzaa.

This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.

1Add the Pen extension

2Motion blocks and the stage

3A square in Scratch

4Two kinds of loop

5Patterns with stamp

6Common bugs